The importance of the duration of contraction on the excitability of the motoneuronal pool was evaluated by measuring the H reflex from soleus muscle after short and long contractions at constant effort equal to 60% maximal voluntary contraction (MVC); the difference between the amplitude of H reflex at rest and after contraction of different duration was evaluated in 10 normal subjects and in one hemiparetic patient. The authors found a significant inhibition of H reflex after contraction of 10 seconds and after 40 seconds and a longer duration of the inhibition after 40 seconds. In the hemiparetic patient the H reflex did not show inhibition which suggests a possible reduction of the presynaptic inhibition, and a lack of modulation of the motoneuronal pool after contraction.