The mechanical effectiveness of short latency reflexes in human triceps surae muscles revealed by ischaemia and vibration.

The resistance to stretch provided by short latency (SL) reflexes in human triceps surae muscles was investigated under three experimental conditions: control, ischaemia, and with 100 Hz vibration applied to the Achilles tendon. Incremental changes in plantar flexion force always showed a strong initial resistance followed by yielding in response to rapid dorsiflexion of the foot about the ankle joint. These changes were attributed to inherent stiffness of the triceps surae muscles. The force curves for each experimental condition diverged during the yield phase some 20 ms after the onset of SL EMG reflexes. During ischaemia, SL EMG reflexes were reduced to 8% of control values and yielding continued until the onset of medium latency EMG activity whereas the yielding was interrupted by SL action in the control situation. The difference between the ischaemia and control force curves was attributed to force recruited by SL reflexes under normal stretch conditions. Vibration reduced the SL EMG reflex amplitude to 20% of control values and produced with it a reduced force response.