Movement, Posture and Equilibrium

8/12/2019 Movement, Posture and Equilibrium

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Progress in NeurobiologyVoi. 38, pp. 35 to 56, 1992 0301-0082/92/$15.00Printed in Great Britain.All rights reserved 1991 PergamonPress plc

M O V E M E N T , PO S T U R E A N D E Q U I L IB R IU M :I N T E R A C T I O N A N D C O O R D I N A T I O N

JEAN MASSIONLaboratory of Fanctional Neurosciences, C.N.R.S., 31, chemin Joseph Aiguier,13402 Marseille Cedex 9, France

Received 27 January 1991)

CONTENTSI. Introduction2. Why arc anticipatory adjustments of posture associated with movements?2.1. Definition of anticipatory postural adjustments2.2. Internal disturbance of posture and equilibrium2.3. Modular approach to posture versus global approach to equilibrium2.4. Anticipatory postural adjustments associated with equilibrium maintenance2.4.1. Axial movements2.4.2. Arm movements2.4.3. Leg movements and equilibrium control2.5. Stabilization of the position of given segments2.6. Coordination between posture and equilibrium3. How are the anticipatory postural adjustments organized?3.1. Reference values3.2. Error detecting sensors3.3. Postural body scheme3.4. Postural adjustments3.4.1. Fixed versus flexible synergies3.4.2. Strategies4. Acquisition and adaptation4.1. Acquisition4.2. Short term adaptation4.3. Long term training4.4. Setting5. Central organization5.1. Postural networks and their control5.2. Modes of coordination between posture and movement5.3. Central organization of the coordination between posture and movement5.3.1. Role of basal ganglia and premotor areas5.3.2. Role of cerebellum5.4. ConclusionAcknowledgementsReferences

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1. I N T R O D U C T I O NSkilful motor performance means producing the

optimal response in a context involvin g external andinternal constraints. The external constraints arethose imposed by the environme ntalconditions underwhich the action is performed such as the gravita-tional forces, the reaction forces from the sup portingsurfaces, imposed accelerations and obstacles.The internal constraints are those which originatefrom the body itself: the geometrical configurationof the body segments, the inertial characteristics ofthe segments and the internal forces associated withmuscle contraction. The central organization of amotor skill takes into account all the external andinter nal constraints. It requires long term le arning asthe result of which these constraints are predicted inthe central control of the motor act.

In an intentional complex multijoint action, suchas grasping an object while standing, many com-ponents can be identified which contribute to theperformance as a whole: reaching the goal withthe hand , gazing at the target, stabilizing posture,maintaining equilibrium. Central control involvesmultiple parallel commands, each fulfilling a givengoal, and these controls are coordinated in order tobe integrated into the same act (Arbib, 1981).When attempting to identify the various com-ponents of the action, one approach consists ofdetermin ing which goals are controll ed at the sametime during the motor act. Actually, two main typesof control with respect to a goal can be identifiedwhich subserve different and opposite functions.The first type of control consists of main tain ing areference value against external or internal disturb-ances. The reference value can be the position of a35


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36 J. MASSlONg iv e n s e g m e n t o r s e v e ra l s e g m e n t s s u c h a s th e a rm ,t h e l e g , t h e t r u n k o r t h a t o f t h e w h o l e b o d y i n t h e c a seo f p o s t u r a l m a i n t e n a n c e . T h e r e f e re n c e v a l u e c a na l s o b e a m o r e a b s t r a c t p a r a m e t e r , a s i n t h e c a s e o fe q u i l i b r i u m c o n t r o l . H e r e t h e v a l u e r e g u l a t e d i n m a nu n d e r s t a t ic c o n d i t i o n s i s t h e p r o j e c t i o n o f t h e b o d yc e n t r e o f g r a v i t y ( C G ) i n s id e t h e s u p p o r t i n g s u r f a c e ,t h e a r e a d e l i n e a t e d b y t h e f e et . A n o t h e r p a r a m e t e rre g u la t e d i s t h e g a z e , w h ic h r e s u l t s f ro m th e c o -o r d i n a t i o n b e t w e e n h e a d a n d e y e p o s i t i o n s .A s e c o n d c a t e g o r y o f c o n t r o l i s t h e d i s p l a c e m e n ta l o n g a t r a j e c t o r y o f o n e o r s e v e r a l se g m e n t s o r t h ew h o l e b o d y t o w a r d s a g i v e n g o a l . It b r e a k s d o w n t h ei n i t i a l p o s t u r e b e f o r e b u i l d i n g u p a n e w o n e , w h i c hw i l l i n t u r n b e s t a b i l iz e d . R e a c h i n g a n d g r a s p i n g b o t hb e lo n g to th i s c a t e g o ry .I n m o s t o f o u r m o t o r a c t s, b o t h o f t he s e c a t e g o r i e so f c o n t r o l a r e c o m b i n e d , s i n c e t h e y i n v o l v e m a i n -t a i n i n g t h e r e f e r e n ce v a l u e s o f th e p a r a m e t e r s t o b er e g u l a t e d s u c h a s e q u i l i b r i u m , t h e p o s i t i o n o f g i v e ns e g m e n t s , g a z e f ix a t io n , a n d a t t h e s a m e t im e p e r -f o r m i n g a m o v e m e n t , s u c h a s r e a c h i n g a n o b j e c t .I n f a c t , t h e m o t o r a c t m i g h t b e c o m p a r e d t o a ni c e b e r g , t h e a p p a r e n t b e i n g t h e m o v e m e n t a n d t h eh i d d e n p a r t , w h i c h i s o f t e n t h e m o s t i m p o r t a n t , b e i n gt h e m a i n t e n a n c e o f r e f e r e n ce v a l ue s . T h e h y p o t h e s i st h a t t w o m a i n c o m p o n e n t s c a n b e i d e n ti f i e d i n a n ym o t o r a c t h a s a l r e a d y b e e n p r o p o s e d b y H e s s ( 19 43 )a n d J u n g a n d H a s s l e r (1 9 60 ) T h e y m a d e a d i s t in c t io ni n m o s t m o t o r a c t s b e t w e e n a t e l e o k i n e t i c a s p e c t ,w h i c h i s t h e g o a l o r i e n t e d m o v e m e n t a n d a n e r e i s -m a t i c a s p e c t, o r p o s t u r a l r e l a t e d c o m p o n e n t , w h i c hp r o v i d e s th e p o s t u r a l s u p p o r t f o r t h e m o v e m e n t w h i l em a i n t a i n i n g e q u i l i b r i u m O n e o f t h e m a in d i f f i c u l t i es a r i s in g w h e n o n ea t t e m p t s t o i n v e s t ig a t e t h e o r g a n i z a t i o n o f a g i v e nm o t o r a c t i s t h a t t h e i n t e r n a l f o r c e s w h i c h g i v e ri s e tot h e m o v e m e n t , a n d w h i c h r e s u l t f r o m m u s c l e c o n t r a c -t io n s , a t t h e s a m e t im e d i s tu rb th e r e fe re n c e v a lu e s tob e r e g u l a t e d , su c h a s t h e p o s t u r e o f b o d y s e g m e n t s o rt h e e q u i l i b r i u m . I t w a s f i r s t o b s e r v e d b y B e l e n k i ye t a l . (1 9 6 7 ) th a t d u r in g a m o v e m e n t s u c h a s r a i s in gth e a rm w h i l e s t a n d in g , t h e f i r s t m u s c le s to b ea c t i v a t e d a r e t h e l e g m u s c l e s i n v o l v e d i n p o s t u r a lc o n t r o l , s h o r t l y ( s o m e 5 0 - 1 0 0 m s ) p r i o r t o t h ep r i m e m o v e r a c t i v a t i o n . T h e se a n t i c i p a t o r y p o s t u r a la d j u s t m e n t s w e r e i n t e r p r e t e d a s f e e d f o r w a r d p a r a l l e lc o m m a n d s a i m e d a t m i n i m i z i n g th e e q u i l i b r i u m d is -t u r b a n c e a s s o c i a t e d w i t h m o v e m e n t p e r f o r m a n c e .S i n c e t h i s p i o n e e r s t u d y , a l a r g e a m o u n t o f d a t a h a v eb e e n c o l l e ct e d w h i c h t h r o w n e w l i g h t o n t h e f u n c t i o no f t h e s e f e e d f o r w a r d a d j u s t m e n t s , t h e i r c e n t r a l o r g a n -i z a t i o n , t h e i r a c q u i s i t i o n a n d t h e i r p a t h o l o g y .

2. W H Y A R E A N T I C I P A T O R YA D J U S T M E N T S O F P O S T U R EA S S O C I A T E D W I T H M O V E M E N T S ?2.1. DEFINITIONOF ANTICIPATORYPOSTURAL ADJUSTMENTS

T h e p o s t u r a l a d j u s t m e n t s a s s o c i a t e d w i t h m o v e -m e n t a r e c a l l e d a n t i c i p a t o r y b e c a u s e t h e i r o n s e to c c u r s p r i o r t o t h e o n s e t o f t h e d i s t u r b a n c e o f p o s t u r ea n d e q u i l i b r i u m r e s u l ti n g f r o m t h e m o v e m e n t ( F i g . 1 ).T h e r e i s s o m e c o n f u s i o n i n t h e l i t e r a t u r e c o n c e r n i n g

MovementI C o n t r o l o fP o t u r eF e e d b a c k g a i n i F e e d f o r w a r d g a i na n d g a t e c o n t r o l [ a n d g a t e c o n t r o l

I

1 P e r t u r b a t i o n 1P O S T U R E . . I M O V E M E N T

FIG. 1. Feedforward and feedback adjustment of posture.Diagra m representing the two m echanisms involved in thecompensation for a postura l perturba tion. In this diagram,the central co ntrol o f posture is indicated by a s triped line.Two phasic mechanisms are minimizing the postural dis-turbance: they operate through a feedback loop and afeedforward control. The feedforward control is actingthrough internal collaterals from the m ovement controlpathways on an adaptive network involved in posturalcontrol. Both mechanisms are under adaptive gate and gaincontrol (from Duress6 e t a l . 1988).t h e m e a n i n g o f a n t i c i p a t o r y . B e c a us e t h e a c ti -v a t i o n o r i n h i b i t i o n o f m u s c l e s in v o l v e d i n p o s t u r a lc o n t r o l s h o r t l y p r e c e d e t h e p r i m e m o v e r o n s e t , a s fo re x a m p l e d u ri n g t h e p e r f o r m a n c e o f a n a r m m o v e -m e n t w h i l e s t a n d i n g ( B e l e n k i y e t a l . 1967; Lee, 1980;Bou isse t an d Za t t a r a , 1981, 1986, 1987b , 1988; Co rd oa n d N a s h n e r , 1 9 8 2 ; F r i e d l i e t a l . 1984 , 1988; Horake t a l . 1 9 84 ; B ro w n a n d F r a n c k , 1 98 7 ; L e e e t a l . 1987;B e n v e n u t i e t a l . 1 9 9 0 ) , i t i s o f t e n c l a im e d th a t t h ep o s t u r a l a d j u s t m e n t s a r e c a l l e d a n t i c i p a t o r y b e c a u s et h e i r o n s e t i n t e r m s o f a c c e l e r a t i o n o r E M G c h a n g e st a k e s p l a c e b e f o r e t h o s e o c c u r r i n g a t t h e l e v e l o f t h em o v i n g a r m . I n f a c t , p o s t u r a l a d j u s t m e n t s a n d t h eo n s e t o f m o v e m e n t c a n b e t i m e l o c k e d , as f o r e x a m p l ei n t h e s t a n d i n g c a t m o v i n g a l e g u n d e r t h e a c t i o n o fc o r t i c a l s t i m u l a t i o n ( G a h 6 r y a n d N i e o u l l o n , 1 98 7;G a h 6 r y a n d M a s s i o n , 1 98 1) o r p e r f o r m i n g a p o i n t i n gt a s k ( A l s t e r m a r k a n d W e s s b e r g , 1 98 5) a n d i n h u -m a n s , w h e n r a i s i n g th e a r m i n r e s p o n s e t o a v i s u alm o v i n g c u e ( L e e e t a l . 1 9 8 7 ) o r d u r in g a b im a n u a ll o a d l i f t i n g t a s k ( H u g o n e t a l . 1982; Dufoss6 e t a l .1 9 8 5 a ; P a u l ig n a n e t a l . 1989).A n t i c i p a t o r y p o s t u ra l a d j u s t m e n t s m e a n t h a tt h e o n s e t o f t h e p o s t u r a l c h a n g e s o c c u r s p r i o r t o t h eo n s e t o f t h e p o s t u r a l d i s t u r b a n c e d u e t o t h e m o v e -m e n t a n d t h a t a f e e d f o r w a r d p o s t u r a l c o n t r o l i sa s s o c i a t e d w i t h t h e m o v e m e n t c o n t r o l w h i c h p r e v e n t st h e p o s t u r e a n d e q u i l i b r i u m d i s t u r b a n c e s a s s o c i a t e dw i t h m o v e m e n t p e r f o r m a n c e f r o m t a k i n g p l a c e . T h et e r m a n t i c i p a t o r y h a s e v e n b e e n u s e d t o d e n o t ea u t o m a t i c p o s t u r a l r e a c t i o n s t o a l o c a l d i s t u r b a n c e( T r a u b e t a l . 1 98 0 ). E v e n i f t h e s e r e a c t io n s a ret r i g g e r e d p a t t e r n e d r e s p o n s e s a c ti n g i n a f e e d f o r w a r dm a n n e r , th e a n t i c i p a t o r y s h o u l d b e a p p l i e d o n l yt o t h e p o s t u r a l a d j u s t m e n t s a s s o c i a t e d w i t h v o l u n t a r ym o v e m e n t s , b e c a u s e t h e y r e s u l t f r o m a n i n t e r n a l


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MOVEMENT, POSTUREAND EQUILIBRIUM 37c o m m a n d a n d n o t f r o m a n e x t er n a l in p u t . T h e f a c tt h a t t h e p o s t u r a l a d j u s t m e n t s o c c u r b e f o r e t h ed i s t u r b a n c e d u e t o t h e v o l u n t a r y m o v e m e n t d o e sno t r u l e ou t t he pos s i b i l i t y t ha t r e a c t i ona l pos t u r a la d j u s t m e n t s o r g a n i z e d i n a fe e d b a c k m o d e m a y t a k ep l a c e i n a l a t e r p h a s e o f t h e m o t o r a c t . A s m e n t i o n e dby F r i e d l i e t a l . ( 1984) , t he a n t i c i pa t o r y a d j us t m e n t sa r e f o l l o w e d b y a b r a k i n g p h a s e i n v o l v i n g th e a n t a g -on i s t i c m us c l e s , a l s o no t e d by C r e nna e t a l . (1987)a nd O dds s on ( 1990) . B ou i s s e t a nd Za t t a r a ( 1987b)m e n t i on t ha t a f t e r t he e a r l y a n t i c i pa t o r y a d j us t -m e n t s , l a t e r r e a c t i on a l a d j us t m e n t s a r e obs e r ve ds how i ng d i f f e r e n t b i om e c ha n i c a l p r ope r t i e s f r om t hea n t i c i pa t o r y one s .

2.2. INTERN AL DISTURBANCE OF POSTUREAND EQUILIBRIUM

T h e p e r f o r m a n c e o f a m o v e m e n t i s l ik e ly t o e n t a ila d i s t u r b a n c e o f p o s t u r e a n d e q u i li b r iu m f o r t w om a i n r e a s ons .Fi r s t , the body i s not a r ig id b lock but a f l exibles ys t e m f o r m e d by m u l t i p l e s e gm e n t s t i e d t oge t he r byt h e m u s c le s s u r r o u n d i n g t h e j o in t s . W h e n a m o v e -m e n t i s pe r f o r m e d by a s t a nd i ng s ub j e c t , t he ge om e -t r y o f t h e b o d y i s c h a n g e d a n d a s a r e s ul t, t h e C Gpr o j e c t i on o n t o t he g r oun d i s d i s p l ac e d . Th i s w a s f i rs to b s e r v e d b y B a b i n s k i ( 1 8 9 9 ) w h o n o t e d t h a t u p p e rt r u n k m o v e m e n t s w e r e a c c o m p a n i e d b y h i p a n dk n e e m o v e m e n t s i n t h e o p p o s i t e d i r e c t i o n i n o r d e rt o m a i n t a i n e q u il ib r iu m . T h e a s y n e r g i e ' , o b s e r v e d inc e r e be l l a r pa t i e n t s w a s c ha r a c t e r i z e d by t he l o s s o ft h e se a s s o c i a te d m o v e m e n t s w h e n b e n d i n g t h e u p p e rt r unk , w h i c h r e s u l t e d i n f a l l i ng . A l s o Ma r t i n ( 1967)s h o w e d t h a t w h e n t h e a r m w a s r a i s e d t o t h e h o r i -z o n t a l , t h e re w a s a s l ig h t b a c k w a r d b e n d i n g o f th eu p p e r t r u n k w h i c h c o m p e n s a t e d f o r t h e f o r w a r d C Gdi s p l a c e m e n t a s s oc i a t e d w i t h t he f o r w a r d d i s p l a c e -m e n t o f t h e a r m .Se c ond l y , t he m ove m e n t i s i n i t i a t e d by i n t e r na lf o r c e s r e s u l t i n g f r o m m u s c l e c o n t r a c t i o n . W h e nr a i s ing t he a r m , t he s e f o r c e s a r e d i r e c t e d f o r w a r d a ndupw a r d . R e a c t i on f o r c e s i n t he oppos i t e d i r e c t i on a r ee x e r te d o n t h e s u p p o r t i n g s e g m e n t s a n d o n t h e r e s t o ft h e b o d y , a n d t h e se l e a d t o p o s t u r a l d i s t u r b a n c e a n di nba l a nc e . The s e dyna m i c r e a c t i on f o r c e s a r e m or ec o n s p i c u o u s w i t h f a s t m o v e m e n t s ( L e e e t a L 1987)be c a us e t he y ne e d g r e a t e r a c c e l e r a t i on i n o r de r t oo v e r c o m e t h e i n e r t i a o f t h e m o v i n g s e g m e n t . S l o wm ove m e n t s t he r e f o r e do no t u s ua l l y i nvo l ve a n t i c i -p a t o r y p o s t u r a l a d j u s t m e n t s ( H o r a k e t a l . 1984,1989b ; C r e nna e t a l . 1987; Od dsso n, 1990).A c a r e f u l a na l ys i s on t he ba s i s o f a c c e l e r om e t r i cr e c o r d i ngs pe r f o r m e d a t t he l e ve l o f the va r i ous j o i n t sw a s c a r r i e d ou t by B ou i s s e t a nd Z a t t a r a ( 1987a , b ,1988) du r i ng un i - a nd b i l a t e r a l a r m r a i s i ng . The s ea u t ho r s m e a s u r e d t he a c c e l e r a t i on f o r c e s a s s oc i a t e dw i t h a r m m ove m e n t s , c a l c u l a t e d t he r e a c t i on f o r c e sa t t he s hou l de r l e ve l a nd m e a s u r e d t he a c c e l e r a t i onf o r c e s a n d t o r q u e s g e n e r a t e d b y t h e a n t i c i p a t o r yp o s t u r a l a d j u s t m e n t s p r i o r t o t h e m o v e m e n t o n s e t .T h e y s h o w e d th a t t h e se a n t i c i p a t o r y f o r c e s w e r ed i r e c t e d i n t he oppos i t e d i r e c t i on t o t he r e a c t i onf o r c e s a s s o c i a t e d w i t h m o v e m e n t p e r f o r m a n c e a n dt hus s e r ve d t o m i n i m i z e t he pos t u r a l d i s t u r ba nc ec a u s e d b y t h e m o v e m e n t .

2.3. MODULARAPPROACH TO POSTtrRE VERSUSGLOBAL APPROACH TO EQUILIBRIUMFr om t he c l a ss i c a l po i n t o f v i e w , a r e fe r e nc epos t u r e , i .e . s t a nc e , i s de t e r m i ne d ge ne t i c a l ly f o r e a c hs pe c i e s . I t s m a i n t e na nc e a nd i t s a da p t a t i on t o t hee nv i r o nm e n t i s ba s e d on a c ha i n o f re fl exe s s t a r t i ngf r om s e ns o r s l oc a t e d i n t he va r i ous body s e g -m e n t s . The he a d a nd ne c k r e f l e xe s de s c r i be d byMagnus (1924) and the r ight ing ref lexes ident i f i ed byR a d e m a ke r ( 1931) a r e e xa m pl e s o f the s e r e fl exe s(see Rober t s , 1978) .The b ody pos t u r e i s bu i l t up m a i n l y a ga i ns t t hef o r c e s o f g r a v i t y . H u m a n s t a nc e is pa r t ic u l a r l ydi f f i cul t to s tudy in th i s respect because i t involveson l y a na r r ow s uppor t ba s i s a nd a n e x t e ns i ve m u l t i -j o i n t c on f i gu r a t i on . I t doe s no t c ons i s t o f a ri g i db l oc k os c i l l a t i ng a r ound t he a nk l e j o i n t l i ke a ni n v e r t e d p e n d u l u m , a l t h o u g h i t c a n s o m e t i m e sbe ha ve i n t h i s w a y ( G ur f i nke l , 1973 ; N a s hne r a ndMc C ol l um , 1985) . I t c ons is t s r a the r o f s upe r i m p os e dm odu l e s f r om t he f e e t t o t he he a d , e a c h l i nke d t o t he

ne x t b y a s e t o f m us c l e s w h i c h ha ve t he i r ow n s pec if icc e n t r a l a nd pe r i phe r a l r e gu l a t i on s e r v i ng t o m a i n t a i nt he m od u l e r e f e r e nc e pos i t i on . Th e r e f e re nc e pos i t i ono f e a c h o f t h e m o d u l e s c a n b e r e g u l at e d i n d e p e n -de n t l y f r om t ha t o f the o t he r s , a nd m odu l e - s pe c i fi cde fi ci ts i n t he pos t u r a l m a i n t e na n c e o f t he he a d o r t het r unk ha ve be e n de s c r i be d i n t he l i t e r a t u r e ( Ma r t i n ,1967) . Fo r e xa m pl e , t he pos t u r a l r e a c t i ons obs e r ve di n t he l eg m us c l e s o f a s ub j e c t s t a nd i ng on a p l a t f o r mos c i l la t i ng a r o un d t he a nk l e j o i n t a r e a i m e d a t s t a b i -l i z i ng t he h i p pos i t i on w i t h r e s pe c t t o t he g r ound ,w ha t e ve r t he he a d o r a nk l e j o i n t pos i t i on ( G ur f i nke le t a l . 1981; Dietz, e t a l . 1989a , b ; G o l l ho f e r e t a l .1989). As the C G i s loca te d a t the l evel of the h ip , th i sf i nd i ng s ugge s t s t ha t s pe c i a l s e ns o r s l oc a t e d a t t ha tl ev el m a y p r o v i d e i n f o r m a t i o n a b o u t t h e C G p o s i t io nw i t h r e s pe c t t o t he g r ou nd . I t ha s r e c e n t l y be e n s how nb y M o u c h n in o e t a l . (1990) in dancers ra i s ing a l egt ha t t he ve r t ic a l a x i s o f the t r un k i s r e gu l a t e d i nde -pe nd e n t l y o f t he l e g pos i t i on . The he a d i t s e l f s upp or t st h r e e t ype s o f s e ns o rs : t he ve s t i bu l a r s y s t e m , w h i c h i ssens i t ive to gravi ty forces ( see Vidal e t a l . 1986; deWa e l e e t a l . 1988 for de ta i l ed an alys i s ) , v i s ion, wh ichi s a b l e t o s t a b il iz e t he he a d a n d t he bo dy w i t h r e s pe c tt o t he e x t e r na l s pa c e , a nd t he ne c k m us c l e p r op r i o -c e p ti v e i n p u t w h i c h c o n v e y s t h e p o s i t i o n o f t h e h e a dw i t h r e s pe c t t o t he t r unk . The he a d c a n t he r e f o r e bes t a b il i ze d a s a f unc t i on o f e i t he r t he ga z e d i r e c t i on( B e r t h o z a n d P o z z o , 1 9 8 8 ; P o z z o e t a l . 1989), thege oc e n t r i c re f e r e nc e o r the t r unk a x i s (N a s hne r e t a l .1989 ; A m bl a r d e t a l . 1990 ; A s s a i a n t e a nd A m bl a r d ,1990a, b).I n a d d i t i o n t o t h i s m o d u l a r o r g a n i z a t i o n o f p o s tu -l a r c o n t ro l , a m o r e g l o b a l c o n t r o l o f t h e w h o l epos t u r e ha s be e n f ound t o e x i s t . The de c e r e b r a t er i g i d it y de s c r i be d by She r r i ng t on ( 1906) is a n e xa m pl es how i ng t ha t a g l oba l c e n t r a l c on t r o l i s e xe r t e d ont he e x t e ns o r a n t i g r a v i t y m us c l e s . Mor i ( 1987 , 1989)ha s r e c e n t l y a na l yz e d t he c e n t r a l o r ga n i z a t i on o f t h i sc on t r o l .A n a dd i t i ona l c ons t r a in t t o t he o r ga n i z a t i on o f pos -t u r e i s p r ov i de d by t he r u l e s f o r e qu i l i b r i um c on t r o l .M a n y p o s i t i o n s c a n b e a d o p t e d b y t h e b o d y s e g m e n tsp r ov i de d t ha t t he C G p r o j e c t i on r e m a i ns i n s i de t he


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38 J. MA ssIor~sup por t ing sur face , i .e . the fee t a rea . A ser ies o f sen-s o r s , v i s ua l , l a by r i n t h i ne , p r op r i oc e p t i ve a nd c u t a -ne ous , p r o v i de i np u t s i gna ls r e gu l a t i ng t h i s r e fe r e nc eva l ue a ga i ns t e x t e r na l o r i n t e r na l d i s t u r ba nc e s .T h i s s h o r t s u r v e y o f t h e o r g a n i z a t i o n o f p o s t u r eand equi l ibr ium ra i ses the ques t ion as to the speci f icgoa l o f t he a n t i c i pa t o r y pos t u r a l a d j us t m e n t s a s s oc i -a t e d w i t h vo l un t a r y m ove m e n t s . A r e t he y r e l a t e d t oe qu i l i b r i um m a i n t e na nc e o r t o t he s t a b i l i z a t i on o fg i ve n pos t u r a l m odu l e s ? The da t a f r om t he l i t e r a t u r ep r ov i de e v i de nc e t ha t t he y a r e i nvo l ve d i n bo t h t ype so f goa l s .

2.4. ANTICIPATORY POSTURAL ADJUSTMENTSASSOCIATED WITH EQUILIBRIUM MAINTENANCE

O n e o f t h e m a i n g o a l s o f th e s e a n t i c i p a to r ypos t u l a r a d j us t m e n t s i s t o m a i n t a i n e qu i l i b r i um ,as sugges ted by Babinski (1899) , Hess (1943) andMar t in (1967) . Here we wi l l analyze these adjus t -m e n t s w he n a s s oc i a t e d w i t h a x i a l m ove m e n t s , a r mm o v e m e n t s a n d l e g m o v e m e n t s .

2.4.1. A x i a l m o v e m e n t sThe a x i a l s yne r g i e s a s s oc i a t e d w i t h upp e r t r unkm ove m e n t s de s c r i be d by B a b i ns k i ( 1899) be l ong t ot h is c a t e go r y . The i r a n a l ys i s w a s r e c e n t l y unde r t a k e no n t h e b a s i s o f k i n em a t i c , f o rc e p l a t fo r m a n d E M Gda t a by O dds s on ( 1988 , 1990) , O dds s on a nd Thor t -e ns s on ( 1986, 1987a , b ) a nd by C r e nna e t a l . (1987,1988) a nd Pe do t t i e t a l . (1989).T h e k i n e m a t i c c h a n g e s a s s o c i a t e d w i t h u p p e rt r u n k m o v e m e n t s a r e p r e c e d e d b y E M G a c t i v a t i o no f the p r i m e m ov e r , na m e l y t he e r e c t o r s p i na e o r t her e c t u s a bdom i n i s , a nd o f l e g m us c l e s s uc h a s t heha m s t r i ng - t r i c e ps s u r a l is i n t he c as e o f ba c kw a r d a nd

t he qua d r i c e ps - ti b i a l is a n t e r i o r i n t ha t o f f o r w a r duppe r t r unk m ove m e n t s . The l e g m us c l e a c t i va t i on i ss i m u l t an e o u s w i t h t h e p r i m e m o v e r a c t i v a t i o n o r e v e npr e c e de s i t. Th i s i nd i c a te s t ha t a f e e d f o r w a r d po s t u r a la d j us t m e n t t a ke s p l a c e ( F i g . 2 ) .K i n e m a t i c a n a l y s i s s h o w s t h a t t h e u p p e r t r u n km ove m e n t s i n one d i r e c t i on a r e a s s oc i a t e d w i t hs i m u l t a n e o u s h i p a n d k n e e m o v e m e n t s i n t h e o p p o -s i te d i rec t ion. A s a resul t o f th i s com plex mu l t i jo in tc o o r d i n a t i o n , t h e d i s p l a c e m e n t o f t h e C G p r o j e c ti o non t o t he g r ou nd r e m a i ns qu i t e l ow ( le ss t ha n 2 c m ) ,w he r e a s a d i s p l a c e m e n t a s l a r ge a s 9 c m m i gh t ha veb e e n e x p e c t e d w i t h o u t t h i s c o o r d i n a t i o n ( C r e n n a e ta l . 1987) . Th i s i nd i c a t e s t ha t t he a n t i c i pa t o r y pos -t u r a l a d j us t m e n t s a s s oc i a t e d w i t h a x i a l m ove m e n t squ i t e e ff ic ie n tl y m a i n t a i n t he C G p r o j e c t i on o n t o t heg r o u n d a t t h e s a m e p l a c e d u r i n g t h e m o v e m e n tpe r f o r m a n c e . I n t he i r s t im u l a t i on o f the r o l e o f t hea n t i c i pa t o r y pos t u r a l a d j us t m e n t s i n s t a b i l i z i ng t hec e n t e r o f g r a v it y p o s it i o n d u r i n g u p p e r t r u n k f o r w a r dm o v e m e n t s , R a m o s a n d S t a r k ( 19 90 ) c o n c l u d e d t h a ti n a b s en c e o f a n t i c ip a t o r y a d j u s t m e n t , t h e C G w o u l dd i s p l a ce i t se l f by a pp r ox i m a t e l y 9 c m ; t he pa s s i veb a c k w a r d s m o v e m e n t r e s u l t i n g f r o m t h e p h y s i c a lc o u p l i n g o f t h e b o d y l i n k s i s n o t q u i c k e n o u g h t oc o m p e n s a t e f o r t h e f a s t f o r w a r d v o l u n t a r y m o v e -m e n t .A n o t he r e xa m pl e o f a x ia l s yne rg i e s a i m e d a t m a i n -t a i n i ng t he s t a b i li t y o f t he C G p r o j e c t i on on t o t heg r o u n d i s p r o v i d e d b y r e s p i r a t o r y m o v e m e n t s : t h er h y t h m i c t r u n k d i s p l a c e m e n t s a r e c o m p e n s a t e d f o rby h i p d i s p l a c e m e n t s i n t he oppos i t e d i r e c t i on a nd a sa r e s u l t no c ha n ge i n t he c e n t e r o f p re s s u r e i s to beobs e r ve d i n pha s e w i t h r e s p i r a t i on w he n t he s ub j e c ti s s t a nd i ng on a f o r c e p l a t f o r m ( G e l f a nd e t a l . 1971;G ur f i nke l a nd E l ne r , 1973) . O n l y und e r pa t ho l og i c a lc ond i t i ons a r e t he s e s yne r g i e s l o s t ( G ur f i nke l a ndElner, 1988).

B C K W R D

. . . . . . . . . .

R A b d4~ . . . . . . .

H a m

i T r T . . . .

I toomVmt0em 10Qm

FIG. 2. Backward upper t runk movement. St ick diagram shovAng ha t the backward m ovement of t h etrunk is accompanied by a forward hip and knee displacement. Continuous line, initial position; stripedline, final position. A set of muscles in the back of the trunk and leg are activated fairly synchronouslyat an early stage after the go signal. Er.S . (erector spinae), Ha m (Ham string), GM (gnstrocnemiusmedius). The antagonist m u s c l e s a r e activated during the braking phase. (R. Abd., r e c t u s a b d o m i n i s ; VM,vastus medialis; TA, tibialis anterior).


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MOVEMENT,POS~ 1tE ANDEQUILIBRIUM 392.4.2. A r m m o v e m e n t s

D o t h e a n t i c i p a t o r y p o s t u r a l a d j u s t m e n t s a s s o c i-a t e d w i t h a r m m o v e m e n t a l s o s e r v e t o m a i n t a i ne q u i l i b r i u m d u r i n g m o v e m e n t o r a r e t h e y i n v o l v e d i np o s t u r a l s t a b i l i z a t i o n o f t h e h e a d a n d t r u n k ? T h e r ee x i s t s e v i d e n c e t h a t o n e o f t h e f u n c t i o n s o f t h e s ea d j u s t m e n t s i s t o s t a b i l iz e t h e C G p o s i t i o n . A c c o r d -in g to B o u i s s e t a n d Z a t t a ra (1 9 8 1 , 1 9 8 7 b , 1 9 8 8, 1 9 9 0 )w h o c l o s e ly e x a m i n e d t h e a c c e l e r a t i o n f o r c e s a t t h el e v e l o f v a r i o u s s e g m e n t s , t h e a n t i c i p a t o r y p o s t u r a la d j u s t m e n t s a s s o c i a t e d w i t h u n i l a t e r a l o r b i l a t e r a la r m e l e v a t i o n c r e a t e a m o v e m e n t w i t h a f o r c e o fi n e r t i a w h i c h w i l l b a l a n c e t h e i n e r t i a l f o rc e s d u e t o t h em o v e m e n t o f t h e m o v i n g l i m b . T h e s e a n t i c i p a t o r ya d j u s t m e n t s , w h i c h a r e s p e ci f ic to e a c h t y p e o f m o v e -m e n t , s h o u l d c o u n t e r b a l a n c e t h e a c c e l e r a t i o n o f t h eC G o f t h e b o d y c a u s e d b y t h e m o v e m e n t ( F i g . 3) .F r i e d l i e t a l . (1 9 8 8 ) r e a c h e d th e s a m e c o n c lu s io n w i tha b i o m e c h a n i c a l a n a l y s is o f t h e g r o u n d r e a c t i o nf o r c e s a n d j o i n t a n g l e s w h e n s t a n d i n g s u b j e c t s w e rep e r f o r m i n g b i l a t e r a l f o r e a r m f l e x i o n m o v e m e n t s .I t c a n b e c o n c l u d e d f r o m t h e s e a n a l y s e s t h a t t h e C Gp o s i t i o n w i t h r e s p e c t t o t h e g r o u n d i s p r o b a b l y

F U F

Z

FIG. 3. In te rpre ta t ion o f the purpose o f the an t ic ipa to rypostu ral adjustments (APA ) associated w ith unilateral (UF )and b ilateral (BF) arm raising. The filled arrows corre spondt o the actual recorded biomechanical data , and the dashedarrows correspond to theoretical parameters . 0 , angulardisplacement of th e up per lim b(s). Aw, ~, , and ~,, tangential,radial and total upper limb acceleration. Rx and ARz,antero-p osterior and vertical acceleration o f the bod y centerof gravity, G.M z, resulting momentum abou t the verticalaxis crossing G . F rom this analysis, i t can be assumed th atAPA tends to create inertia l forces which, when the timecomes, will counterbalance the dis turbance to po sturalequilibrium due to the forthcoming intentional movement(Boulsset and Zattara , 1987b).

r e g u l a t e d b y m e a n s o f t h e s e a d j u s t m e n t s , e v e n i f it sd i s p l a c e m e n t c a u s e d b y t h e r e a c t i o n f o r c e s sh o u l d b era th e r i n s ig n i f i c a n t (R a m o s a n d S ta rk , 1 9 90 ), a n dt h a t a t l e a s t o n e o f t h e i r g o a l s i s t o m a i n t a i n e q u i -l i b r iu m ( s e e h o w e v e r S e c t io n 2 . 5 ) . O n e in t e re s t in gp o i n t i s t h e f a c t t h a t i n a r e a c t i o n t i m e p a r a d i g mw h e r e a r m r a i s i n g w a s p e r f o r m e d w i t h o u t a n d w i t h al o a d , t h e o n s e t o f p o s t u r a l a d j u s t m e n t ( s o - c a ll e dm o t o r l a t e n c y ) w a s f ix e d w i t h r e s p e c t to t h e g os i g n a l w h e r e a s t h e a r m m o v e m e n t o n s e t ( d e l t o i da c t i v a t i o n ) w a s d e l a y e d w h e n a l o a d w a s a d d e d( B o u i s se t a n d Z a t t a r a , 1986 1 9 88 , 1 9 90 ; Z a t t a ra a n dB o u i s s e t , 1 9 8 6 a, b ) . T h i s i n c re a s e d r e a c t io n t im e i sd u e t o a l o n g e r d u r a t i o n o f t h e a n t i c i p a t o r y p o s t u r a la d j u s t m e n t . A s a c o n s e q u e n c e , t h e v e l o c i ty o f t h e C Gd i s p l a c e m e n t p r o d u c e d b y t h e a n t i c i p a t o r y p o s t u r a la d j u s t m e n t i s i n c r e a s e d a t t h e t i m e o f m o v e m e n to n s e t a n d t h e l a r g e r d i s t u r b a n c e d u e t o t h e l o a d w i l lb e c o m p e n s a t e d f o r . T h e i n c r e a s ed d u r a t i o n o f t h ea n t i c i p a t o r y p o s t u r a l a d j u s t m e n t w h e n a l o a d i sa d d e d d u r i n g a r m m o v e m e n t w a s n o t o b s e r v e d b yB e n v e n u t i e t a l . (19 9 0) . H o w e v e r , t h e s e a u th o r s u s e da n e l b o w f l e x i o n m o v e m e n t , w h i c h d i s t u r b s p o s t u r ea n d e q u i l i b r i u m .A n o t h e r i n t e r p r e t a t i o n f o r t h e f u n c t i o n o f t h ea n t i c i p a t o r y a d j u s t m e n t s a s s o c i a t e d w i t h a r m m o v e -m e n t s h a s b e e n p u t f o r w a r d b y B r o w n a n d F r a n c k(1 9 8 7) a n d b y L e e e t a l . (1 9 9 0 ) . In L e e a n d c o w o rk e r s 'p a r a d i g m ( L e e e t a l . 1 99 0 ), t h e s u b je c t w a s a s k e d top u l l a l ev e r w i t h b o t h h a n d s a n d t o e x e r t v a r i o u sf o r c e s . A p r e p a r a t o r y p o s t u r a l a d j u s t m e n t i n v o l v i n ga h i p f l e x i o n w a s o b s e r v e d , t h e d u r a t i o n o f w h i c hi n c r e a s e d b e f o re m o v e m e n t o n s e t w i t h t h e f o r c e t o b ee x e r t e d b y t h e m o v e m e n t . A l t h o u g h t h e i n c r e a s i n gf o r c e o f t h e p u l l w a s a s s o c i a t e d w i t h a n i n c r e a s e da g o n i s t p u l s e l e v e l , t h e in c re a s e d in t e n s i ty o f t h ep o s t u r a l a d j u s t m e n t m e a s u r e d f r o m t h e a n k l e t o r q u er e s u l t e d f r o m a n i n c r e a s e i n t h e d u r a t i o n o f E M Gb u r s t f r o m t h e p o s t u r a l m u s c l e s ( u p t o 8 0 0 m s e cb e f o r e t h e m o v e m e n t ) . T h e s e f i n d i n g s a r e i n l i ne w i t ht h o s e o f Z a t t a r a a n d B o u i s s e t ( 1 98 6 a , b ) a n d B o u i s s eta n d Z a t t a r a ( 1 98 8 ) in d i c a t i n g t h a t t h e d u r a t i o n o f t h ea n t i c i p a t o r y p o s t u r a l a d j u s t m e n t i n c r e a s e s w i t h t h el o a d t o b e r a i s e d b y t h e a r m . L e e e t a l . (1 9 9 0 ) h a v es u g g e s t e d h o w e v e r t h a t t h e s e p r e p a r a t o r y a d j u s t -m e n t s a r e n o t s p e c if i c al l y r e l a t e d t o e q u i l i b r i u m c o n -t r o l b u t t h a t t h e y a l so d i r e c t l y p r o v i d e a d d i t i o n a lf o r c e f o r p e r f o r m i n g t h e m o v e m e n t . T h e f r o n t i e rb e t w e e n p o s t u r e a n d m o v e m e n t i s t h u s n o t q u i t ec l e a r , a n d t h e s e a d j u s t m e n t s c a n b e s a i d t o b e p a r to f t h e m o v e m e n t c o n t r o l . A n o t h e r r e m a r k b y t h e sea u t h o r s ( L e e e t a l . 1 9 9 0 ) i s t h a t t h e s e p o s tu ra la d j u s t m e n t s o c c u r r i n g l o n g b e f o r e t h e m o v e m e n to n s e t s h o u l d b e d i f f e r e n ti a t e d f r o m t h o s e w h i c h o c c u rd u r i n g t h e s a m e m o t o r a c t o n l y s h o r t l y b e f o r e t h ep r i m e m o v e r o n s e t . T h e f i r s t m i g h t b e q u a l i f i e d a sp r e p a r a t o r y p o s t u r a l a d j u s t m e n t s ( s ee G a h 6 r y , 1 98 7) ,w h e r e a s t h e s e c o n d a r e m o r e d i r e c t l y a s s o c i a te d w i t hm o v e m e n t o n s e t a n d m i g h t b e t e r m e d a n t i c i p a t o r yp o s t u r a l a d j u s t m e n t s . T h e f r o n t i e r b e t w e e n t h e s e t w oc a t e g o r i e s is n o t a l w a y s v e r y c l e a r c u t , h o w e v e r .

2.4.3. L e g m o v e m e n t s a n d e q u il i b ri u m c o n t r o lT h e c o n t r o l o f e q u i l i b r i u m d u r i n g l e g m o v e m e n t si s i n te r e s t i n g to a n a l y z e b e c a u s e t h e m o v i n g l i m b i s


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40 J. MASSIONi n v o lv e d i n s u p p o r t i n g t h e b o d y . A n y l e g m o v e m e n tt he r e f o r e , c ha nge s t he s uppor t c ond i t i ons a nd e n t a i l sa s h i ft o f t h e C G p o s i t io n p r i o r t o m o v e m e n t o n s e t.A ge n e r a l c ha r a c t e r is t i c o f l e g m o ve m e n t s i s t ha t t he yc ons i s t o f a s e que nc e w he r e t he C G f i rs t ha s t o bed i s p l a c e d t ow a r d t he r e m a i n i ng s uppor t i ng l i m bs ,a n d t h e m o v e m e n t o n s e t i s d e l a y e d u n t i l t h e C Gdi s p l a c e m e n t ha s r e a c he d a g i ve n va l ue .The f i r s t r e po r t on t he a n t i c i pa t o r y pos t u r a la d j us t m e n t s a s s oc i a t e d w i t h l e g m ove m e n t s w a s pub -l i s he d on hum a ns by A l e xe i e v a nd N a i de l ( 1972) .The s e a u t ho r s no t e d t ha t p r i o r t o a nk l e do r s i f le x i on o rvent rof lexion on one s ide , a t ib ia l i s anter ior (TA) ort r ic e ps s u r a l i s a c t i va t i on o c c u r r e d i n t he o t h e r l e g .Se ve r a l s t ud ie s on t he a n t i c i pa t o r y p os t u r a l a d j us t -m e n t s a c c o m p a n y i n g l eg m o v e m e n t s h a v e b e e nc a r r i e d o u t o n q u a d r u p e d s . D u r i n g c o n d i t i o n e ds i ng le l e g l if t ing i n t he c a t , t he m ove m e n t i s p r e c e de db y a d i s p l a ce m e n t o f t h e C G t o w a r d t h e c e n te r o f at r i a ng l e f o r m e d by t he t h r e e r e m a i n i ng s uppor t i ngl im bs . A de t a i le d a na l ys is o f t he ba c k d i s p l a c e m e n tdu r i ng t he l e g l i f t i ng ( D uf os s 6 e t a l . 1982) as wella s a m o d e l f o r t h e p o s t u r a l c h a n g e s ( F r o l o v e t a l .1988) ha ve be e n pub l i s he d . Th i s C G d i s p l a c e m e n t i si n i t i a t e d by a n e x t e ns o r t h r us t e xe r t e d by t he t r i c e pso f t he m ov i ng l eg , w h i c h g i ve s ri se t o a f o r c e i nc r e a s eu n d e r t h a t l e g p r i o r t o u n l o a d i n g ( A l s t e rm a r k a n dSasaki , 1983; Bi r juk ova e t a l . 198 9) . This in i t ia lt h r us t i s i n t e r p r e t e d a s p r ov i d i ng a n a c c e l e r a t i on o ft he C G t ow a r d t he oppos i t e s i de .I n t e r e s ti ng l y , t he s a m e s e que nc e i s obs e r ve d i n m a nwhe n f lexing a l eg (Ro gers an d P ai , 1990) . W i th fas tm ov e m e n t s , a n i n i ti a l d i s p l a c e m e n t o f t he c e n t e r o fp r e s s u r e ( C P) t ow a r d t he f le x ing l e g i s obs e r ve d w h i c hi s i n i ti a t ed by a n e a r l y a c t i va t i on o f t he g l u t e usm e d i us o f t ha t l e g . Th i s i n it ia l t h r us t s e e m s t o be a l s oc o r r e l a t e d w i t h t he c e n t e r o f m a s s a c c e l e r a t i ont ow a r d t he oppos i t e l e g . Wi t h s l ow m ove m e n t s , t hei n it ia l t h r us t o f t he m ov i ng l e g i s a bs e n t a nd t he kne eextensor ac t iv i ty in the suppor t ing leg f i r s t increases .S i m i l a r re s u l ts w i t h f a s t l e g m o ve m e n t s w e r e ob t a i ne db y M o u c h n i n o e t a l . (1990) in exper iments where thes ub j e c ts w e r e a s ke d t o m ove a l e g l a t e ra l l y t o a he i gh tof 45 as fas t as poss ible . T he f i r st s t ep in th i s m ot ora c t c ons i s ts o f d i s p l a c ing t he b ody w e i gh t on t o t he s up -po r t i ng l e g , by e x t e r na l l y r o t a t i ng t he s uppor t i ng l e ga r ou nd t he a nk l e j o i n t ( a n t e r o pos t e r i o r a x i s) . The r ea ga i n , a n i n i t i a l t h r us t e xe r t e d by t he m ov i ng l e go c c u r s p r io r t o t h e C G d i s p la c e m e n t t o w a r d t h e s u p -po r t i ng l e g : a bu r s t f r om t he ga s t r oc ne m i us m e d i us( G M) oc c u r s i n t he m ov i ng l e g be f o r e t he C P t h r us t .

A n o t h e r t y p e o f l e g m o v e m e n t o c c u r s w h e ns t a nd i ng on t i p - t oe ( L i ps h i t s e t a l . 1981; C16ment e ta l . 1984 ; N a r done a nd Sc h i e ppa t i , 1988 ; D i e ne r e ta l . 1990) . The t o e e x t e ns i on i s p r e c e de d by a f o r w a r dC G d i s p l a c e m e n t i n i t i a t e d by a t r i c e ps i nh i b i t i on ,w h i c h i s s om e t i m e s a c c om pa n i e d by a TA a c t i va t i on .The r e ve r se t a ke s p l a c e w i t h r oc k i ng o n t he he e ls . A ss ugge s t e d by N a r do ne a nd S c h i e pa t ti ( 1988), t hef o r w a r d C G d i s p la c e m e n t m a y b e n e c e ss a r y to c o u n -t e r a c t t he ba c kw a r d g r ound r e a c t i on f o r c e a s s oc i a t e dw i t h e x t e ns i on on t i p - t oe . I n a dd i t i on , t h i s C G d i s -p l a c e m e n t i s ne e de d t o ho l d t he pos i t i on w he n s t a nd -i ng on t i p - t oe i s m a i n t a i ne d .I n m os t c a s e s , t he pos t u r a l a d j us t m e n t a s s oc i a t e dw i t h l e g m o ve m e n t s s e r ve s t o d i s p l a c e t he C G p r o j e c -

t i on t ow a r d a p l a c e w h i c h i s c om pa t i b l e w i t h e qu i -l i b r ium m a i n t e na nc e du r i ng t he d i s p l a c e m e n t o f t hem ov i ng l i m b . H e r e w e ha ve a m o t o r a c t w h i c h i sc ha r a c t e r i z e d by a s e que n t i a l c on t r o l w he r e t w odi f ferent goal s a re ach ieved success ive ly, the f i r s tc ons i s t i ng o f d i s p l a c i ng the C G p r o j e c t i on t ow a r d t hef e e t s u r f a c e o f the s up por t i ng l e g o r t ow a r d t he t ip -t oe s o r t ow a r d t he he e ls a nd a s e c ond o f r a i s ing t hem ov i ng l e g . The m ove m e n t ons e t i s de l a ye d un t i l t heC G d i s p l a c e m e n t i s a c h i e ve d o r a bou t t o be a c h i e ved .2.5. STABILIZATION OF THE POSITION OF GIVEN

SEGMENTST h e s e c o n d p u r p o s e s e r v e d b y a n t i c i p a t o r ypos t u r a l a d j us t m e n t s i s t ha t o f st a b i li z ing t he pos i t i ono f s e gm e n t s s uc h a s t he he a d , t r unk o r l im bs du r i ngm o v e m e n t p e r f o r m a n c e .O ne e xa m pl e i s p r ov i de d by t he pos t u r a l c ha nge sw h i c h e ns u r e t ha t t he body w e i gh t i s s uppor t e da ga i ns t i nc r e a s e d ve r t i c a l f o r c e s . The s e pos t u r a lc ha nge s w e r e f ir s t de s c r i be d i n I o f f 6 a nd A n dr e ye v

( 1969) i n qua d r upe ds w he n one l e g be c om e s un -s uppor t e d . A d i a gona l s t a nc e t a ke s p l a c e , w he r e byt h e b o d y s u p p o r t i s p r o v i d e d b y t w o d i a g o n a l l yoppo s i t e l im bs . D ur i ng t h is s t a nc e , a n i nc r e as e i n t hee x t e n s o r a c t i v a t i o n c a n b e o b s e r v e d u n d e r t h e t w os uppor t i ng l i m bs , w he r e a s a de c r e a s e i n t he e x t e ns o ra c t iv i ty o c c u r s u n d e r t h e u n l o a d e d l im b s ( G a h r r y a n dMa s s i on , 1981) . I n o r de r t o unde r s t a nd t he goa l s o ft he s e pos t u r a l a d j us t m e n t s , one s hou l d r e m e m be rt h a t t h e s t a n d in g q u a d r u p e d is c o m p a r a b l e t o a f o u rl e gge d t a b l e e qu i ppe d w i t h j o i n t s l i nke d by s p r i ngsw h i c h s i m u l a t e t he m us c l e s ( G r a y , 1944) . Whe n onel i m b be c om e s uns uppor t e d , a b i pe da l s t a nc e de ve l -ops , on t w o d i a gona l l y oppos i t e l e gs , f o r m e c ha n i c a lr e a s ons . H ow e ve r , due t o t he s p r i ng p r ope r t i e s o f thel e g m us c l e s , t he i nc r e a s e d w e i gh t s uppor t e d by t het w o d i a g ona l l y oppos i t e s u ppor t i ng l egs w il l p r ob a b l yl o w e r t h e b a c k a n d l o w e r t h e C G p o s i t i o n ( G a h r r ye t a l . 1982; Gahr ry , 1987) . This , p e r s e has no effecton e qu i l i b r i um c on t r o l be c a us e t he C G pos i t i onr e m a i ns a t t he s a m e p l a c e bu t m i gh t c ha nge t hepos i t i on o f t he ba c k s e gm e n t s . The f e e d f o r w a r dp o s t u r a l a d j u s t m e n t a s s o c i a t e d w i t h m o v e m e n t p e r -f o r m a nc e p r e ve n t s t h i s ba c k l ow e r i ng by i nc r e a s i ngthe s t if fness o f the l eg muscles . A cco rdin g toL a c q u a n i t i e t a l . (1990) the geomet r ica l axi s of thel i m bs a nd o f t he ba c k r a t he r t ha n t he c e n t e r o f g r a v i t yp r o j e c t i on i s a h i gh l y r e gu l a te d va l ue i n qua d r upe ds .The d i a gona l s t a nc e ha s be e n de s c r i be d bo t h i nc o n n e c t i o n w i t h m o v e m e n t s i n d u c e d b y c o r t i c a ls t i m u l a t i on i n c a t s ( G a hr r y a nd N i r ou l l on , 1978 ;G a h r r y e t a l . 1980) , a nd du r i ng c ond i t i one d m ove -m e n t s i n dogs ( I o f f 6 a nd A nd r e ye v , 1969 ; Io f f 6 e t a l .1988) . I t i s a l so to be seen af ter imposed s tanced i s t u r ba nc e s ( D uf os s 6 e t a l . 1982, 1985b) . Anothere x a m p l e o f f e e d f o rw a r d p o s t u r a l c o n t r o l a i m e d a tc om pe ns a t i ng f o r a n i nc r e a s e i n t he l e g s uppor t ha sbe e n de s c r i be d by A l s t e r m a r k a nd We s s be r g ( 1985) i ns t a nd i ng c a t s : a t r ic e ps a c t i va t i on o f t he s upp or t i ngl e g t i m e l oc ke d w i t h b i c e ps a c t i va t i on o f t he r e a c h i ngl e g w a s obs e r ve d i n a t a r ge t r e a c h i ng t a s k .Fe e d f o r w a r d s t a b i l i z a t ion o f g i ve n s e gm e n t s s uc ha s t he he a d , t r unk o r f o r e a r m a ga i ns t t he d i s t u r b -a n c e s a s s o ci a te d w i th v o l u n t a r y m o v e m e n t s c a n a l s o


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MOVEMENT, POSTUREAND EQUILIBRIUM 4 1b e o b s e r v e d i n m a n . T h e r e a s o n f o r t h is f e e d f o r w a r ds t a b i l iz a t i on r e qu i r e s t o be d i s c us s e d i n r e l a t ion t o t heno t i o n o f the r e f e r e nc e f r a m e s w h i c h a r e u t i li z e d i nt he o r ga n i z a t i on o f m o ve m e n t ( Pa i l la r d , 1971, 1990) .The f i rs t o f t he s e i s t he g e oc e n t r i c r e f e r e nc e f r a m e ,b a s e d o n t h e g r a v i t y v e c t o r a n d t h e r e a c t io n f o r c e s o ft h e s u p p o r t i n g s u r f a c e . I t i s p r e p o n d e r a n t i n t h eb u i ld i n g u p o f p o s tu r e a n d e q u i li b ri u m . T h e s e c o n di s t he e goc e n t r i c r e f e r e nc e f r a m e ba s e d on t he pos -i t ion o f the va r i ou s bo dy s e gm e n t s a t a g i ve n ti m e .A m o n g t h e s e s e g m e n ts , t h e p o s i ti o n o f t h e h e a d a n dt r u n k i s p r e v a l e n t i n t h e o r g a n i z a t i o n o f m o v e m e n t si n pe r i pe r s ona l s pa c e . The t h i r d on e i s t he e xoc e n t r i cr e f e r e nc e fr a m e , i n w h i c h t he e x t e r na l s pa c e is u s e d a sa r e f e r e nc e va lue . Th e l i nk be t w e e n t he e x t e r na l s pa c ea nd t he e goc e n t r i c r e f e r e nc e f r a m e i s a c h i e ve d byr e p r e s e n t i ng a g i ve n po i n t i n s pa c e f i r s t i n t e r m s o fr e ti n a l c o o r d i n a te s , t h e n i n t e r m s o f h e a d a n d t r u n kc o o r d i n a t e s d u e t o t h e p r o p r i o c e p t i v e c h a i n w h i c hl in k s t h e v a r i o u s b o d y s e g m e n t s f r o m t h e e y e t o t h efee t (Rol l and Rol l , 1988; Rol l e t a l . 1989; Soecht inga nd F l a nde r s , 1989a , b ; C a m i n i t i e t a l . 1990). In th i sc onn e c t i on , t he r e p r e s e n t a t i on o f a t a r ge t pos i t i ona n d t h e c a l c u la t i o n o f t h e m o v e m e n t t r a je c t o r y t o b em a d e t o r e a c h i t w o u l d b e e r r o n e o u s i f t h e p r o p r i o -c e p t i ve c ha i n l i nk i ng e ye a nd he a d t o t r unk w e r ei n t e r r up t e d o r b i a s e d . Th i s oc c u r s a f t e r ne c k de -a f f e r e n t a t i on i n t he m onke y ( C ohe n , 1961) o r c a t

( M a n z o n i e t a L 1973) o r a f t e r ne c k m us c l e v i b r a t i oni n m a n ( B i gu e r e t a l . 1988) . In the l a t t e r exper iment ,v i b r a t i on ga ve r is e t o a n i l l u s o r y pos i t i on o f lum i noust a r ge t f i xa t e d i n t he da r k . Th i s i l l u s o r y s h i f t w a si n t e r p r e t e d a s r e s u l t i ng f r om t he a r t i fi c ia l I a a f f e r e n ti npu t e l i c i t e d by v i b r a t i on , w h i c h m a y ha ve p r ov i de dw r o n g i n f o r m a t i o n a b o u t t h e h e a d p o s i t i o n w i t hr e s p e c t t o t h e t r u n k . I n a d d i t i o n , t h e m o v e m e n tt r a j e c t o r y t o w a r d t h e t a r g e t l e d u n d e r t h e s e c o n -d i t i o n s t o m i s r e a c h i n g , b e c a u s e t h e m o v e m e n t w a sd i r e c t e d t ow a r d t he i l l u s o r y t a r ge t . A s t he pos i t i ono f s e gm e n t s s uc h a s t he he a d s e r ve s a s a r e f e r e nc ep o s i t i o n f o r t h e m o v e m e n t s t o b e m a d e i n p e r i p e r -s ona l s pa c e , a p r i o r i t y t a s k f o r t he c e n t r a l ne r vouss ys t e m i s t ha t o f s t a b i l i z i ng t he pos i t i on o f t he s es e g m e n t s d u r i n g m o v e m e n t p e r f o r m a n c e .A n e x a m pl e w he r e t he pos i t i on o f a s e gm e n t i st a ke n a s a r e f e r e nc e f r a m e a nd s t a b i l i z e d du r i ng av o l u n t a r y m o v e m e n t is p r o v i d e d b y t h e a n t i c i p a to r ya d j u s t m e n t o f f o r e a r m f le x o rs d u r i n g a b i m a n u a l l o a dl i f t i ng t a s k ( H ugon e t a l . 1982; D u f os s 6 e t a l . 1985a;P a u l i g n a n e t a l . 1989) ( F i g . 4 ) . The un l oa d i ng , t hep o s t u r a l f o r e a r m b y a v o l u n t a r y m o v e m e n t o f t h es ub j e c t ' s o t he r a r m i s a c c om pa n i e d by a n a n t i c i pa t -o r y i nh i b i t i on o f t he pos t u r a l f o r e a r m f l e xo r s, w h i c hi s t i m e l oc ke d w i t h t he ons e t o f b ic e ps c on t r a c t i on i nt h e v o l u n t a r y f o r e a r m . T h e a n t i c i p a t o r y a d j u s t m e n tw a s obs e r ve d i n a de a f f e r e n t e d pa t i e n t ( Fo r ge t a nd

I M P O S E D U N L O D I N G V O L U N T R Y U N LO D IN G

O O N T R O

F m

N t l l

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m ~ m

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i m mw ~i ] ,

Fio. 4. Comparison between imposed unloading and voluntary unloading. The sitting subject maintainsa loaded forearm (1 kg) in a horizontal position. The load is lifted either by the experimenter (imposedunloading) or by the voluntary mov ement o f the other hand (voluntary arm). The vert ical dashed l ineshows the onset of unloading. On the left, imposed unloading: the exp erimenter lifted the weigh t supportedby the postural fo rearm. O n the right, voluntary unloading: the sub ject himself lifted the weight with hisother hand. No te that during voluntary unloading, the posit ion of the postural forearm w as maintained anda deactivation o f the postural forearm flexors occurred befo re the onset of unloading. Av erage of 20 trials.

JPN 38/I--D


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MOVEMENT, POSTUREAND EQUILIBRIUMG A T I N G I N P U T . , R E P E R T O I R E I

B O D Y 8 C H E M E . IN B T R U G T I O N 8 . _ l O FS U P P O R T C O N D I T IO N 8 ) D I B C O N T I N U O U S l 8 Y N E R G I E 8

43

P O S T U R ER E F E R E N C E ~ J C O N T R O L L E RG O N T I N U O U 8 IF E E D B A C K . . .

V I S U A L . L A B Y R I N T H I N E . P R O P R I O C E P T I V E )FIG. 5. Control o f posture by continuous and discontinuous feedback. Note that three sources of input(visual, labyrinthine, and proprioceptive) contribute to the feedback regulation of whole body posture.The reference value f or equilibrium control usually correspond s to the center o f gravity position withrespect to the ground, but it can change w hen explicit or implicit instructions a re received, as for examplewhen a subject is holding a glass full of liquid. The gated inp ut preselects the appro priate set of synergiesas a function of the b ody scheme, learning, an d the suppor t conditions.

t i o n i s a r e g u l a t e d v a l u e i s a l s o p r o v i d e d b y t h eo b s e r v a ti o n s t h at d u r i n g l i m b m o v e m e n t i n m a n a n di n c a t, a d i s p l a c e m e n t o f th e C G p r o j e c t i o n p r e c e d est h e o n s e t o f m o v e m e n t i n o r d e r t o p r e s e r v e e q u i -l i b r iu m o r to in c re a s e s t a b i l i t y (R o g e r s a n d P a i , 1 9 90 ;M o u c h n i n o e t a l . 1990; Dufoss6 e t a l . 1 9 8 2 ; F ro lo ve t a l . 1 9 8 8 ) . O th e r r e fe re n c e v a lu e s m a y b e c o m es t a b i li z e d a s t h e r e s u l t o f i n s t r u c ti o n s . F o r e x a m p l e ,M a r s d e n e t a l . ( 1 9 8 1 ) h a v e m e n t i o n e d t h a t w h e nb e a r i n g a c u p o f t e a , t h e s t a b i l i z e d v a l u e w a s t h ep o s i t i o n o f t h e c u p o f t e a i n s p a c e ( s ee a l s o D r o u l e za n d B e r t h o z , 1 98 6) . I t w a s r e c e n t ly sh o w n ( Q u o n i a me t a l . 1 9 9 0 ) th a t w h e n s u b je c t s w e re in s t ru c te d tos t a b i l i z e th e f in g e r t i p in s p a c e u s in g v i s io n , a t a c t i l ec u e o r e v e n m e n t a l r e p r e s e n t a t i o n , p o s t u r a l r e a c t i o n sw e re o rg a n iz e d in s u c h a w a y a s to s t a b i l i z e th ep o s i t i o n o f t h e f in g e r in s p a c e , a s i n s t ru c te d .T h e r e f e r en c e v a l u es c a n a l s o b e a s e g m e n t o r p a r to f t h e b o d y . D r o u l e z ( 1 9 88 ) a n d D r o u l e z a n d B e r t h o z( 19 8 6) h a v e s u g g e s t e d i n a d d i t i o n t h a t a t o p o l o g i c a lo r g a n i z a t i o n o f t h e r e f e re n c e v a lu e s t o b e s t a b i l i z e dm a y r e s u l t f r o m i n s t r u c t i o n . O n e e x a m p l e i s t h a t o fa m a n w a l k i n g a n d r e a d i n g a p a p e r w h e r e o n e o f t h es t a b i l i z e d r e fe re n c e s i s t h e d i s t a n c e b e tw e e n th e p a p e ra n d t h e m a n ' s h e a d .

3.2. ERR OR DETECTING SENSORSU n d e r n o r m a l g r a v i t y c o n d i t i o n s , t h e v e r t i c a lg e o c e n t r i c r ef e r e n c e v a l u e i s p r o v i d e d b y t h r e e m a i nt y p e s o f i n p u t s , l a b y r i n t h i n e , v i s u a l a n d p r o p r i o c e p -t iv e , w h ic h a l s o s e rv e a s e r ro r d e t e c t in g s ig n a l s w i thr e s p e c t t o t h a t r e f e r e n c e v a l u e . W h e n a r t i f i c i a l l ye l i c i t e d , t h e s e in p u t s g iv e r i s e to a s lo w p o s tu ra lc h a n g e w h i c h m i m i c k s t h e p o s t u r a l c o r r e c t i o n s t h a tw o u l d h a v e o c c u r r e d i f t he s e r e c e p t o r s h a d b e e ns t i m u l a t e d i n a n a t u r a l w a y . F o r e x a m p l e , b i l a t e r a lv i b r a t i o n o f t h e A c h i l l e s t e n d o n w h i c h a r t i f i -c i a l ly s t i m u l a t e s t h e I a a f f e r e n t s f r o m t h e g a s t r o -

c n e m i u s m u s c l e g i v es r i se t o a w h o l e b o d y b a c k w a r d

s w a y a s i f t h e se i n p u t s w e r e d u e t o t h e s t r e t c h i n go f t h e m u s c l e b y a f o r w a r d s w a y t h a t n e e d s t o b ec o r re c t e d (E k lu n d , 1 97 2 ; R o l l , 1 9 81 ). C o m p a r -a b l e p o s t u r a l c h a n g e s w e r e o b s e r v e d i n r e s p o n s e t oa n o d i c s t i m u l a t i o n o f th e l a b y r i n t h ( L u n d a n dB ro b e rg , 1 9 8 3 ; G u r f in k e l e t a l . 1 9 8 8 a ) a n d m o v i n gv i s u a l s c e n e s ( s e e D ic h g a n s e t a l . 1972; Les t iennee t a l . 1977).O t h e r t y p e s o f s e n s o r s a r e a l s o i n v o l v e d i n t h ed e t e c t i o n o f e r r o r s in p o s t u r a l a n d e q u i l i b r i u ms t a b i li t y . C u t a n e o u s p l a n t a r i n p u t s p l a y a n i m p o r t a n tr o l e b o t h i n d o g s ( B r o o k h a r t e t a l . 1965) and inm a n ( M a g n u s s o n e t a l . 1990; Asa i e t a l . 1990). Thei d e a h a s b e e n p u t f o r w a r d t h a t g r a v i c e p t o r s m a y b ei n v o l v e d a t t h e l e v e l o f t h e l o w e r p a r t o f t h e t r u n k( G u r f i n k e l e t a l . 1 9 81 ) o r t h e jo in t s i n v o lv e d in s t a n c e( D i e t z e t a l . 1 9 89 b ). S lo w c o n t in u o u s a n d f a s t p h a s i cm o d e s o f p o s t u r a l a d j u s t m e n t s , h a v e b e e n d e s c r i b e d( s e e S e c t io n 3 . 4 ), w h ic h s u g g e s t s t h a t s p e c i f ic s e n s o r sm a y b e i n v o l v ed in e a c h m o d e o f p o s t u r a l a d j u s t m e n t( A m b l a r d e t a l . 1 9 85 ) b u t t h i s q u e s t io n s t i l l r e m a in sl a rg e ly u n re s o lv e d .

3.3. POSTURALBony SCHEMET h e c o n c e p t o f t h e b o d y s c h e m e i s n o t s p e ci f ic t op o s t u r e . B o d y r e p r e s e n t a t i o n h a s b e e n p r e v i o u s l yin v e s t ig a t e d b y H e a d (1 9 2 0 ) a n d M i t t e l s t a e d t (1 9 6 4 ,1 9 83 , 1 9 90 ). A s p e c i fi c i n t e rn a l r e p re s e n ta t io n fo rp o s t u r e h a s b e e n p r o p o s e d b y C 1 6 m e n t e t a l . (1984)a n d L e s t i e n n e a n d G u r f i n k e l ( 1 9 8 8 ) o n t h e b a s i s o fe x p e r i m e n t s c a r r i e d o u t d u r i n g s p a c e f l i g h t s u n d e rm i c r o g r a v i t y , w h e r e t h e g e n e r a l c h a r a c t e r i s t i c s o fs t a n c e w e re r e c o v e re d a f t e r a f e w d a y s in f l i g h tw h e n t h e s u b j e c t w a s a s k e d t o a d o p t a n e r e c t p o s t u r ew i th h i s f e e t f i x e d o n to th e f lo o r o f t h e s p a c e c a b in .T h e s e a u t h o r s s u g g e s t e d t h a t t h e v e r y s t a b l e b o d yr e p r e s e n t a t i o n m a y b e p a r t l y g e n e t i c a ll y d e t e r m i n e da n d p a r t l y a c q u i r e d t h r o u g h l e a r n i n g . I t i n c l u d e s

a r e p r e s e n t a t i o n o f t h e v e r t i c a l it y b a s e d o n l a b y -


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44 J. MASSlONr i n t h i ne , p r op r i oc e p t i ve a nd v i s ua l i npu t s a nd on t hepe r c e p t i on o f t he l ong i t ud i na l t r unk a x i s ( Mi t t e l-s t a e d t a nd F r i c ke , 1988) ; i n a dd i t i on , t h i s r e p re s e n -t a t i on i nc l ude s t he ge om e t r y o f t he bod y a nd i t sdyna m i c s . The r e p r e s e n t a t i on o f t he ge om e t r y isba s e d on m us c l e p r op r i oc e p t i ve I a i npu t s w h i c hc onv e y t he pos i t i on o f a g i ve n s e gm e n t w i t h r e s pe c tt o t he o t he r s . A c c o r d i ng t o R o l l a nd R o l l ( 1988) a ndR o l l e t a l . 0989) , t he s p i nd l e i npu t f o r m s a c on t i nu -ous k i ne m a t i c c ha i n f r om t he f e e t to t he e ye s . A r ti f i-c i a l l y s t i m u l a t i ng t he p r op r i oc e p t o r s a t a ny l e ve l i nt he c ha i n by t e ndon v i b r a t i on c ha nge s t he i n t e r na lr e p r e s e n t a t i on o f t he body ge om e t r y a nd g i ve s r i s et o i ll u s ions o f m ove m e n t o r t o pos t u r a l a d j us t -m e n t s , de pe nd i ng on t he c on t e x t ( R o l l a nd R o l l ,1988; La ckn er , 1988). A nice i llus t ra t ion of theo r g a n i z a t i o n o f t h e p o s t u r a l a d j u s t m e n t a s a f u n c t i o no f t he p r op r i oc e p t i ve i npu t w a s r e c e n t l y p r ov i d e dby G ur f i nke l e t a l . ( 1988a ) . The pos t u r a l r e a c t i on t oa nod i c s t i m u l a t i on o f t he ve s t i bu l a r s y s t e m va r ie sa s a f unc t i on o f t he pos i t i on o f the he a d w i t h r e s pe c tt o t h e t r u n k ( L u n d a n d B r o b e r g , 1 9 8 3 ) . G u r f i n k e le t a l . ( 1 9 8 8 a ) h a v e s h o w n t h a t i n f o r m a t i o n a b o u tt h e h e a d p o s i t i o n w a s p r o v i d e d b y t h e n e c k m u s c l eI a i npu t s , be c a us e du r i ng un i l a t e r a l v i b r a t i on o f ne c km u s c l e s , w h i c h m i m i c k s h e a d r o t a t i o n t o w a r d t h eoppos i t e s i de , t he pos t u r a l r e a c t i on t o ve s t i bu l a rs t i m u l a t i on i s t ha t w h i c h w ou l d be obs e r ve d i f t heh e a d w a s a c t u a l l y r o t a t e d . D y n a m i c i n f o r m a t i o na l s o c o n t ri b u t e s t o t h e p o s t u r a l b o d y s c h em e ; t h es e ns o r s i nvo l ve d he r e ha ve no~ ye t be e n c l e a r l yi de n ti fi ed , how e ve r . The y p r ov i de i n f o r m a t i on a b ou tt he s upp or t c on d i t i ons a s w e ll a s t he m a s s a nd i ne r ti ao f the va r i ous s e gm e n t s . The p6s t u r a l a d j us t m e n t si nduc e d by a d i s t u r ba nc e o f pos t u r e a r e o r ga n i z e d a sa f u n c t i o n o f th e s u p p o r t c o n d i t io n s a s s h o w n b yB e l e nk i y e t a l . ( 1967) , C or do a nd N a s hne r ( 1982) a ndM a r s d e n e t a l . (1981).I n t e r e s t i ng l y , t he pos t u r a l body s c he m e i s ve r ys t a b l e e v e n u n d e r m i c r o g r a v i t y c o n d i t i o n s a l t h o u g ht he ve s t i bu l a r g r a v i t a t i ona l s e ns o r s a nd t he p r op r i o -c e p t i ve i npu t no r m a l l y s ub j e c t e d t o g r a v i t y f o r c e s nol onge r c onv e y i n f o r m a t i on r e l a t ing t o t he ve r t i c a lgrav i ty forces (C16ment e t a l . 1984) . Af ter a few daysunde r m i c r og r a v i t y , t he ve r ti c a l a x is o f t he bo dy w i t hrespect to the f loor i s again rebui l t , when the fee t a rea t t a c he d t o t he f l oo r , a nd i n a dd i t i on , t he pos i t i on o ft he c e n t e r o f m a s s s e e m s t o be r e gu l a t e d unde r b o t hs t a t i c ( Le s t i e nne a nd G ur f i nke l , 1988) a nd dyna m i c( M a s s i o n e t a l . 1991) cond i t ions . T his f inding i s qui tes u r p r is i ng , be c a us e t he ne e d f o r t he c e n t e r o f m a s spos i t i on t o be r e gu l a t e d de pe nds on e qu i l i b r i umc o n t r o l u n d e r g r a v i ty c o n d i ti o n s , w h e r e t h e C Gp r o j e c t i o n o n t o t h e g r o u n d m u s t r e m a i n w i t h i n t h esuppor t ing sur face , i .e . the fee t . This sugges t s tha t thepos i t i on o f t he c e n t e r o f m a s s w i t h r e s pe c t t o the fee ti s o r ha s be c om e a s t a b l e r e f e r e nc e va l ue w h i c h i sr e gu l a t e d i nde pe nde n t l y f r om t he g r a v i t y c ons t r a i n t s .

3.4. POSTURALADJUSTMENTSI t w a s s ugge s t e d t ha t t w o m o d e s of pos t u r a l a d j us t -m e n t m a y o p e r a t e , d e p e n d i n g o n w h e t h e r t h e i n p u tmessages ind ica te tha t a cont inuous or a t rans ientp o s t u r a l d i s t u r b a n c e i s o c c u r r i n g ( A m b l a r d e t a l .1 9 8 5 ; D i e n e r a n d D i c h g a n s , 1 9 8 6 , 1 9 8 8 ) . T h e d is t i n c-

t i on be t w e e n o r i e n t a t i on a nd s t a b i l i z a t i on m a de byA m b l a r d e t a l . ( 1985) on t he ba s i s o f t he v i s ua l i npu ta l so s u g g e st s th a t t w o m o d e s o f p o s t u r a l c o n t r o l d oe x is t. T he f i r st m od e c ons i s t s o f a s l ow c on t i nu ousp o s t u r a l c o r r e c t i o n t h r o u g h a c l o s e d l o o p f e e d b a c ks ys te m . Exa m pl e s o f t he s e c o r r e c t ions a r e p r ov i de dby t he pos t u r a l s w a ys i nduc e d by a r t i f i c i a l s t i m u-l a t i on o f t he e r r o r de t e c t i ng s e ns o r s , u s i ng m us c l et e ndon v i b r a t i on , a nod i c l a by r i n t h i ne s t i m u l a t i on o ra moving vi sual scene .T h e s e c o n d m o d e o f p o s t u r a l a d j u s tm e n t s i sobs e r ve d w i t h f a s t d i s t u r ba nc e s o f pos t u r e . Th e s e n -s o r y i npu t s r e l a t i ng t o t he d i s t u r ba nc e i nduc e pha s i co p e n l o o p p o s t u r a l r e a c ti o n s . T h e p h a s i c a n t i c i p a t o rypos t u r a l a d j us t m e n t s a nd t he pha s i c pos t u r a l r e a c -t i ons bo t h be l on g t o th i s s e c ond m ode , bu t t he f o r m e ra r e e l i c i t e d by a n i n t e r na l c om m a nd s i gna l a nd t hel a t t e r by a n e x t e r na l one . I t s hou l d how e ve r bem e n t i one d t ha t o n l y t he in i ti a l pa r t o f t he pos t u r a lr e a c ti o n s a r e o r g a n i z e d i n a n o p e n l o o p m o d e . D i e n e ra nd D i c hga ns ( 1988) va r i e d bo t h t he ve l oc i t y a nd t hea m p l i t u d e o f t he p l a t f o r m d i s t u rb a n c e i n s t a n d in ghum a n s ub j e c ts a nd no t e d t ha t t he ve l oc i t y i npu tt r i gge r e d a n ope n l oop r e a c t i on , w he r e a s t he a m pl i -t u d e i n p u t o p e r a t e d i n a c lo s e d lo o p m o d e . T h e s a m es e q u e nt ia l o p e n l o o p a n d c l o se d l o o p c o m p o n e n t s a st hos e p r e s e n t i n a pos t u r a l r e a c t i on w e r e p r e v i ous l yde s c r i be d i n a v i s ua l p l a c i ng r e a c t i on by H e i n a ndHeld (1967) .

3 . 4 .1 . F i x e d v e r s u s f l e x i b l e s y n e r g i e sO n e o f t h e h y p o t h e s e s p r o p o s e d b y N a s h n e r ( 19 77 )a n d C o r d o a n d N a s h n e r ( 1 9 8 2 ) i n m a n a n d b yG a h6r y a nd Ma s s i on ( 1981) i n t he c a t w a s t ha t theree x i s ts a r e pe r t o i r e o f s yne r g i e s p r ov i d i ng a s t a b l em us c l e pa t t e r n a nd t ha t t h i s r e pe r t o i r e m a y be u t i -l i z e d by s e ns o r y i npu t s a s s oc i a t e d w i t h a n e x t e r na ld i s t u r ba nc e a n d by i n t e r na l i npu t s a s s oc ia t e d w i t hv o l u n t a r y m o v e m e n t a s w e l l . T h i s o r g a n i z a t i o nw o u l d r e d u c e t h e n u m b e r o f d e g r ee s o f f r e e d o m a n ds i m p li fy t h e p r o b l e m o f m o t o r c o n t r o l i n t h e d o m a i no f pos t u r a l a d j us t m e n t i n l i ne w i t h t he c onc e p t s o fB e r ns t e i n ( 1967) a nd G e l f a nd e t a l . (1971). Thishypo t he s i s w a s ba s e d on t he f a c t t ha t a r e s t r i c t e dn u m b e r o f m u s c l e p a tt e r n s c o u l d b e o b s e r v e d d u r i n gbo t h pos t u r a l r e a c t i ons a nd a n t i c i pa t o r y pos t u r a la d j u s tm e n t s a n d t h a t s o m e o f th e m w e r e c o m m o n t ob o t h t y p e s o f p o s tu r a l a d j u s t m e n t s. A c c o r d i n g t oC o r d o a n d N a s h n e r ( 1 9 8 2 ) , c o m p a r a b l e p a t t e r n sw e r e obs e r ve d w he n pu l l i ng a l e ve r by t he a r m and

w h e n i n d u c i n g a b a c k w a r d s w a y o f t h e s u p p o r t i n gp l a t fo r m . T h e d i a g o n a l p a t t e r n o f s u p p o r t o b -s e r ve d i n qua d r upe ds bo t h unde r s i ng l e l i m b d i s t u r b -a n t e . ( D uf os s 6 e t a l . 1982, 1985b) and dur ingm o v e m e n t s e l i c i t e d by c o r t i c a l s t i m u l a t i on ( G a h~r ya n d N i e ou l l on , 1978 ; G a h6r y e t a l . 1980) m i gh t bet a k e n as another e xa m pl e o f t he fi xe d pa t t e r ns oc c u r -r i ng u n d e r i m p o s e d or c e n t r a l l y i nduc e d d i s t u r ba nc e s .The pos s i b i l i t y t ha t f i xe d s yne r g i e s m a y be theba s i s o f pos t u r a l r e a c t i ons o r a n t i c i pa t o r y po s t u r a la d j u s t m e n t s w a s f u r t he r dis ussed b y ~ 1 9 8 4 )w h o p r o p o s e d t ha t f o r e a c h m us c l e pa t t e r n , a f i xe dsynergy shou ld be ident i f i ed on the bas i s o f t h er e p r oduc i b i l i t y o f t he s pa t i a l d i s t r i bu t i on o f t hepa t t e r n . I n f a c t , a s f a r as the a n t i c i pa t o r y a d j us t -


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46 J. MASSIONt he i n it i al s upp or t c ond i t i ons . The a nk l e a nd h i ps t r a t e g i e s c ons t i t u t e d d i f f e r e n t w a ys o f r e a c h i ng t hes a m e goa l s , i . e . m a i n t a i n i ng t he C G pos i t i on w i t hr e s pe c t t o t he f e e t. The f o r m e r s t r a t e gy i s a s s oc i a t e dw i t h t h e w h o l e b o d y m o v e m e n t a r o u n d t h e a n k l ej o i n t ( i n ve r te d p e n d u l u m m o d e l ) , a n d t h e s e c o n d o n ew i t h h i p f l e x i on o r e x t e ns i on i n r e s pons e t o f o r w a r do r b a c k w a r d i m p o s e d b o d y s w a y . M a c p h e r s o n ( 19 90 )ha s de f i ne d t he s t r a t e g i e s i n t e r m s o f m e c h a n i c s . I tc a n be de f i ne d in t e r m s o f k i ne m a t i c s ( La c qu a n t i e ta l . 1990) . A s t r a t e gy i s i m p l e m e n t e d by a pa t t e r n o fm u s c l e a c t iv a t i o n o r s y n e r g y w h i c h d e p e n d s m a i n l yo n t h e b o d y s u p p o r t a t t h e t im e o f th e d i s t u rb a n c e .D o t h e c o n c e p t s o f s t ra t e g y a n d s y n e r g y al s o a p p l yt o t he a n t i c i pa t o r y pos t u r a l a d j us t m e n t s ? The a ns w e rs e e m s t o be ye s , a t l e as t i n t he c a s e o f t he a n t i c i pa t o r ypos t u r a l a d j us t m e n t s i nvo l ve d in e qu i li b r i um c on t r o l .U n d e r n o r m a l g r a v i t y c o n d i t i o n s , t h e d i s p l a c e m e n to f u p p e r t r u n k a n d l o w e r s eg m e n t s in o p p o s i t ed i r e c t i ons c ou l d be qua l if i e d a s a s t r a t e g y a i m e d a tc on t r o l l i ng e qu i l i b r i um ( Ma s s i on e t a l . 1 9 9 1 ). U n d e rm i c r og r a v i t y c ond i t i ons , t he s a m e ge ne r a l k i ne m a t i c sw e r e o b s e r v e d d u r i n g u p p e r t r u n k m o v e m e n t s a son e a r t h . Th i s i s s u r p r i s i ng , be c a us e t he ne e d t or e gu l a t e t he c e n t e r o f m a s s pos i t i on w i t h r e spe c t t ot he f e e t d i s a ppe a r s unde r m i c r og r a v i t y . Th i s m e a nst h a t t h e s t r a t e g y w h e r e b y t h e C M p o s i t i o n i sr e gu l a t e d r e m a i ns qu i t e s t a b l e e ve n unde r m i c r o -g r a v i t y ( se e Le s t ie nne a n d G ur f i nke l , 1988) . The w a yi n w h i c h t h i s s t r a t e gy i s i m p l e m e n t e d by a m us c l es y n e r g y c h a n g e s , h o w e v er . D u r i n g b a c k w a r d u p p e rt r unk m ove m e n t s , t he e a r l y So l a c t i va t i on w h i c hoc c u r s on e a r t h i s r e p l a c e d by a n e a r l y TA a c t i va t i onw hi c h i s r e s pons i b le f o r t he k ne e f l e x ion . A f t e r r e t u r nt o Ea r t h c ond i t i ons , t he e a r l y TA a c t i va t i on pe r s i s t sf o r a f e w da ys a nd t he n t he p r e v i ous So l a c t i va t i onr e a ppe a r s . A s ho r t t e r m a da p t a t i on p r oc e s s t he r e f o r eoc c u r s a f t e r t he r e t u r n t o Ea r t h . I n t h i s e xa m pl e , t hes a m e k i ne m a t i c s t r a t e g y i s u t i li z e d du r i ng uppe rt r u n k m o v e m e n t s a i m e d a t s t a b i l i z i n g t h e C M w i t hr e s pe c t t o t he f e e t a nd t h i s s t r a t e gy i s i m p l e m e n t e dby d i f f e r e n t m us c l e s s yne r g i e s de pe nd i ng on t he pa r -t i c u l a r b i om e c ha n i c a l c ons t r a i n t s e nc oun t e r e d unde rn o r m a l g r a v i ty a n d m i c r o g r a v i t y c o n d i ti o n s .

4 . A C Q U I S I T I O N A N D A D A P T A T I O NA n t i c i p a t o r y p o s t u r a l a d j u s t m e n t s a r e m o s t l ya c qu i r e d by l e a r n i ng be c a us e t he i r o r ga n i z a t i onde pe nd s on t he p r e v i ous e xpe r ie nc e o f t he pos t u r a l

d i s t u r b a n c e a s s o c i a t e d w i t h t h e m o v e m e n t p e r f o r m -a nc e . P r e v i ous e xpe r i e nc e a nd t r a i n i ng a l s o ha vei m por t a n t e f f e c t s on pos t u r a l r e a c t i ons ( s e e H or a ka n d N a s h n e r , 1 98 6; H o r a k e t a l . 1989a) . However ,a n t i c i pa t o r y pos t u r a l a d j us t m e n t s d i f f e r f r om t hepos t u r a l r e a c t i ons i n t ha t t he i r o r ga n i z a t i on i s ba s e don e xpe r i e nc e i n pe r f o r m i ng i n t e n t i ona l a c t i ons .A n t i c i pa t o r y l e g m us c l e pos t u r a l a d j us t m e n t s a r ea c q u i r e d d u r i n g e a r l y c h i l d h o o d ( W o o l l a c o t t e t a l .1987 ; H a a s e t a l . 1989) . I t w a s obs e r ve d by vonH o f s t e n a n d W o o l l a c o t t ( p e r s o n a l c o m m u n i c a t i o n )t ha t e a r l y a c t i va t i on i n t r unk m us c l e s w h i l e r e a c h i ngc o u l d b e o b s e r v e d a s e a r l y a s 9 m o n t h s o f a g e. O n c el e a r ne d , t he a n t i c i pa t o r y a d j us t m e n t s r e m a i n s t a b l et h r oughou t l i f e a nd t he i r i m pa i r m e n t i n t he e l de r l y

s e e m s t o be due t o m u l t i f a c t o r i a i de f e c t s ( H or a k e ta L 1989b ; Woo l l a c o t t e t a l . 1988; Wool lacot t , 1990;S t e l m a c h e t a l . 1989).The ge ne r a l p r oc e s s unde r l y i ng t he a c qu i s i t i ono f a n a n t i c i pa t o r y pos t u r a l a d j us t m e n t i m p l ie s thet r a n s f o r m a t i o n o f f e e d b a c k p o s t u r a l c o r r e c t io n s i nf e e d f o r w a r d c on t r o l a s s oc i a t e d w i t h t he vo l un t a r ym ove m e n t w h i c h i s c a us i ng t he pos t u r a l d i s t u r ba nc e .The ge ne r a l m ode l f o r t h i s t r a ns f o r m a t i on i nvo l ve sa n a da p t i ve ne t w or k w h i c h bu i l d s up a n i n t e r na li m a ge o f t he d i s t u r ba nc e t o be m i n i m i z e d o r t hec on t r o l ne e de d t o c a nc e l i t. Th e m o de l s by I t o ( 1990)a nd H o l l e r ba c h ( 1982) f o r t he r e gu l a t i on o f a m ov e -m e n t t r a j e c t o r y a r e ba s e d f o r e xa m pl e on t he i nve r s ed y n a m i c s o r t h e d y n a m i c s m a y a c c o u n t f o r t h ebu i l d i ng up o f a n t i c i pa t o r y a d j us t m e n t s .

4.1. ACQUISITIONV e r y f e w s t ud i e s ha ve de a l t w i t h t he a c qu i s i t i ono f a n t i c i pa t o r y pos t u r a l a d j us t m e n t s . I o f f 6 e t a l .( 1988) ha ve s how n how e ve r t ha t a ne w pos t u r a l

pa t te rn in asso cia t ion wi th fore l im b f lexion el ic i ted bys k i n e l e c t r i c a l s t i m u l a t i on c ou l d be l e a r ne d by t hequa d r upe d a f t e r l ong t e r m t r a i n i ng . The a c qu i s i t i ono f t he s e ne w pa t t e r ns r e qu i r e s a n i n t a c t m o t o r c o r t e xa nd / o r py r a m i da l t r a c t ( I o f f r , 1973) .I n b i m a nua l l oa d l i f t i ng t a s ks w he r e t he m a i n t e n -a n c e o f t h e f o r e a r m p o s t u r e d u r i n g u n l o a d i n g i sa c h i e ve d by a n a n t i c i pa t o r y d e a c t i va t i on o f t he f o r e -a r m f l e xo r s , a s s oc i a t e d w i t h t he vo l un t a r y m ove m e n to f t he o t he r a r m , t he a c qu i s i t i on o f the a n t i c i pa t o r ya d j us t m e n t s ha s be e n e xp l o r e d ( D uf os s 6 e t a l . 1985a;P a u l i g n a n e t a l . 1989) . Fo r t h i s pu r pos e , t he l oa dr e le a s e w a s t r i gge r e d by s w i t c h i ng o f f a n e l e c t ro -m a gne t , w h i c h c ou l d be t r i gge r e d by e x t e r na l s i gna l sr e l a t e d o r o t he r w i s e t o vo l un t a r y m ove m e n t pe r -f o r m e d by t he o t he r a r m . A s a r e s u lt o f t h is i nve s ti -ga t i on , i t w a s f ound t ha t no a n t i c i pa t o r y a d j us t m e n tw a s a c q u i r e d i n th e a b s e n c e o f v o l u n t a r y m o v e m e n t( f o r e xa m pl e , w he n a s o und i nd i c a t e d t he p r e c is e t im eo f u n l o a d in g ) . M o r e o v e r , d i st a l m o v e m e n t s , s u c has press ing a key t r igger ing the load re lease , inducedn o a n t i c ip a t o r y f o r e a r m a d j u st m e n t s. O n l y p r o x i m a le l b o w m o v e m e n t s , w h a t e v e r t h e p a r a m e t e r c o n -t r o l l e d ( f o r c e , d i s p l a c e m e n t o r bo t h ) , i nduc e d t hea c qu i s i t ion o f t he a n t i c i pa t o r y a d j us t m e n t o f f o r e a r mpos t u r e . The a n t i c i pa t o r y pos t u r a l a d j us t m e n t w a se l a bo r a t e d du r i ng 40 - 60 t r i a l s a nd t he r e a f t e r s t a b i l -i z e d ( Pa u l i gna n e t a l . 1989) ; but the ef f ic iency of ther ef le x c o r r e c t i on o f the f o r e a r m po s i t i on d i s t u r ba nc ei nc r e a s e d a s s oon a s t he oc c u r r e nc e o f t he d i s t u r ba nc ec ou l d be p r e d i c t e d . Th i s he l d t r ue bo t h w he n d i s t u r b -a nc e s w e r e i m pos e d by t he e xpe r i m e n t e r a nd w he nt h e y w e r e in d u c e d b y a d i s ta l m o v e m e n t o f t h es ub j e c t ' s o t he r ha nd ( Pa u l i gna n e t a l . 1989) . Thega t i ng o f t he r ef le xes obs e r ve d i n a s i t ua t i on i nvo l v i ngp r e d i c t i on i s i n a g r e e m e n t w i t h p r e v i ous da t a ( s e eH o r a k e t a l . 1989a; Prochazka , 1989) .I n b i m a nua l t a s ks , w he r e l oa d i ng r a t he r t ha n un -l oa d i ng i s pe r f o r m e d , a n a n t i c i pa t o r y f o r e a r m a d j us t -m e n t i s a l s o obs e r ve d . I n t he b i m a nua l pa r a d i gmde s c r i be d by B e n i s ( 1990) one h a nd r e l ea s e s t he l oa df r om va r i ous he i gh t s a nd t he o t he r r e c e i ve s t he l oa d ;a nd i n t ha t by J oha ns s on a nd We s t l i ng ( 1988) oneha nd r e l e a s e s a l oa d a nd t he o t he r ha nd e xe r t s a g r i p


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MovE~,mtcr,POSTUR ND EQUILIBRIUM 47f o r c e on a c y l i nde r w h i c h r e c e i ve s t he l oa d . I n t he s es i tu a t io n s , t h e p a r a m e t e r s o f t h e v o l u n t a r y m o v e m e n ta r e no t d i r e c t l y re s pons i b l e f o r t he d i s t u r ba nc e o f t hef o r e a r m p o s t u r e . T h e d i s t u r b a n c e d e p e n d s i n f a c t o nt h e l o a d a n d o n t h e h e i g h t f r o m w h i c h t h e l o a di s r e le a s e d . The r e s u l ts s how e d t he oc c u r r e nc e o f a na n t i c i p a t o r y a d j u s t m e n t o f f o r e a r m p o s i ti o n d e p e n d -i ng on t he k i ne t i c e ne r gy o f the l oa d , i nvo l v i ng a ve r ya c c u r a t e p r ed i c t io n o f th e ti m e o f l a n d i n g .S u r p r is i n gl y , t h e c o n t a c t o f a l o a d d r o p p e d o n t ot h e f o r e a r m m a i n t a in i n g a p o s i t io n i s p r e c e d e d b y a na n t i c i p a t o ry a d j u s t m e n t o f t h e f o r e a r m p o s i ti o n e v e nw he n t he l oa d r e l e a s e i s t r i gge r e d by t he e xpe r -i m e n t e r . Th i s i s i n d i s a g r e e m e n t w i t h p r e v i ous f i nd -i ngs on t he a c qu i s i t i on o f t he a n t i c i pa t o r y pos t u r a la d j us t m e n t s i n t he l oa d l i f t i ng t a s k ( Pa u l i gna n e t a l .1989) . Fo r e xa m pl e , L a c qua n i t i a n d M a ' io l i ( 1989a , b )h a v e o b s e r v e d t h a t a n a n t i c i p a t o r y a d j u s t m e n t o ft he f o r e a r m pos i t i on w he n a l oa d i s r e l e a s e d f r omva r i ous he i gh t s by a s w i t c h c on t r o l l e d by t he e xpe r -i m e n t e r c a n c a u s e a d i s t u r b a n c e o f t h e h a n d p o s i ti o n .T h e a n t i c i p a t o r y a d j u s t m e n t w a s c o r r e l a t e d w i t ht he k i ne t i c e ne r gy o f the l oa d . I n t he e xa m pl e , t hea n t i c i p a t o r y a d j u s t m e n t r e s u lt s f r o m a c o m p l e x c o m -p u t a t i o n c o n c e r n i n g t h e k i n e t ic e n e r g y o f t h e l o a d a sa f unc t i on o f the he i gh t o f t he r e le a se . Th i s s i t ua t i onc a n b e c o m p a r e d t o s o m e e x t e n t t o t h e p o s t u r a la d j us t m e n t s w h i c h oc c u r be f o r e l a nd i ng ( Me l v i l l -J o n e s a n d W a t t , 1 9 7 1 ; G r e e n w o o d a n d H o p k i n s ,1976 ; Mc K i n l e y a nd Sm i t h , 1983 ; D yhr e - Pou l s e n a ndLaursen, 1984) .To c onc l ude , t he a n t i c i pa t o r y pos t u r a l a d j us t -m e n t s a r e u s ua l l y bu i l t up unde r c ond i t i ons w he r ea v o l u n t a r y m o v e m e n t c a u s e s p o s t u r a l d i s t u r b a n c ea n d n o t w h e n t h e d i s t u r b a n c e i s i m p o s e d b y a ne x t e r na l s ou r c e . H ow e ve r , unde r s pe c i f i c c ond i t i onsw he r e t he k i ne t i c e ne r gy o f t he i m pa c t c a n be e s ti -m a t e d o n t h e b a s is o f i n f o r m a t i o n p r o v i d e d b yp r op r i oc e p t i ve o r v i s ua l c ue s , a n t i c i pa t o r y pos t u r a la d j u s t m e n t s c a n b e o b s e r v e d i n t h e a b s e n c e o f v o l u n -t a r y m o v e m e n t s .

4.2. SHORT TERM ADAPTATIONB o t h a n t i c i p a t o r y a n d r e a c t i o n a l c h a n g e s i n t h ep a t t e r n o f p o s t u r a l a d j u s t m e n t s c a n b e o b s e r v e d a ss o o n a s t h e e n v i r o n m e n t a l c o n s t r a i n t s a r e c h a n g e d .Fo r e xa m pl e , a c ha nge i n t he s uppor t c ond i t i ons i sa c c o m p a n i e d b y a c h a n g e i n t h e E M G p a t t e r n w h i c hc ha r a c t e r i z e s t he pos t u r a l a d j us t m e n t . Th i s w a s ob -s e r ve d i n the c a s e o f t he a n t i c i pa t o r y a d j us t m e n t s

a s s o c i a t e d w i t h a r m m o v e m e n t s w h e n b e n d i n g o n aw a l l ( B e l e nk i y e t a l . 1967; Benvenut i e t a l . 1990),w he r e t he a n t i c i pa t o r y l e g m us c l e a d j us t m e n t s w e r em a r k e d l y r e d u c e d . W h e n a s u p p o r t i s t a k e n w i t h t h eha nd , t he pos t u r a l a d j us t m e n t p r e v i ous l y de t e c t e d int h e l e g m u s c l e s w a s o b s e r v e d t o o c c u r i n t h e a r mm u s c l e s ( C o r d o a n d N a s h n e r , 1 9 8 2 ; M a r s d e n e t a l .1981) . Whe n s t a nd i ng on a na r r ow s uppor t , t he e a r l ya c t i va t i on o f t he d i s t a l l e g m us c l e s a s s oc i a t e d w i t hb a c k w a r d u p p e r t r u n k m o v e m e n t d i s a p p ea r s . T h epha s i c a c t i va t i on o f the s e m us c l e s g i ve s ri se t o ho r i -z on t a l s he a r f o r c e s li a b l e t o c a us e i m ba l a nc e ( Pe do t t ie t a l . 1989) . Th i s s ho r t t e r m a da p t a t i on i s t he r e s u l to f l o n g t e r m t r a in i n g . E n c o u n t e r i n g m i c r o g r a v i t y i sa n o t h e r e x a m p l e w h e r e a c h a n g e i n t h e e n v i r o n -

m e n t a l c ons t r a i n t s m od i f i e s t he a n t i c i pa t o r y pos t u r a la d j us t m e n t , s i nc e c ha nge s i n t he pa t t e r n o f t he a n t i c i-pa t o r y pos t u r a l a d j us t m e n t s oc c u r du r i ng s pa c e f l i gh tw h e n t he f e e t a r e a t t a c he d t o t he f l oo r (C 16m en t e t

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