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SUMMARY1. A cat preparation was used to study the modulation of stretch reflexes during locomotion. The brain stem was transacted and locomotion was induced by electrical stimulation of the mesencephalic locomotor region below the level of transaction. Three legs walked normally on a treadmill, while the fourth leg, which was denervated except for the soleus muscle, was held fixed. Brief length perturbations were applied to the soleus muscle at various phases of the stepping cycle.2. The stretch reflex in this muscle was deeply modulated during the step cycle, and reached its peak at or before the peak in soleus e.m.g. activity associated with the locomotion. A similar variation was observed when the sciatic nerve was stimulated electrically at a strength which elicited reflex activity (H wave), but did not directly elicit motor nerve activity (M wave). Variation in the reflex during electrical stimulation could not be accounted for by cyclic variation in fusimotor activity or in the afferent volley, but must be due to post-synaptic changes in a-motoneurones or in the presynaptic inputs to them.3. Large changes were also observed in intrinsic muscle stiffness during the step cycle. The maximum stiffness occurred near the time the limb would normally strike the ground during locomotion. A high stiffness would be useful in reducing the amount that the limb would yield under the weight of the body during the extension phase of the step cycle. The variation of the stretch reflex in parallel with stiffness suggests that reflexes could assist in this load compensation. The variation is not consistent with the idea that the stretch reflex is used to compensate for changes in intrinsic muscle properties, so that the total system behaves more like a spring of constant stiffness than does muscle alone.

SummaryThe load-velocity relations were determined at various contractile forces in the small bundle dissected from frog semitendinosus muscle.

Voluntary contraction of the skeletal muscles is controlled by two mechanisms : One changes the number of active motor units and the other changes the firing rate of individual motor units. It has been generally believed that the former is the main factor in coarse control and the latter in fine control. Recently, MILNER-BROWN et al. (1973 a, b) investigated quantitatively the relation between motor unit activity and voluntary isometric force in the human first dorsal interosseus muscle and concluded that the change in the firing rate of motor units ("rate coding") plays a major part in the control of muscle force. Similar results were