Electrical and mechanical H-max-to-M-max ratio in power- and endurance-trained athletes

The aim of this study was to compare the mechanical and electromyographic ( EMG) characteristics of soleus motor units activated during maximal H refle x and direct M response among subjects with different histories of physical activity. Power-trained athletes produced stronger twitches, with a higher rate of twitch tension buildup and relaxation, than their endurance counte rparts for both maximal H-reflex and maximal M-wave responses. The maximal H-reflex-to-maximal M-wave ratios for both force output (twitch) and EMG wa ve amplitude were significantly lower in power-trained than endurance-train ed athletes. However, power-trained athletes exhibited a significantly grea ter twitch-to-EMG ratio for the reflexly activated motor units with respect to the entire motor pool, whereas endurance-trained athletes had comparabl e twitch-to-EMG ratios for both reflexly and directly activated units. Powe r training increases the force output of the whole ensemble of the motor un its, thereby compensating for the lower efficacy of the reflex transmission between Ia spindle afferent input and soleus alpha -motoneuron. On the oth er hand, the lower level of force evoked by the reflexly activated units in endurance-trained athletes is associated with a greater motor pool reflex excitability. Therefore, endurance-trained athletes produce the necessary f orce by recruitment of more slow-twitch units than do other subjects for co mparable levels of force and type of task.