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The role of stretch reflexes during active movements
Brain Research, 181 (1980) 493-497 :(3 Elsevier/North-Holland Biomedical Press
493
The role of stretch reflexes during active movements
J. DAVI-D COOKE
Department of Physiology, University of Western Ontario, London, Ont. (Canada) (Accepted September 13th, 1979)
Key words: stretch reflexes - - active movements - - limb models
H a m m o n d ' s 7 remarkable observation in 1956 that the E M G responses induced by sudden limb perturbation could be modified by the 'will' or 'intent' of the subject has led to questions concerning the function of the stretch reflexes. It has, for example, been suggested that the early, presumably segmentally mediated E M G response following a perturbation may not generate sufficient force to aid in overcoming the perturbation 9. This role was ascribed to the longer latency responses, and was therefore termed the 'functional stretch reflex'. Underlying this name was the idea that the 'function' of stretch-induced reflex muscle activation was to return the muscle to its original length or the limb to its original position. This view, which has recently been questioned8,12 sees the stretch reflex(es) as forming part of an automatic length servol0,11,15.
What, however, can be the function of the reflex responses to a sudden perturbation during an active movement? In this case the limb position at the time of the perturbation may not be the position which is 'desired'. In general, the stretch reflexes could not act as part of a simple length servo system as there is no muscle length (limb position) around which they could be acting. Described here are initial experiments which indicate that the stretch reflexes during active movement may be involved in returning the limb to the trajectory it would have taken had it not been perturbed. Experiments were performed on 4 normal human subjects. The subjects were seated comfortably and grasped a vertical rod attached to a manipulandum handle. Their arm was held horizontally and was supported just distal to the elbow. The manipulandum handle was pivoted at one end, the subject's elbow being positioned below the pivot point. Angular position and velocity of the handle were recorded by a precision potentiometer and a tachometer respectively. Subjects performed step-tracking movements following with the handle a target displayed as a vertical bar on an oscilloscope. Handle position was displayed as a vertical line. Target positions were not mechanically detectable and were not bounded by mechanical stops. Surface electromyograms (EMGs) were recorded from the biceps and triceps muscles. The E M G s were full-wave rectified and all data was digitized online using a PDP-1 l computer with an effective sampling rate of 500 Hz (1 K H z with 2 point block averaging).
494
A
B
/
f . . . . . . . 0
~L .
.
500
I
II
I
II I
O00msec
r
0
500
I000
msec
", 500
O00msec
C
il o
T 400
// 200
0
200
X
400 m s e c
Fig. t. Effect of brief torque perturbations on limb trajectories. A: data from movements made without (left-hand traces) and with (right-hand traces) application of a 50 msec torque pulse during the movements. The dashed vertical line in the right-hand trace marks the onset of the force pulse. The records shown (top to bottom) are angular position, angular velocity, biceps and triceps EMGs and phase planes. The phase planes plot limb velocity (ordinate) as a function of limb position (abscissa) and are generated from left to right during the evolution of the movement. Note that both perturbed and non-perturbed phase planes are over-plotted in the right-hand traces. Vertical calibration lines represent 50° for position and 60°/see for velocity. Calibration bars on the phase planes represent 15"~ and 50°/sec. S.D. are shown surrounding the position traces. B: biceps EMGs and phase planes derived from movements for which the subject moved as rapidly as possible between targets. Movement onset is indicated by the arrow below the EMG records and the time of application of a 50 msec torque pulse opposing the movement is indicated by the dashed vertical line. Note that following the perturbation the limb's trajectory is markedly different from the control trajectory. Calibration bars represent 15¢' and 50°/sec. C: shown is the difference in the integrated biceps EMGs of perturbed and non-perturbed movements. The open bars were derived from the slower movements (A) and the solidbars from the rapid movements (B). The vertical dashed line indicates the onset of the perturbation and the thin horizontal bars indicate the movement duration. The ordinate is in arbitrary units: positive values indicate that the integrated EMG in the perturbed movements was greater than that during the nonperturbed movements.
S h o w n in Fig. I A (left-hand traces) are records o f typical movements. Th e m o v e m e n t s were m a d e in one s m o o t h traverse f r o m target to target with little v ari at i o n f r o m m o v e m e n t to m o v e m e n t . T h e E M G
pattern associated with these
m o v e m e n t s is similar to that described by previous a u t h o r s a,4,~,14. P h ase p l a n e plots o f these m o v e m e n t s are also s h o w n in Fig. IA. These plots, o f a r m velocity as a function o f a r m position, have been described previously 2 and a r e sh o w n here as they are relatively responsive to small changes in m o v e m e n t trajectory. In Fig. I A (right-hand traces) are s h o w n records o f average m o v e m e n t s and phase planes with a brief
495 p e r t u r b a t i o n a p p l i e d to the a r m during the movements. P e r t u r b a t i o n s were applied r a n d o m l y during a series o f m o v e m e n t s and were o f the same m a g n i t u d e during any one series. The p e r t u r b a t i o n elicited discrete E M G responses which, on the average, had onset latencies o f 30 and 85 msec. It should particularly be n o t e d in Fig. 1A that the l i m b ' s trajectory quickly r e t u r n e d to the control or n o n - p e r t u r b e d trajectory following the p e r t u r b a t i o n 2. As is seen in Fig. 2A and B, this return to the control trajectory did not occur in an a n a l o g model o f the limb in which the limb was considered as a simple d a m p e d spring with mass. This model has been described previously 2 and is related to the m o d e l describing m a i n t e n a n c e o f fixed limb or head positions p r o p o s e d by other investigatorsl, ~,13. M o v e m e n t s in the m o d e l were p r o d u c e d by step changes in the resting spring constant. The earlier studies with this model suggested that it p r o v i d e d an a d e q u a t e qualitative description o f a r m trajectory during step-tracking m o v e m e n t by humans. One obvious difference between the model and the h u m a n subjects was the occurrence o f reflex responses to the p e r t u r b a t i o n . The possibility was thus considered that the r a p i d return to the original trajectory seen in the h u m a n subjects was p r o d u c e d by the reflex responses to the p e r t u r b a t i o n . Two tests o f this possibility were made. In the first, a 'pseudo-reflex' was a d d e d into the model. T h a t is, at a fixed delay following the p e r t u r b i n g force, a second force was applied with a direction o p p o s i t e to the p e r t u r b i n g force. As can be seen in Fig. 2 C - F , the trajectory following perturbation could be made to return to the original trajectory by suitable m a n i p u l a t i o n o f the m a g n i t u d e o f this 'pseudo-reflex'. The second test was to study the trajectories o f rapid arm movements in h u m a n s in which p e r t u r b a t i o n s p r o d u c e no, or at least greatly reduced, reflex responses 4. D a t a from such m o v e m e n t s are shown in Fig. 1B. F o r these trials the subject was requested to move 'as quickly as possible' between the target zones. As can be seen from the phase planes from the movements, a p p l i c a t i o n o f a p e r t u r b a t i o n p r o d u c e d a m a j o r change in the m o v e m e n t trajectory which did not return to the control trajectory A
B
C
pos~
Fig. 2. Phase plane trajectories derived from an analog model of the limb. A : a control trajectory of a movement generated in the model by a step change in the resting spring stiffness. This trajectory is also shown in B as is the trajectory of a movement in which a brief (50 msec) perturbation was applied just after the start of the movement. The perturbation, which opposed the movement, decreased the movement velocity and displaced the phase plane trajectory below the control trajectory throughout its course. The dashed curves in C-F show the effect on the trajectory of a reflex response to the perturbation. This reflex was modelled as a force initiated 50 msec following the perturbation and assisting the movement. The magnitude of the reflex was progressively increased from C to F with its duration constant at 50 msec. Note that by appropriate adjustment of the magnitude of the reflex force the trajectory following the perturbation can be made to match the control trajectory (D).
496
'
t
C
t 0
2(X) ' 4(~0 ' 6(2(] t~sec
40deg
Fig. 3. Effect of brief torque perturbations during 'fast' movements. In A-E are shown (left-hand traces) biceps EMGs from individual flexion movements made when the subject was required to move as rapidly as possible from target to target. The EMG records have been aligned at the start of the biceps activity which preceded movement onset. A 50 msec torque pulse was applied at the time indicated by the arrows. The corresponding phase planes are shown to the right of each trace as well as the average phase plane derived from the non-perturbed control movements. EMG responses to the perturbation are evident in traces A, C and E and not in traces B and D. Correspondingly, the perturbation produced a more pronounced disruption of the movement trajectory (phase planes) in those trials where no reflex response to the perturbation was present. Note that the traces do not represent consecutive trials.
following the p e r t u r b a t i o n . I n the E M G records shown in Fig. 1B, a small reflex response to the p e r t u r b a t i o n is seen. As is i n d i c a t e d in Fig. IC, however, the reflex responses e v o k e d d u r i n g these r a p i d m o v e m e n t s were c o n s i d e r a b l y smaller t h a n those e v o k e d d u r i n g the slower m o v e m e n t s o f Fig. 1A. R e c o r d s from i n d i v i d u a l m o v e m e n t s when the subject was asked to move as r a p i d l y as possible i n d i c a t e d that reflex responses t o a p e r t u r b a t i o n o c c u r r e d o n l y in s o m e m o v e m e n t s (Fig. 3). I t should be noted t h a t in those trials in which a reflex response occurred (Fig. 3A, C and E) the t r a j e c t o r y following p e r t u r b a t i o n returned m o r e closely to the c o n t r o l t r a j e c t o r y than it did when the reflex responses were a b s e n t (Fig. 3B a n d D). The a b o v e o b s e r v a t i o n s suggest that the reflex responses to s u d d e n p e r t u r b a t i o n d u r i n g m o v e m e n t m a y be involved in re-establishing the l i m b ' s trajectory. This would. p r e s u m a b l y , be o f value in t h a t the limb w o u l d rapidly be r e t u r n e d to a state which c o u l d be predicted by the C N S on the basis o f p a s t experience a n d for on t h e basis o f k n o w l e d g e o f the limb m e c h a n i c a l properties. A s the trajectories o f such well-learned m o v e m e n t s m a y depend, in large p a r t , on the l i m b m e c h a n i c a l p r o p e r t i e s ~. k n o w l e d g e o f the limb mechanics w o u l d allow the C N S to p r e d i c t the state o f the l i m b t h r o u g h o u t the m o v e m e n t . S u p p o r t e d by the M e d i c a l Research Council o f C a n a d a ( G r a n t MA-6699).
497 1 Bizzi, E., Polit, A. and Morasso, P., Mechanisms underlying achievement of final head position, J. Neurophysiol., 39 (1976) 435-444. 2 Cooke, J. D., Dependence of human arm movements on limb mechanical properties, Brain Research, 165 (1979) 366-369. 3 Cooke, J. D. and Brown, S. H. C., 'Non-guided' limb movements in normal humans, Proc. Canad. Fed. Biol. Soc., 21 (1978) 90. 4 Desmedt, J. E. and Godaux, E., Ballistic skilled movements: load compensation and patterning of the motor commands. In J. E. Desmedt (Ed.), Cerebral Motor Control in Man: Long Loop Mechanisms, Progress in Clinical Neurophysiology, Vol. 4, Karger, Basel, 1978, pp. 21-55. 5 Fel'dman, A. G., Change in the length of the muscle as a consequence of a shift in equilibrium in the muscle-load system, Biofizika, 19 (1974) 534 538. 6 HaIlett, M., Shahani, B. T. and Young, R. R., E M G analysis of stereotyped voluntary movements in man, J. Neurol. Neurosurg. Psyehiat., 38 (1975) 1154-1162. 7 Hammond, P. H., The influence of prior instruction to the subject on an apparently involuntary neuro-muscular response, J. Physiol. (Lond.), 132 (1956) 17-18. 8 Houk, J. C., Regulation of stiffness by skeletomotor reflexes, Ann. Rev. Physiol., 41 (1979) 99-114. 9 Melvill Jones, G. and Watt, D. G. D., Observations on the control of stepping and hopping movements in man, J. Physiol. (Lond.), 219 (1971) 709 727. 10 Merton, P. A., Speculations on the Serro Control of Movement in the Spinal Cord, Little, Brown, Boston, 1953, pp. 183-198. 11 Merton, P. A., The properties of the human muscle servo, Brain Research, 71 (1974) 475-478. 12 Nichols, T. R. and Houk, J. C., Improvement in linearity and regulation of stiffness that results from actions of stretch reflex, J. NeurophysioL, 39 (1976) 119-142. 13 Polit, A. and Bizzi, E., Characteristics of motor programs underlying arm movements in monkeys, J. Nearophysiol., 42 (1979) 183 194. 14 Stetson, R. H. and McDill, J. A., Mechanism of the different types of movement, P.~:vchol. Monographs, 32 (1923) 18-45. 15 Wiesendanger, M., Comments on the problem of transcortical reflexes, J. Physiol. (Paris), 74 (1978) 325 330.
493
The role of stretch reflexes during active movements
J. DAVI-D COOKE
Department of Physiology, University of Western Ontario, London, Ont. (Canada) (Accepted September 13th, 1979)
Key words: stretch reflexes - - active movements - - limb models
H a m m o n d ' s 7 remarkable observation in 1956 that the E M G responses induced by sudden limb perturbation could be modified by the 'will' or 'intent' of the subject has led to questions concerning the function of the stretch reflexes. It has, for example, been suggested that the early, presumably segmentally mediated E M G response following a perturbation may not generate sufficient force to aid in overcoming the perturbation 9. This role was ascribed to the longer latency responses, and was therefore termed the 'functional stretch reflex'. Underlying this name was the idea that the 'function' of stretch-induced reflex muscle activation was to return the muscle to its original length or the limb to its original position. This view, which has recently been questioned8,12 sees the stretch reflex(es) as forming part of an automatic length servol0,11,15.
What, however, can be the function of the reflex responses to a sudden perturbation during an active movement? In this case the limb position at the time of the perturbation may not be the position which is 'desired'. In general, the stretch reflexes could not act as part of a simple length servo system as there is no muscle length (limb position) around which they could be acting. Described here are initial experiments which indicate that the stretch reflexes during active movement may be involved in returning the limb to the trajectory it would have taken had it not been perturbed. Experiments were performed on 4 normal human subjects. The subjects were seated comfortably and grasped a vertical rod attached to a manipulandum handle. Their arm was held horizontally and was supported just distal to the elbow. The manipulandum handle was pivoted at one end, the subject's elbow being positioned below the pivot point. Angular position and velocity of the handle were recorded by a precision potentiometer and a tachometer respectively. Subjects performed step-tracking movements following with the handle a target displayed as a vertical bar on an oscilloscope. Handle position was displayed as a vertical line. Target positions were not mechanically detectable and were not bounded by mechanical stops. Surface electromyograms (EMGs) were recorded from the biceps and triceps muscles. The E M G s were full-wave rectified and all data was digitized online using a PDP-1 l computer with an effective sampling rate of 500 Hz (1 K H z with 2 point block averaging).
494
A
B
/
f . . . . . . . 0
~L .
.
500
I
II
I
II I
O00msec
r
0
500
I000
msec
", 500
O00msec
C
il o
T 400
// 200
0
200
X
400 m s e c
Fig. t. Effect of brief torque perturbations on limb trajectories. A: data from movements made without (left-hand traces) and with (right-hand traces) application of a 50 msec torque pulse during the movements. The dashed vertical line in the right-hand trace marks the onset of the force pulse. The records shown (top to bottom) are angular position, angular velocity, biceps and triceps EMGs and phase planes. The phase planes plot limb velocity (ordinate) as a function of limb position (abscissa) and are generated from left to right during the evolution of the movement. Note that both perturbed and non-perturbed phase planes are over-plotted in the right-hand traces. Vertical calibration lines represent 50° for position and 60°/see for velocity. Calibration bars on the phase planes represent 15"~ and 50°/sec. S.D. are shown surrounding the position traces. B: biceps EMGs and phase planes derived from movements for which the subject moved as rapidly as possible between targets. Movement onset is indicated by the arrow below the EMG records and the time of application of a 50 msec torque pulse opposing the movement is indicated by the dashed vertical line. Note that following the perturbation the limb's trajectory is markedly different from the control trajectory. Calibration bars represent 15¢' and 50°/sec. C: shown is the difference in the integrated biceps EMGs of perturbed and non-perturbed movements. The open bars were derived from the slower movements (A) and the solidbars from the rapid movements (B). The vertical dashed line indicates the onset of the perturbation and the thin horizontal bars indicate the movement duration. The ordinate is in arbitrary units: positive values indicate that the integrated EMG in the perturbed movements was greater than that during the nonperturbed movements.
S h o w n in Fig. I A (left-hand traces) are records o f typical movements. Th e m o v e m e n t s were m a d e in one s m o o t h traverse f r o m target to target with little v ari at i o n f r o m m o v e m e n t to m o v e m e n t . T h e E M G
pattern associated with these
m o v e m e n t s is similar to that described by previous a u t h o r s a,4,~,14. P h ase p l a n e plots o f these m o v e m e n t s are also s h o w n in Fig. IA. These plots, o f a r m velocity as a function o f a r m position, have been described previously 2 and a r e sh o w n here as they are relatively responsive to small changes in m o v e m e n t trajectory. In Fig. I A (right-hand traces) are s h o w n records o f average m o v e m e n t s and phase planes with a brief
495 p e r t u r b a t i o n a p p l i e d to the a r m during the movements. P e r t u r b a t i o n s were applied r a n d o m l y during a series o f m o v e m e n t s and were o f the same m a g n i t u d e during any one series. The p e r t u r b a t i o n elicited discrete E M G responses which, on the average, had onset latencies o f 30 and 85 msec. It should particularly be n o t e d in Fig. 1A that the l i m b ' s trajectory quickly r e t u r n e d to the control or n o n - p e r t u r b e d trajectory following the p e r t u r b a t i o n 2. As is seen in Fig. 2A and B, this return to the control trajectory did not occur in an a n a l o g model o f the limb in which the limb was considered as a simple d a m p e d spring with mass. This model has been described previously 2 and is related to the m o d e l describing m a i n t e n a n c e o f fixed limb or head positions p r o p o s e d by other investigatorsl, ~,13. M o v e m e n t s in the m o d e l were p r o d u c e d by step changes in the resting spring constant. The earlier studies with this model suggested that it p r o v i d e d an a d e q u a t e qualitative description o f a r m trajectory during step-tracking m o v e m e n t by humans. One obvious difference between the model and the h u m a n subjects was the occurrence o f reflex responses to the p e r t u r b a t i o n . The possibility was thus considered that the r a p i d return to the original trajectory seen in the h u m a n subjects was p r o d u c e d by the reflex responses to the p e r t u r b a t i o n . Two tests o f this possibility were made. In the first, a 'pseudo-reflex' was a d d e d into the model. T h a t is, at a fixed delay following the p e r t u r b i n g force, a second force was applied with a direction o p p o s i t e to the p e r t u r b i n g force. As can be seen in Fig. 2 C - F , the trajectory following perturbation could be made to return to the original trajectory by suitable m a n i p u l a t i o n o f the m a g n i t u d e o f this 'pseudo-reflex'. The second test was to study the trajectories o f rapid arm movements in h u m a n s in which p e r t u r b a t i o n s p r o d u c e no, or at least greatly reduced, reflex responses 4. D a t a from such m o v e m e n t s are shown in Fig. 1B. F o r these trials the subject was requested to move 'as quickly as possible' between the target zones. As can be seen from the phase planes from the movements, a p p l i c a t i o n o f a p e r t u r b a t i o n p r o d u c e d a m a j o r change in the m o v e m e n t trajectory which did not return to the control trajectory A
B
C
pos~
Fig. 2. Phase plane trajectories derived from an analog model of the limb. A : a control trajectory of a movement generated in the model by a step change in the resting spring stiffness. This trajectory is also shown in B as is the trajectory of a movement in which a brief (50 msec) perturbation was applied just after the start of the movement. The perturbation, which opposed the movement, decreased the movement velocity and displaced the phase plane trajectory below the control trajectory throughout its course. The dashed curves in C-F show the effect on the trajectory of a reflex response to the perturbation. This reflex was modelled as a force initiated 50 msec following the perturbation and assisting the movement. The magnitude of the reflex was progressively increased from C to F with its duration constant at 50 msec. Note that by appropriate adjustment of the magnitude of the reflex force the trajectory following the perturbation can be made to match the control trajectory (D).
496
'
t
C
t 0
2(X) ' 4(~0 ' 6(2(] t~sec
40deg
Fig. 3. Effect of brief torque perturbations during 'fast' movements. In A-E are shown (left-hand traces) biceps EMGs from individual flexion movements made when the subject was required to move as rapidly as possible from target to target. The EMG records have been aligned at the start of the biceps activity which preceded movement onset. A 50 msec torque pulse was applied at the time indicated by the arrows. The corresponding phase planes are shown to the right of each trace as well as the average phase plane derived from the non-perturbed control movements. EMG responses to the perturbation are evident in traces A, C and E and not in traces B and D. Correspondingly, the perturbation produced a more pronounced disruption of the movement trajectory (phase planes) in those trials where no reflex response to the perturbation was present. Note that the traces do not represent consecutive trials.
following the p e r t u r b a t i o n . I n the E M G records shown in Fig. 1B, a small reflex response to the p e r t u r b a t i o n is seen. As is i n d i c a t e d in Fig. IC, however, the reflex responses e v o k e d d u r i n g these r a p i d m o v e m e n t s were c o n s i d e r a b l y smaller t h a n those e v o k e d d u r i n g the slower m o v e m e n t s o f Fig. 1A. R e c o r d s from i n d i v i d u a l m o v e m e n t s when the subject was asked to move as r a p i d l y as possible i n d i c a t e d that reflex responses t o a p e r t u r b a t i o n o c c u r r e d o n l y in s o m e m o v e m e n t s (Fig. 3). I t should be noted t h a t in those trials in which a reflex response occurred (Fig. 3A, C and E) the t r a j e c t o r y following p e r t u r b a t i o n returned m o r e closely to the c o n t r o l t r a j e c t o r y than it did when the reflex responses were a b s e n t (Fig. 3B a n d D). The a b o v e o b s e r v a t i o n s suggest that the reflex responses to s u d d e n p e r t u r b a t i o n d u r i n g m o v e m e n t m a y be involved in re-establishing the l i m b ' s trajectory. This would. p r e s u m a b l y , be o f value in t h a t the limb w o u l d rapidly be r e t u r n e d to a state which c o u l d be predicted by the C N S on the basis o f p a s t experience a n d for on t h e basis o f k n o w l e d g e o f the limb m e c h a n i c a l properties. A s the trajectories o f such well-learned m o v e m e n t s m a y depend, in large p a r t , on the l i m b m e c h a n i c a l p r o p e r t i e s ~. k n o w l e d g e o f the limb mechanics w o u l d allow the C N S to p r e d i c t the state o f the l i m b t h r o u g h o u t the m o v e m e n t . S u p p o r t e d by the M e d i c a l Research Council o f C a n a d a ( G r a n t MA-6699).
497 1 Bizzi, E., Polit, A. and Morasso, P., Mechanisms underlying achievement of final head position, J. Neurophysiol., 39 (1976) 435-444. 2 Cooke, J. D., Dependence of human arm movements on limb mechanical properties, Brain Research, 165 (1979) 366-369. 3 Cooke, J. D. and Brown, S. H. C., 'Non-guided' limb movements in normal humans, Proc. Canad. Fed. Biol. Soc., 21 (1978) 90. 4 Desmedt, J. E. and Godaux, E., Ballistic skilled movements: load compensation and patterning of the motor commands. In J. E. Desmedt (Ed.), Cerebral Motor Control in Man: Long Loop Mechanisms, Progress in Clinical Neurophysiology, Vol. 4, Karger, Basel, 1978, pp. 21-55. 5 Fel'dman, A. G., Change in the length of the muscle as a consequence of a shift in equilibrium in the muscle-load system, Biofizika, 19 (1974) 534 538. 6 HaIlett, M., Shahani, B. T. and Young, R. R., E M G analysis of stereotyped voluntary movements in man, J. Neurol. Neurosurg. Psyehiat., 38 (1975) 1154-1162. 7 Hammond, P. H., The influence of prior instruction to the subject on an apparently involuntary neuro-muscular response, J. Physiol. (Lond.), 132 (1956) 17-18. 8 Houk, J. C., Regulation of stiffness by skeletomotor reflexes, Ann. Rev. Physiol., 41 (1979) 99-114. 9 Melvill Jones, G. and Watt, D. G. D., Observations on the control of stepping and hopping movements in man, J. Physiol. (Lond.), 219 (1971) 709 727. 10 Merton, P. A., Speculations on the Serro Control of Movement in the Spinal Cord, Little, Brown, Boston, 1953, pp. 183-198. 11 Merton, P. A., The properties of the human muscle servo, Brain Research, 71 (1974) 475-478. 12 Nichols, T. R. and Houk, J. C., Improvement in linearity and regulation of stiffness that results from actions of stretch reflex, J. NeurophysioL, 39 (1976) 119-142. 13 Polit, A. and Bizzi, E., Characteristics of motor programs underlying arm movements in monkeys, J. Nearophysiol., 42 (1979) 183 194. 14 Stetson, R. H. and McDill, J. A., Mechanism of the different types of movement, P.~:vchol. Monographs, 32 (1923) 18-45. 15 Wiesendanger, M., Comments on the problem of transcortical reflexes, J. Physiol. (Paris), 74 (1978) 325 330.