Acta Physiol. scand. 1968. 72. 72-80 From the Departments of .%viation and Naval Medicine, Faculty of Medicine.

Karolinska Institutet, Stockholm, Sweden

Antigravity Effects of Leg Exercise By

G. ROSENHAMER

Received 8 May 1967

Abstract

ROSENHAMER, G. Antigravity effects of leg exercise. Acta physiol. scand. 1968. 72. 72-80,

‘ro investigate the effects of leg esercise on the overall tolerance to exaggerated gravitational stress, 8 subjects were esposed for 13 min to a threefold increase of the force of gravity in centrifuge experiments ( + 3 G,) while in the sitting position, both during motionless resting and when exercising a t 600 kpm/niin on a bicy-cle ergometer. In all subjects, continuous leg exercise prevented the occurrence of circulatory collapse. which otherwise became imminent in 5 subjects after 5 to 12 inin in the resting condition as evidenced by blackout or rapid in- crease in heart rate. Subjectively, exercise was experienced as less stressful than the resting condition. The improved circulatory stability with exercise was also reflected by uniformity of individual heart rate responses, the group mean value increasing from 152 to 160 beats/min during the 6th to 12th min.

Cardiovascular tolerance in man to motionless standing is limited, and it is well known that the force of gravity eventually acts to bring about a progressivc fall of the systemic arterial pressure at head level followed by circulator). collapse (for review, see Gauer and Thron 1965). Border-line tolerance is reached morr rapidly when the acting force is magnified through use of the human centrifuge. Thus, exposure of unprotected individuals to a 4 to 7-fold increase of the forcr of normal Travity in the sitting position generally causes loss of vision or consciousness within 4~ 6 sec because of insufficient perfusion prcswre a t eye or head leyel (\l’ood et al. 1946. Henry et nl. 1951 i. Y‘hese consequences of gravity and accelerative forces acting in the head-to-foot direction arc largely due to increased hydrostatic pressure differencrs in the vascular tree. I,eg esercisc in the crrct position, as in walking, or sitting as in bicycling, incrcaxs the systemic arterial pressure at normal gravity and so exerts a protective action on orthostatic tolerance.

So far no investigations have been carried out as to the effects of graded leg cxercise on the cardiovascular tolerance to forcrs stronger than that cxerted by normal gravity. Such experiments seem to be of considerable intercst, since the

EXERCISE AND G-STRESS 73 TABLE I. Individual data‘

Subj. Age, Weight, Height, Heart rate, beats/min PWC,,OZ, no. years kg cm kpmlmin

lying standing at 6th min of at rest rest after 8 exercise at

min 600 kpm/minz

20 24 23 21 22 24 24 26

70 68 50 74 63 78 58 72

181 185 173 186 172 191 162 185

55 68 81 62 60 55 63 59

74 94

107 102 75 92 79 95

128 105 142 125 117 109 127 113

1070 1350 850

1020 1090 1360 1040 1250

All functional data obtained at normal gravity. PWC,,, = physical working capacity at pulse rate 170.

(Sjostrand 1947, IYahlund 1948), sitting position (Fig. 1).

normally protective effect of leg exercise on orthostatic tolerance might theoretically be cancelled by the greater hydrostatic pressure that develops in the dependent leg muscles during increased gravitational stress. In the present study the effects of leg exercise in the sitting position on the electrocardiographic, heart rate and subjective responses to a simulated increase of gravity to three times its normal value were investigated.

Subjects, Methods and Procedure Subjects. Experiments were performed on 8 healthy, male students. All were familiar with tbe subjective sensations experienced during runs in the centrifuge. Heart rates immediately pripr to the runs were not higher than those normally observed at rest in the sitting position, arjd no one showed other signs of apprehension or anxiety prior to or during the runs. Individulal dimensional and functional data are given in Table I. Physical working capacities were average, except in one subject (no. 3 ) . Methods. All subjects were studied while seated in the centrifuge cabin suspended 7 3 m from the center of rotation (for description of the centrifuge, see Gotzlinger and Helsing 1955, Bjurstedt 1957). Throughout all runs the subject leant against a backrest, inclined 13” frokn the vertical and had his occiput in contact with a headrest. His feet were fastened on tbe pedals of an elect rodynamically braked bicycle ergometer, which was modified so that the axis of the pedals was at the level of the seat. On starting the centrifuge, the cabin swings out freely so that the resultant of the gravitational and centrifugal G vectors remains aligned wilh the head-seat axiG; (sagittal plane) of the body. The final magnitude of the resultant G vectpr was preset to 3 1s units‘; the resultant G profile as a function of time had the shape of a single plateau. In this manner the subject could thus be exposed to a force three times greatbr than that exerted by normal gravity. Fig. 1 shows the magnitude and direction of G in relatign to body geometry prior to and during the centrifuge runs. G-suits were not used. Ergometer. An t lectrically braked bicycle ergometer (Holmgren and Mattsson 1954) whs used in which th? work load could be preset and was largely independent of pedal rpm.

Synonyms often used to indicate the magnitude and direction of the force employed: “posi- tive” or “headward” acceleration, + 3 G,.

74 G. ROSENHAMER

I-! Js

n

Fig. 1. G-forces and body contour u i th the subject seated in the centrifuge cabin: left arrow indicates the direction and magnitude of the force with the centri- fuge standing still. right arrow with the centrifuge running.

Tlir increase of power dissipation in the ergometer resulting from the increased G-force \vas cstiinated by driving the generator of the ergometer as a motor a t a rate of 350 rpm (ix. the speed of the generator axis in work tests at the standard pedal rate of 60 rpm) , and measuring the fnergy expenditure of the motor at increasing G-levels. The increase in expenditure measured in this way was found not to exceed 5 kpm/min a t 3 G. Rlvcfrocardiogrnnis were recorded with a direct writing recorder (Mingograph 42. Elerna. Stockholm) at a paper speed of 50 mmisec. Leads I, 11. 111: aVR, a\T+ a\-F> CR 1-2-4-5-7 were recorded before the centrifuge runs, with the subject in recumbent and standing (after 8 mirr) positions, respectively, and then while sitting in the cabin. These leads were also rrrorded for 3 min following the centrifuge runs. During the exposures to 3 G, leads CH 2-4-5-7 (indifferent electrode on the forehead, cf . Holmgren and Strandell 1961 1 Tvere re- corded for the last 15-20 sec of each min, whether the subject was working or not.

Thp electrocardiograms were examined and classified according to Gre\vin ( 1948) and Strandell (1963). ST-depressions were measured from a horizontal line through the end of the preceding P-R segment. The lower border of the recorded lines w a ~ used as a reference for the measurements.

HPnrt rntc was recorded on photographic paper by means of an instantaricoui cai tiio- tachometer Sturm and Wood 1947).

Blood Znctic acid tvas determined in “arterialized” finger blood within 2 min o f cessation of G exposures. by the colorimetric method of Barker and Summerson (1941 as modified b y Strom I 1949 I .

Physirrrl zoorking rnpaci ty . At normal gravity the approximately linear relationship between heart rat? and work load was used to obtain, by extra- or intrapolation, the rate of work in kprnhin corresponding to n heart rate of 170 heat!/min (PWCim; Sjostrand 1947. \Yahluncl 1949).

O r t h o ~ t o f i c t t ’ r t . Heart rate and ECG were recorded after 8 min of motionlevs starding againqt a wall.

t’rocrdrcre. .\I1 subjects were exposed to 3 G in two centrifuge runs, each of 13 rnin d u r a t i o ~ ~ . lh roughout one run the subject rested, with his feet supported by the ergometer pedals, and was instructed to relax and to remain completely motionless. In the other he iested dui-ins the first min and then exercised a t a constant load of 600 kpm/min for 1 2 rnin. i.e. until the end of thc run. The rate of pedalling w-as GO/min. Both runs were preceded h y at least GO Inin rest at 1 G. The sequence between the 3 G rest and 3 G cxercise runs \cas rotated amoiie the subjects. Whcreas the heart rate and ECG were recorded continuously during both tylles of runs. blood samples for lactic acid determination were only taken immediately before and aftcr the I U I l S .

A centriiuge r u n could rapidly be terminated in the event of signs of intolerance. or when- ever the subject desired. For this purpose voice communication between the subject and the investisator was possible at all times. In addition, the subject’s condition was checked hy constant surveillance of the subject’s face and upper part of his torso by closed-circuit tele- \ision. and by monitoring audible heart rate and electrocardiographic tracings. The temper- ature i n the centrifuge cabin varied ketwen 23“ and 25” C during both rest and ererrise.

EXERCISE AND G-STRESS 75

Results

Subjective reactions. The subjective sensations associated with the 3 G rest experi- ments were generally relatively stressful. This was due only to a minor degree to the increased weight of different body parts, and more to a progressively increasing feeling of fatigue and, in six of the eight subjects, dimming of peripheral or central vision following the first 4 to 5 min. In 5 cases, rapidly increasing dimming of vision and impending loss of consciousness necessitated a shortening of the 3 G rest exposures. In three of these cases the runs were discontinued during the 6th to 7th min, and in two cases during the 12th rnin (see Fig. 2 ) .

By contrast, when leg work was performed for the last 12 of the 13 min at 3 G, no case of dimmed vision occurred, and no run had to be shortened. 3 G exercihe was experienced by all subjects as less exhausting and unpleasant than 3 G re$t. Moving the pedals was not subjectively experienced as more difficult at 3 G than bt normal G. One type of discomfort, however, was more pronounced during exercise, viz., a dull, aching sensation in the neck which increased moderately during the course of the run. Movement of the head did little to relieve the pain. There was no pain in the lower back or lower extremities, and in none of the experiments wps substernal pain reported.

Electro-cardiographic findings. Control ECG:s at rest and at normal G in the sitting position were all normal. In subject no. 4 the T-waves in leads 11, 111, CR5 and CR7 were slightly diphasic or isoelectric during the orthostatic test. The changes in this case were classified with Strandell (1964) as moderate but not marked. No ectopic beats 01- other marked alterations in the ECG:s were observed either at 3 G rest or 3 G exercise. Thus, in no case during or after work did ST-junction deprqs- sions exceed 0.5 mm, or changes to horizontal or downward sloping ST-segments appear. No marked flattening or the T-waves was observed. According to Strandell’s nomenclature, no ECG during exercise was classified as “intermediate” or “suspected abnormal”. The subject who had shown “moderate” changes in the orthostatic test displayed no such changes during the centrifuge runs.

Heart rate. Individual heart rates during 3 G rest and exercise, averaged over 1-min periods, are shown in Fig. 2. In the rest experiments at 3 G all subjects showed higher heart rates after 2 rnin than after 8 min motionless standing at normal G ( c f . Table I ) . ‘The range of individual responses at 3 G rest was rather wide, how- ever. Thus, 2 subjects (no. 3 and 7 ) showed rapidly increasing heart rates over the last 2 to 3 min of the run before it had to be terminated in the 6th rnin because of impending unconsciousness. In two other cases (no. 6 and 8) where the exposuys had to be terminated for the same reason, the heart rate showed a rapid increase only during the last few seconds, as observed on the original photokymographic recordings. There seemed to be no correlation between the incidence of blackout and the increments in heart rate observed after 8 rnin standing at normal G or after 2 to 4 rnin exposure to 3 G rest. During the course of the 3 G exercise runs, a

76 G. ROSEUH4MbR

200, I I I I I I I I , 1 HEART

beats/min

160 -

140 -

120 -

100 - -

3G, REST

.L TIME, minutes .-+ I , I , I , I , I , I a

0 2 4 6 8 I 0 1 2 1 4

180 -

160

140 -

120 -

-

HEART RATE beats/min

Fig. 2. Heart rate5 (individual averages for consecutive minutes of observationi in 8 subjects in the sittinp rlosition at 3 G lest

36 , EXERCISE . .

(upper graph) , arid at 3 G ener- cise at 600 kpm/rnin (lower graph): with individual responses numbered for identification. \.al- ues olitained during the first 2 min have been ommitted. Circles indicate elective termination of experiment hecause of impending

0 2 6 8 10 12 14 loss of consciousnecs.

progir\si\e increase of the heart rate occurred In all raaes 3 h e mean \aliie of indi\idual time averages over the 6th min of exercise was 152 beats min. and had increased b\ 4 beats by the 9th min (range 1 to 121 and by another 4 bent< [ranee 2 t o 14) b) the 12th niin

Blood luctatc-.l-. Individual values are shown in Table 11. Exposure to ‘3 G rcst to1 13 min resulted in no systemic change of the resting control ~ a l u e s obtained .it normal G With 600 kpmlmin for the last 1 2 min a t 3 G, the average lactate con-

EXERCISE AND G- STRES S

TABLE 11. Arterial lactate concentrations (mM/1) obtained a t 1 G rest (control) and within 2 mi01

77

following 3 G rest and 3 G exercise

Subject No. 1 G rest 3 G rest 3 G 600 kpm/rnin

1 2 3 4 5 6 7 8 Mean SE

.77

.78 1 .oo 1.08 1.40 1.07 .65 .75 .94 .09

.60

.79 1.62

.93

.63 1.15 .93 .95 .60

-

1.40 3.13 3.48 1.80 2.03 1.30 2.28 3.45 2.36 .3 1

Highest value based on duplicate determinations on each of two consecutive blood samples.

centration attained a value of 2.36 meq/l, as compared to 0.95 meq/l for 3 G rest. No individual value at 3 G exercise exceeded 3.5 meq/l. There seemed to be 40

correlation between the heart rate and lactate concentration during exercise.

Discussion

L e g exercise avd G tolerance. In the experiments with exposure to 3 G duridg motionless resting, the occurrence of blackout with impending unconsciousne$s necessitated termination of the centrifuge runs in 5 of the 8 subjects before the prC- set time period of 13 min had elapsed. By contrast, exposure to the same G level was experienced as less stressful when combined with leg exercise, and vision and consciousness were well maintained in all subjects throughout the runs. These are clear indications that leg exercise involves physiological adjustments favourable to G tolerance. Loss of central and peripheral vision during headward acceleration, which precedes loss of consciousness, and which may be used as an index of G tolerance (GaucLr and Zuidema 1961), is generally attributed to a reduction of the arterial perfusion pressure at eye level to values inadequate to overcome intraoculqr pressure (Lamkert 1945). The absence of visual impairment suggests that exercie elevated the ari erial pressure sufficiently to maintain an effective blood supply to the eyes and brain.

With headward acceleration, the increased effective weight of blood causeS the arterial pressure to fall in the upper portion of the body and to rise in the dependefit regions (Wood et al. 1946). The pressure reduction at head level is partly due t;o the increased hydrostatic pressure difference between the head and heart, and partly to a curtailment of the cardiac output secondary to sequestration of blood in

78 G . ROSEXHAMER

distensible veins in dependent regions of the body. In this situation. rrflrs cori- striction of resistance vessels in these regions would counteract downward shunting of arterial flow. and tend to improve the supply of blood to the hraiii (Rro\\-n, Wood and Lambert 1949, cf . Gauer and Thron 1965).

\Vith leg exercise, local blood flow must increase to preserve an adequate oxygen supply to the working muscles. The observation that no impairment in vision and consciousness appeared at 3 G when combined with exercise indicates that the total cardiac output was sufficiently increased not only to permit a n increase of the blood flow through the dependent legs, but also to maintain a n adequate blood supply to thr eyes and the brain. The observed improvement of the circulation at head l r \ d may also to some extent have been due to compensatory, exercise-induced u t r r i a l vasoconstriction in body regions which were not actively engaged in the \\ark per- formed, such as occurs at normal gravity (see Kushmer 1961). Thus. a more forreful vasoconstriction during exercise, c.g., in the splanchnic region may ha\.e been super- imposed icpon the initial vasoconstriction induc.ed by acceleration.

HPart rat(*. The increase in the resting heart rate during exposure to 3 G wnb of thc

same order of magnitude as observed in previous investigations by Bjurstedt. Han;- son and Strom (1959) and by Wood ~t al . (1961 ) . Presumably. the response of the heart rate was compensator) for a reduced stroke volume, secondary to irnpairrd cardiac filling ( r f . Wood et el. 1961 I . While the initial c.ompens.atory incrrasc of the heart rate generally levelled off after the first 2 min, a rapid serondal-y i isr occurred i n -1. of the 5 cases in which blackout or unronsciousnprs w a s imminrnt. This responsr pattern was evidently indicative of impending failure of compensatory adjustments.

By contrast. the observed further elevation of the heart rate during exercise I Fig. 2 ) must have been combined with partial restoration of cardiac filling, leading to improvement of the stroke volume and cardiac output. Enhancement of cardiac. filling would in turn result from the repetitive action of the leg muscle putxlp as long as pooling of blood in distensible intra-abdominal vessels is countrractcd. ( ' .g .

by adequate external support from the abdominal muscle wall.

ECG reaction. Thc absence of abnormal ECG patterns during exercise at hi311 G is of particular interest in view of the debated question whether increasrtl gr;i\.i- tational stress may involve risks for coronary insufficiency ( c f . Gauer and Ziiidema 1961. Sirkcr 1961 ) . That no signs of functional disturbances were re\,ealed under resting conditions is in accordance with observations by Bjurstedt. Hansson a n d Strom (19.591. The present results indicate that any potrntial coronary insufficiency a t 3 G was in any case not aggravated by moderate esercize.

O a y g r n transport. Though a rise in the cardiac output implied an increment in t h r oxygen transport capacity of the circulation, it may be questioned whether thr resulting oxygen s~ipply to the working muscles was adequate in relation to thf.

EXERCISE AND G-STRESS 79

metabolic need. For instance, one might expect that G-induced vasoconstriction in the leg muscles interfered with the oxygen supply during exercise. However, the observed blood lactate concentrations indicated that such interference must have been slight. This is of considerable interest, if it is realized that already at normal gravity a change from supine to upright position causes a significant reduction of

cardiac output not only at rest but also during leg exercise (Bevegird, Holmgren and Jonsson 1960).

The improvement of circulatory tolerance to 3 G caused by exercise indicates rise in the arterial pressure at head level. It is concluded that the shunting of arterial flow to the dependent portions of the body produced by the combined action of large hydrostatic pressures and strongly increased blood flow through working leg muscles did not occur at the expense of a sufficient arterial blood flow at head level. This indicates a substantial increase in the cardiac output.

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