SUMMARY

1. Ankle joints were subjected to sinusoidal movements at a range of amplitudes and frequencies. Records were made ofelectromyograms (e.m.g.s) in calf muscles, and of the forces at the joints. 2. When the leg is relaxed, the ankle joint resists an imposed sinusoidal movement with a small approximately sinusoidal force. It is stiffer in its resistance to small movements than to large ones, and this resistance is greater when the joint is dorsiflexed than when it is plantarflexed. 3. If the subject exerts a steady mean flexing force, the imposed sinusoidal movement generates reflex activity which may be recorded as a modulation of the gastrocnemius and soleus e.m.g.s. 4. The e.m.g. response to the sinusoidal movement occurs later in cycles of movement at high than at low frequencies, as one could expect of a reflex pathway that involves a delay. The results suggest that this delay is between 50 and 60 ms, and we conclude that under these circumstances spinal stretch reflexes are playing the important part