Surface EMG and Achilles tendon reflexes during a foot movement in a reaction time task.

A forewarned simple visual reaction time (RT) experiment was conducted to study the relation between surface EMG of the soleus muscle and Achilles tendon reflex amplitudes during the initiation of a voluntary plantar flexion of the right foot. In two conditions the tension of the response-involved soleus muscle was manipulated: (1) gradual isometric contraction towards the end of the foreperiod (FP) and (2) complete relaxation during the FP. Gradual contraction of the involved soleus muscle produced faster RTs. It was found in both conditions that reflex amplitudes and EMG of the involved soleus muscle were equally informative about the growing motor excitation during the execution of the response. In the tension condition reflex amplitudes in the involved leg were smaller than the reflex amplitudes in the non-involved leg prior to the initiation of the movement, 0-50 msec after the expiration of the FP. The EMG, however, showed a reversed involved/non-involved difference in this condition. In the relaxation condition, neither the EMG nor the reflex amplitudes showed a significant involved/non-involved difference. It was concluded from these results that changes in Achilles tendon reflex amplitudes should only cautiously be interpreted as an indication of motor preparation.