Stretch and Hoffmann reflexes during phasic voluntary contractions of the human soleus muscle

Measurements were made of EMG responses in human soleus muscle, following torque disturbances of the foot during phasic voluntary plantarflexions. These were compared with measurements of the Hoffmann reflex during identical voluntary movements. We observe that stretch adequate to evoke a stretch reflex in the relaxed limb often fails to evoke any response during a phasic contraction until about 120 msec after the onset of the stimulus. During this period the H-reflex is facilitated. These findings are discussed in terms of the servo-theory of stretch reflex function and the concept of trans-cortical stretch reflexes.