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1. The presence of a form of stretch reflex, previously described in the arm by other authors, has been confirmed in the gastrocnemius muscle of the human leg. The electromyographic (e.m.g.) manifestation of this reflex occurred 120 msec (S.E. of mean = 3·5 msec) following a sharply applied, and maintained, dorsiflexing force to the foot. This form of response is referred to in this article as the Functional Stretch Reflex (FSR).
2. To determine the contribution of the FSR to the control of normal leg movement, the e.m.g. activity in the above muscle was monitored during single downward steps of 12·7, 25·4 and 38·1 cm and during repetitive, rhythmic, hopping movements on one foot.
3. It was found that e.m.g. activity associated with steps to the ground began 141 msec (S.E. of mean = 8·5 msec) before contact with the ground and ended 131 msec (S.E. of mean = 7·6 msec) after contact, when the e.m.g. usually became temporarily inactive.
4. It is inferred from these results that the muscular deceleration associated with landing was brought about by the release of a pre-programmed pattern of neuromuscular activity which was inaccessible to reflex activity resulting from the mechanical event of landing, rather than by a stretch reflex.
5. It was found that subjects chose their preferred frequency of hopping with great accuracy and consistency. The mean value obtained was 2·06 Hz (S.E. of mean = 0·02 Hz).
6. At the preferred frequency, e.m.g. activity began 84 msec (S.E. of mean = 9·6 msec) before and terminated 263 msec (S.E. of mean = 10 msec) after contact with the ground.
7. It is inferred that in rhythmical hopping and perhaps also in running, each landing is effected, as in single steps, by a predetermined pattern of neuromuscular activity. However, when hopping at the preferred frequency, the take-off phase of muscular activity is timed to make maximal use of the FSR, i.e. between 120 and 260 msec after initial contact.
8. The results emphasize the importance of pre-programming complex muscular contractions suitable for opposing sudden passive stretching forces, and of initiating them prior to the onset of these forces.
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