Perceptual Motor Control

Perceptual-Motor Contributions to Static and Dynamic Balance Control in ChildrenV. HatzitakiV. ZisiI. KolliasE. KioumourtzoglouDepartment of Physical Education and Sports SciencesAristotelio University of ThessalonikiGreece

ABSTRACT. The authors addressed balance control in childrenfrom the perspective of skill development and examined the rela-tionship between specific perceptual and motor skills and staticand dynamic balance performance. Fifty 11- to 13-year-old chil-dren performed a series of 1-legged balance tasks while standingon a force platform. Postural control was reflected in the maxi-mum displacement of the center of mass in anteriorposterior andmediolateral directions. Simple visual, discrimination, and choicereaction times; sustained attention; visuomotor coordination;kinesthesis; and depth perception were also assessed in a series ofperceptual and motor tests. The correlation analysis revealed thatbalancing under static conditions was strongly associated with theability to perceive and process visual information, which is impor-tant for feedback-based control of balance. On the other hand,when greater task demands were imposed on the system underdynamic balancing conditions, the ability to respond to the desta-bilizing hip abductionsadductions in order to maintain equilibri-um was associated with motor response speed, suggesting the useof a descending, feedforward control strategy. Therefore, likeadults, 11- to 13-year-old children have the ability to select vary-ing balance strategies (feedback, feedforward, or both), dependingon the constraints of a particular task.Key words: balance, feedback vs. feedforward control, perceptual-motor skills

ostural control has been closely associated with theability to correctly perceive the environment through

peripheral sensory systems, as well as to centrally processand integrate proprioceptive, visual, and vestibular inputs atthe level of the central nervous system (CNS). That abilityenables the CNS to form appropriate muscle synergiesneeded so that equilibrium can be maintained (Teasdale,Lajoie, Bard, Fleury, & Courtemanche, 1992). Althoughthere is considerable evidence concerning the importance ofsensory organization abilities for balance control in children(Assaiante & Amblard, 1992; Sundermier & Woollacott,1998; Sundermier, Woollacott, Roncesvalles, & Jensen,

2000; Sveistrup & Woollacott, 1996), the exact nature ofthose mechanisms in terms of skill development is not yetwell explored. The focus in most studies has been on look-ing at postural responses to visual or proprioceptive pertur-bations (Assaiante, & Amblard, 1992; Sundermier et al.,2000) or to sensory conflict situations created by the com-bination of the two, such as the moving room paradigm(Sveistrup & Woollacott, 1996). On the other hand, bothresearchers and clinicians rely on qualitative scales ofmovement performance such as the Bruininks-Oseretskytest for motor proficiency or the Motor Skills Inventory(Werber & Bruininks, 1988) to assess motor skill develop-ment and balance in children. Although those scales areuseful for establishing an overall level of performance ascompared with norms, they do not provide any insight intohow one might use specific perceptual and motor skills topredict behavior on static or dynamic balance tasks. In thepresent study, we attempted to address balance control inchildren from the perspective of skill development and toexamine how specific perceptual and motor skills are relat-ed to static and dynamic balance performance.

Before considering perceptual and motor skill contribu-tions to balance control, one must properly address the issueof the different strategies used by the CNS in order to main-tain equilibrium, depending on the nature of the task, staticor dynamic. For example, quiet standing is said to be con-trolled by sensory feedback on the basis of a closed-loopsystem (Nashner, 1976) in which the center of foot pressuremoves in phase with the center of mass (Winter, Patla,

Correspondence address: Vassilia Hatzitaki, Department ofPhysical Education and Sports Sciences, Aristotelio University ofThessaloniki, Thessaloniki, 540 06, Greece. E-mail address:[email protected]

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Prince, Ishac, & Gielo-Perczak, 1998), and integration ofvisual and proprioceptive inputs is needed for that control(Massion & Woollacott, 1996). The importance of visualcues in maintenance of static posture has been well demon-strated, particularly in children, who use them to visuallymonitor their body during posture (Gallahue & Ozmun,1998; Riach & Hayes, 1987). In addition, proprioceptiveinformation from virtually all areas of the body is co-processed and integrated at a central level so that it can con-tribute to a stable posture (Jeka, Oie, & Kiemel, 2000;Kavounoudias, Gilhodes, Roll, & Roll, 1999; Roders,Wardman, Lord, & Fitzpatrick, 2001; Slijper & Latash,2000).

On the other hand, balancing under dynamic task condi-tions requires the use of feedforward control (Horak &Nashner, 1986; Lacquaniti & Maioli, 1989). With feedfor-ward control, postural disturbances are predicted, and thosepredictions result in anticipatory postural adjustments(APAs) that enable the mover to maintain stability (Massion,1992). In that case, equilibrium control is of a more reflex-ive nature and depends on the ability to rapidly transformperturbations of proprioceptive or vestibular origin intoproper motor responses, an ability that has been linked to anadequately functioning reaction time process (Lord, Clark,& Webster, 1991; Woollacott, Shumway-Cook, & Nashner,1986). The importance of cognitive function for the organi-zation and integration of available sensory informationunder both static and dynamic balancing conditions has alsobeen well acknowledged (Schmidt, 1982; Shumway-Cook& Woollacott, 2000; Vuilerme, Nougier, & Teasdale, 2000;Woollacott, Moore, & Hu, 1992). Nevertheless, the differen-tial contributions of skills such as depth perception and reac-tion time or cognitive skills such as attention to the perfor-mance of static or dynamic balance tasks have not beenlargely explored. Because task constraints are different forstatic and dynamic balance, we hypothesized that, depend-ing on whether task constraints were static or dynamic, dif-ferent perceptual and motor skills would be associated withbalance, reflecting feedback or feedforward mechanisms ofbalance regulation, respectively.

The selection of the appropriate balance strategy in eachcase not only depends on task constraints and environmen-tal demands but is also a function of neural maturation andexperience. A feedback-based system in the control of bal-ance appears very early in life, as confirmed by experimen-tal evidence showing that postural response synergies trig-gered by sensory perturbations are present as early as 1531months and have latencies comparable with those of adults(Shumway-Cook & Woollacott, 1985). On the other hand, achilds ability to apply feedforward control and initiate anAPA to upcoming perturbations greatly depends on the abil-ity to control gravity and inertial forces (Berger, Quintern,& Dietz, 1987; Grasso, Assaiante, Prevost, & Berthoz,1998; Schmitz, Martin, & Assaiante, 1999) and to move thehead independently of the trunk (Assaiante & Amblard,1995), skills that develop later, between 6 and 10 years of

age. However, the presence of a developmental anticipatorybehavior is task specific and is shaped through exerciseand training in the specific environment (Haas & Diener,1988, p. 58), and depends on whether the perturbation isexternally imposed or self-initiated (Riach & Hayes, 1987).

We designed the present study to examine whether 11- to13-year-old children are capable of selecting the appropri-ate balance strategy depending on task constraints, static ordynamic. The childs ability to select a feedforward- orfeedback-based control mechanism would be reflected inthe differential contribution of specific perceptual-motorskills to balance performance depending on whether theparticular task conditions are static or dynamic. Childrenperformed a series of static and dynamic balance exercisesduring one-legged stance on a force platform, whichallowed us to quantify balance characteristics. They werethen assessed in a series of perceptual-motor tests involvingreaction time, depth perception, sustained attention, kines-thesis, and visuomotor coordination measures. We used amultiple correlation approach to investigate balance on thebasis of a multilayered or multifactorial spectrum of select-ed motor, perceptual, and cognitive skills.

Method

ParticipantsFifty boys (11 1.7 years of age) volunteered to partici-

pate in the present study following informed consent. Noneof the children had a pathological disorder associated witheither the visual or vestibular system or had been involvedin sports activities in which a single-limb stance was used.At the beginning of the experimental session, participantswere asked to declare their dominant kicking leg. All bal-ance exercises requiring one-legged stance were performedon the limb contralateral to the dominant kicking leg.

Experimental Task and ProtocolStatic and dynamic balance tests. We achieved static bal-

ance conditions by asking participants to stand on the forceplatform on one leg (single-limb stance). The nonsupport-ing limb was flexed at the knee, with the foots plantar sur-face stabilized on the knee of the supporting leg (storkbalance; see Figure 1a). At the go signal, each child wasrequired to elevate his heel and to keep his balance for aminimum duration of 5 s, narrowing his base of support toan area of a few centimeters.

In the dynamic balance test, the children had to stand onthe force platform on one leg while performing two differ-ent exercises with the nonsupporting limb (Figure 1b and c).The first exercise involved repetitive hip flexion and exten-sion of the swinging limb, and the second required the chil-dren to perform hip abductionadduction in a similar way.Hip flexionextension induced a perturbation in the anteri-orposterior (x) direction, and hip abductionadductionsimilarly introduced a perturbation in the mediolateral (y)direction. Participants were instructed to perform both exer-

V. Hatzitaki, V. Zisi, I. Kollias, & E. Kioumourtzoglou

162 Journal of Motor Behavior

cises as fast as possible and at their full range of motion.Dynamic balance exercises were performed repeatedly overa time period of 5 s. Each participant performed two test tri-als following a practice trial, and, on the basis of maximumforce output, we selected the better of the two trials for fur-ther analysis. Those trials in which the child was unable tosustain his balance for a period of 5 s were excluded fromfurther analysis. Five of the children tested were unable tokeep their balance for 5 s in one of the static or dynamic bal-ance exercises, and as a result of that inability, their datawere not considered in the subsequent analysis.

Perceptual-motor tests. In addition to balance testing, thechildren performed the following series of tests:

1. They performed a simple reaction time test to a visualstimulus (RT visual) by using the Reaction Test software ofthe VIENNA Test Instrument System (Schuhfried, 1996).Participants were seated comfortably behind a monitor,with the index finger of their dominant hand resting on atouch-sensitive key. As soon as a black circle turned yellow,they had to press a bar button located 8 cm above the rest-ing key. The release of the resting key, with consequentpressing of the reaction button at stimulus appearance, wasconsidered a correct reaction. Reaction time was defined asthe interval between the onset of the stimuli and the releaseof the resting key. After oral instructions and four subse-quent correct responses in the practice phase, the childrenperformed 24 trials. Intertrial interval was varied between2.5 to 6.5 s. The mean reaction time of the correct reactionswas recorded.

2. The children performed a simple reaction test to anauditory stimulus (RT auditory) with a different software

version of the VIENNA Test Instrument System. The pro-cedure and recording were the same as before. However, theauditory stimulus in that test was a tone (frequency of 2000Hz) presented on earphones via the systems interface.

3. The children performed a discrimination reaction timetest (DRT) with another version of the Reaction Test soft-ware. A red light, a combined red-yellow light, a tone, andcombinations of the three were presented alternatively onthe monitor. The children had to react only to the criticalstimulus, which was the simultaneous onset of red and yel-low lights. The response procedure and recording wereexactly the same as with the simple reaction time test.

4. The children performed a choice reaction time test(CRT) with the Determination Test software of the VIEN-NA Test Instrument System. The children were seatedbehind a monitor and had to react to seven different visualand two different auditory stimuli by using their upper andlower limbs. They had to press one of five round coloredbuttons, arranged in a semicircle on the working panel, assoon as a circle of the corresponding color appeared on thescreen. Finally, they were also required to press a left orright foot pedal as soon as a white rectangle appeared on thecorresponding side of the screen. The frequency of stimuluspresentation was controlled by the childs working speed.After oral instruction and separate instruction phases for thethree types of stimuli (round circle, white rectangles, andtones) and the combination of the three, the children per-formed the test for 4 min. The mean reaction time (in mil-liseconds) and the total number of correct and incorrectresponses were recorded.

5. The children performed a sustained attention test

Static and Dynamic Balance in Children

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FIGURE 1. Schematic representation of the static and dynamic balance tasks: a. Static bal-ance. b. Dynamic balance I: flexionextension. c. Dynamic balance II: abductionadduction.

(ATN), using the continuous attention test software of theVIENNA Test Instrument System. The children were seat-ed comfortably behind a monitor, with the index finger ofthe dominant hand resting on the lower bar button of aworking panel. A row of seven triangles, pointing upward ordownward, was presented on the screen with irregularjumps. The children had to press the bar button as soon asthey realized that three of the triangles were pointing down-ward (critical stimulus). After successive completion of apractice phase, the children performed the test for 10 min.During that period, 80 critical stimuli out of 400, with alength of 1.5 s each, were presented. The numbers of cor-rect and incorrect responses as well as the mean reactiontime (in milliseconds) for those responses were recorded.

6. The children performed a line-tracking test (LT) withthe Motor Performance Series of the VIENNA Test System.Using a stylus, the children had to track a path on a verticalwork board without touching the sides or the rear. After onepractice trial, they performed one trial, and the total time oftrial completion and numbers of errors were recorded.

7. The children performed a depth perception test (DPF,DPB) with the Electric Depth Perception Tester (Takei Sci-entific Instruments, Tokyo, Japan). The children were seat-ed, and their chin was stabilized so that they could see a par-ticular visual field through a slot located 120 cm away ateye level. Through that slot, they could see a dowel movingforward or backward at a speed of 50 mm/s, and they had topress a button as soon as they realized that the dowel wasaligned with two other dowels located bilaterally 20 cmaway from the moving dowel. Although the test instrumentis designed so that depth perception can be measured, it isset up so that performance outcome also involves a signifi-cant motor reaction time as well as an anticipatory compo-nent. The influence of motor response speed on perfor-mance was eliminated, however, because the movement ofthe experimental dowel was very slow (50 mm/s); in addi-tion, the dowels motion was always available to the chil-drens sight, suggesting involvement of an anticipatorymechanism as part of the response. To further eliminate thepossibility that a delay in pressing the button would beattributed to an incorrect motor response, we gave the chil-dren one practice trial to make sure that they were acquaint-ed with the motor task and could control their movement sothat they could get the result they wanted. A second practicetrial was given in instances of bad movement control. Afterpractice, participants proceeded to perform four trials ineach moving direction of the dowel (forward and back-ward). Distance error from the alignment point was record-ed for each trial in millimeters, and a mean of those valueswas calculated separately for the forward and backwardmovements.

8. The children performed a kinesthetic sense test withthe kinesthesiometer (Lafayette Instrument Co., Lafayette,IN). The children were seated in parallel with a horizontallever that rotated about the vertical axis. The childrenplaced their dominant forearm on the lever so that the axis

of rotation was aligned with the elbow joint. At the startingposition, the forearm was flexed 90 at the elbow. Offeringmanual help, the instructor guided the children arm so thatit moved the lever 60 into extension, which signified aposition that corresponded to a 150 elbow angle. Thatmovement was repeated for a second time, with the childrenblindfolded. They were then instructed to move the lever tothe desired target position. which corresponded to a 150elbow angle. After two practice trials, one with eyes openand the other blindfolded, they performed eight test trialsblindfolded. Lever position was recorded for each trial, andthe mean absolute deviation from the 150 angle was calcu-lated (mean error).

Analysis of BalanceAn AMTI force platform (Model OR6-5-1000; Advanced

Mechanical Technology, Inc., Watertown, MA) recorded ata rate of 50 samples per second the ground reaction forces(Fx, Fy, and Fz) exerted on the support foot during perfor-mance of the balance exercises. A representative trial,which depicts the ground reaction force components duringperformance of the flexionextension exercise, is shown inFigure 2. We used a simplified method based on force plat-form measurements in order to estimate the maximum dis-placement of the center of mass (COM) in the anteriorpos-terior (x), mediolateral (y), and vertical (z) directions duringperformance of the balance exercises (Shimba, 1984; Zat-siorksy & King, 1998). In that method, one uses the threeground reaction force components (Fx, Fy, and Fz) in orderto estimate COM position. Because the ground reactionforce component in each direction is proportional to theacceleration of the COM in the respective direction, its sec-ond integral represents the COM displacement in eachdirection. One estimates the integration constants by usinga technique, called zero-point-to-zero-point integration,which is based on the postulate that center of foot pressure(COP) and COM positions coincide when Fx is zero. Thedouble integration of x (t) = Fx(t)/m starts at the time whenFx is zero and continues until Fx is again zero. The sameprocedure continues from one zero point to the next zeropoint. That technique was analytically described by Zat-siorsky and King (1998) and was found to be in good agree-ment with estimations of the COM position from kinematicdata for situations in which dynamics are not significant(e.g., quiet stance, postural disturbances), confirming thevalidity of the suggested algorithm (Caron, Faure, &Brenire, 1997; Zatsiorsky & King, 1998). We subtractedthe positive value of the maximum COM displacement esti-mated by using the aforementioned technique from the sub-sequent maximum negative COM position in order to esti-mate the maximum amplitude of the COM oscillation in theanteriorposterior (A/P), mediolateral (M/L), and verticaldirections. That value was used as an index of postural sta-bility during performance of static and dynamic balancetasks.



Statistical AnalysisWe investigated the relationship between balance and

perceptual-motor abilities by using multiple correlation andlinear regression analysis techniques. All reaction time,depth perception, attention, kinesthesis, and visuomotorcoordination measures examined in the present study wereinitially placed in a simple correlation (Pearson productmoment), with the postural stability index defined as themaximum amplitude of the COM oscillation in each of thethree directions. Following correlation, we formulated amultiple linear regression model to identify those perceptu-al-motor variables that could predict static and dynamic bal-ance. We included in the equations only those variablesfound to be significantly correlated with balance, and weused stepwise regression to test the models power in pre-dicting static and dynamic balance from the perceptual-motor abilities examined in the present investigation.

ResultsStatic Balance

During performance of the static balance test (the storkbalance), the maximum shift of the COM had mean valuesof 5.2 cm, 4.6 cm, and 3.5 cm in the A/P, M/L, and verticaldirections, respectively (Table 1). The analysis revealed thatpostural stability, expressed by the maximum amplitude ofthe COM oscillation in the M/L plane, was highly associat-ed with depth perception (r = .475, p < .01) when the visu-al stimuli were moving away from the children (backwarddirection; see Table 2). A positive correlation indicated thatthe greater the COM displacement, that is, the greater theinstability in the M/L plane, the greater the amount of spa-tial error in the depth perception test. Balance in the M/Ldirection, as expressed by the same index, was also signifi-

cantly correlated with the number of errors in the LT test (r = .482, p < .01), indicating that children with greateramplitude shift of the COM in the M/L direction were proneto a greater number of errors in the visuomotor coordinationtest. Finally, the amplitude of the COM oscillation in theA/P direction was moderately, yet significantly, correlatedwith the total number of incorrect responses (r = .299, p .05) in the CRT.

Using variables (LT and DPB) that showed a significant


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FIGURE 2. A representative trial showing ground reaction forces (Fx, Fy, Fz) recorded on thesupport foot during performance of the flexionextension exercise.

TABLE 1Mean (M) and Standard (SD) Deviation

of the Maximum Amplitude of the Centerof Mass Displacement in Each Direction

During Performance of the Staticand Dynamic Balance Exercises

Flexion- Abduction-Static Extension Adduction

M SD M SD M SD

A/P direction (x).0529 .0319 .0428 .0218 .0396 .0100

M/L direction (y).0460 .0246 .0507 .0153 .0751 .0268

Vertical direction (z).0358 .0256 .0547 .0228 .0584 .0365

Note. N = 45. All displacements are in meters. A/P = anteriorpos-terior, M/L = mediolateral.

relationship to the amplitude of the COM oscillation in theM/L direction, we formulated a linear regression model toenable us to predict performance in the static balance testfrom the depth perception and visuomotor coordinationmeasures (Table 3). The stepwise regression analysisrevealed that a model that included both variables as pre-dictors could explain 31.7% of the variance in the COM dis-placement in the M/L direction, and that prediction washighly significant, F(2, 42) = 9.29, p < .001. The variationwas accounted for by the number of errors in the line track-ing, t = 2.35, p < .05, and depth perception, t = 2.213, p .05, did not enter theregression equation, mainly because of its significant rela-tionship to DRT (r = .784, p < .01), indicating that the twovariables explained the same amount of variation in thedata.

DiscussionIn the present study, we investigated the contribution of

selected perceptual, cognitive, and motor skills to balancecontrol in children between the ages of 11 and 13 years.Two different balance conditions were tested. The firstrequired unperturbed standing balance under static condi-tions (quiet one-leg standing), and the second involved reac-tions to postural self-induced disturbances evoked by rapidhip flexionextension or abductionadduction. The resultsconfirmed the hypothesis that, depending on the imposedenvironmental constraints and nature of the balance task,static or dynamic, the selection of the appropriate balancestrategy and therefore the perceptual, motor, and cognitiveprocesses involved in maintaining equilibrium can be quite

different (Assaiante & Amblard, 1995; Haas & Diener,1988). Moreover, we found that 11- to 13-year-old childrenare capable of applying strategies that significantly resem-ble those used by adults for maintaining equilibrium understatic or dynamic conditions.

The significant relationship of static balance with depthperception and with visuomotor coordination suggests thatone-leg standing is regulated by a feedback-based mecha-nism in which sensory information from visual cuesbecomes important for the continuous adjustments; more-over, corrections of the required posture that are needed sothat the COM can be kept within the narrow boundariesdefined by the base of support. Other investigators (Ghez,1991; Nashner, 1976; Robertson, Collins, Elliott, & Starkes,1994) have proposed a slow sensory integration processsuch as the one just described, which is based on a closed-loop system, as the regulating mechanism for quiet stance.In addition, an increasing body of evidence suggests thatinformation processed through vision, particularly fromperipheral visual cues providing exteroceptive informationabout the environment, is the most reliable source of per-ceptual information for balance control, especially in chil-dren (Assaiante & Amblard, 1995; Garland & Barry, 1990;Grasso et al., 1998; Riach & Hayes, 1990; Williams,Davids, Burwitz, & Williams, 1994). However, the abilityto interpret and use peripheral visual cues is associated withincreasing accuracy and consistency of eye movements,which is acquired with age (Starkes & Riach, 1990) and isnot achievable in younger children. For that reason, childrenbetween 6 and 7 years of age rely mostly on vestibularinformation about head position relative to the supportingsurface; that information becomes progressively more avail-able to the equilibrium control centers as a result of theacquisition of the head stabilization in space strategy (Assaiante & Amblard, 1995; Assaiante, Marchand, &Amblard, 1989; Forssberg & Nashner, 1982). The relativecontribution of peripheral vision in equilibrium controlincreases from 8 to 9 years of age to adulthood (Assaianteet al., 1989). In adults, peripheral visual cues as well asvisual motion cues have been reported to play a particular-ly important role in postural control (Amblard, Crmieux,Marchand, & Carblanc, 1985). The results of the presentstudy confirmed the important role of peripheral visual cuesfor postural stabilization in 11- to 13-year-old children aswell. In addition to that finding, we showed that the rela-tionship between balance and visual motion cues was sig-nificant only under conditions involving motion of theexperimental dowel in the backward direction. That findingmay indicate the higher contribution of vision during cor-rective backward body sway, when visual cues appear to bemoving away from the eyes. Exteroceptive information canbe available through the kinesthetic system as well,although it has been suggested that vision provides an easi-er anchoring for that information to the gravito-inertia ref-erence frame (Patla, 1999). In the present study, poor corre-lation of kinesthesis with both static and dynamic balance


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TABLE 4Stepwise Regression of Dynamic Balance(Amplitude of Center of Mass Oscillationin the x Direction) on Simple Visual (SRT)and Discrimination (DRT) Reaction Times

in the Dynamic Balance,Abduction/Adduction Exercise

Model: COM = int + 1 DRT, 43 df,MS = .000787, F = 9.15, p < .004**, R2 = .176

Hypothesis testing about the model parameters

Parameters t p < Partial RDRT .419 3.026 .004** .419 inSRT .210 .938 .354 .143 out

Note. COM = amplitude of maximum center of mass oscillation inthe anterioposterior (x) direction, and in and out indicate in or outof the regression equation (model).

measures was observed. That finding can be attributed tothe fact that all balance exercises were performed with eyesopen, although, on the other hand, kinesthesis was tested inthe absence of vision. The predominance of visual over pro-prioceptive cues for static and dynamic equilibrium is notnew (Assaiante & Amblard, 1995; Garland & Barry, 1990;Williams et al., 1994). We speculate that under the condi-tions involving vision, the afferent contributions of thekinesthetic system to static balance control are seriouslylimited.

In contrast to one-leg quiet stance, in which a significantreliance on visual cues was found, and in accordance withprevious studies (Do, Brenire, & Brenguier, 1982; Woolla-cott et al., 1986), balance performance under dynamic con-ditions involving a self-induced perturbation in the M/Lplane was predominantly associated with an adequatelyfunctioning reaction time process. Short reaction times areknown to be associated with preparatory adjustments in agiven perturbation and to reduce the disturbance of theCOM during performance of postural tasks (Riach &Hayes, 1987). According to the results of the present study,the ability to rapidly respond to a destabilization induced byrapid hip abductionadduction in order to maintain equilib-rium is greatly dependent on reaction time. That findingsuggests that postural reactions to rapid hip abduc-tionsadductions are preprogrammed in an open-loop fash-ion, whereas the child has the ability to apply feedforwardcontrol in his other movement and therefore to reliablycoordinate posture with the performed task. Balance controlin that case is temporally organized in a descending order,from head to toe (Assaiante & Amblard, 1995). Recentexperimental evidence suggests that such an anticipatory,open-loop (feedforward) mode of balance control appears ataround the age of 7 years and continues through adulthood(Grasso et al., 1998; Ledebt, Bril, & Brenire, 1998). Incontrast, a feedback-based control corresponding to anascending organization of balance control from foot to head(in posture) and hip to head (in locomotion) is more char-acteristic of children under the age of 6 (Assaiante &Amblard, 1995). According to the same authors, the ascend-ing, descending, and mixed strategies all co-exist in adults,and the selection of one or another and their relevant con-tributions greatly depend on the particular task conditions.For example, in difficult balance situations, the descendingstrategy (from head to toe) is always dominant. On the basisof our present findings, we speculate that dominance of thatstrategy is also true for children between the ages of 11 and13 years, when, according to studies reported in the litera-ture, the complexity of the CNS with respect to balancecontrol strongly resembles that of an adult (Berger et al.,1987; Ledebt et al., 1998). We showed that balancing understatic conditions was strongly associated with the ability toperceive and process visual information, an ability that isimportant for feedback-based control of balance (ascendingstrategy). On the other hand, when greater task demandswere imposed on the system under dynamic balancing con-

ditions, the ability to respond to the destabilizing hip abduc-tionsadductions in order to maintain equilibrium was asso-ciated with the motor response speed, implying the possibleinvolvement of a descending, feedforward strategy in thecontrol of balance. The significant association of dynamicbalance with discrimination reaction time suggests, howev-er, that although postural reactions to hip abductionadduc-tion perturbations seem to be preprogrammed in an open-loop fashion, the use of sensory information or cognitiveprocessing at a later stage to modulate the response is notprecluded. Further evidence in support of cognitive contri-butions to the control of both static and dynamic balancewas provided by the significant relationship between thenumbers of errors in the choice reaction time test and inboth static and dynamic balance indices. Because the num-ber of incorrect responses in the choice reaction time testrepresented the speed of information processing in theresponse selection stage, that relationship suggests that cog-nitive interventions targeted to speed up processes of a cer-tain stage may also be involved in both static and dynamicequilibrium control (Thomas, Thomas, & Gallagher, 1993).Those findings are in agreement with previous studiesshowing that postural control might rely not only on periph-eral sensory information but also on higher-level systemsresponsible for integrating sensory information (Teasdale etal., 1992; Woollacott et al., 1992; Woollacott et al., 1986).There is no doubt, however, that the relationship betweenpostural control and cognitive processes is rather complex,and researchers must perform further research to substanti-ate any findings concerning cognitive contributions to pos-tural control.

Another interesting finding of the present study is thatmotor response speed, as reflected in reaction time mea-sures, seems to be an important element in dynamic balancecontrol only when postural disturbance occurs in the frontal(M/L) plane. In contrast, when destabilization occurred inthe A/P plane because of rapid hip flexionextension, noassociation between the amplitude of the COM oscillationand reaction time was noted. There are several possible rea-sons for that task-specific contribution of reaction time indynamic balance control. First of all, reactive control of bal-ance, through a reflex-activated feedback loop, might not bethe only mechanism used by the CNS to control the positionof the COM in response to external perturbations (Rietdyk,Patla, Winter, Ishac, & Little, 1999). Alternatively, it is pos-sible that the CNS uses muscle tone to set joint stiffness tocontrol body posture. In the sagittal plane, there is stiffnesscontrol at the ankle plantarflexors, whereas in the frontalplane, the hip abductorsadductors provide that control. Onthe basis of those findings, we speculate that the inherentproperties of the muscles and passive tissue around theankle could have provided an immediate resistance to theA/P perturbation, eliminating the need for voluntary activemuscle activation during performance of the flexionexten-sion dynamic exercise. Riach and Hayes (1987), who havealso shown that the maturation of postural responses in the



two planes may follow different courses in children, pro-vided further evidence in support of that hypothesis. Thatconclusion was based on experimental evidence showingthat the initiation of APAs was more often in the lateral thanin the sagittal plane, indicating the stronger associationbetween reaction time and balance control in the lateralplane.

ConclusionBalance control during performance of different posturo-

kinetic activities in children not only is an age-dependentprocess associated with neural maturation but is also asso-ciated with task or environmental constraints. The results ofthe present study suggest that, like adults, 11- to 13-year-old children have the ability to select varying balance strate-gies depending on the constraints of a particular task. Bal-ancing under static conditions was shown to be greatlyassociated with the ability to perceive and process visualinformation, an ability that is important for feedback-basedcontrol of balance. On the other hand, when greater taskdemands were imposed on the system under dynamic bal-ancing conditions, the ability to respond to the destabilizinghip abductionsadductions so as to maintain equilibriumwas associated with motor response speed, implying thepossible involvement of a descending feedforward mecha-nism in the control of balance.

ACKNOWLEDGMENTThe authors thank Patricia McKinley as well as the two anony-

mous reviewers for their helpful comments and suggestions on anearlier draft of the present article. The present study was partiallysupported by a research grant from the National ScholarshipsFoundation.

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Submitted December 23, 1999Revised October 11, 2000

Second revision March 1, 2001



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