The aim of this study was to compare the mechanical and electromyographic (EMG) characteristics of soleus motor units activated during maximal H reflex 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 counterparts 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 wave amplitude were significantly lower in power-trained than endurance-trained athletes. However, power-trained athletes exhibited a significantly greater twitch-to-EMG ratio for the reflexly activated motor units with respect to the entire motor pool, whereas endurance-trained athletes had comparable twitch-to-EMG ratios for both reflexly and directly activated units. Power training increases the force output of the whole ensemble of the motor units, thereby compensating for the lower efficacy of the reflex transmission between Ia spindle afferent input and soleus alpha-motoneuron. On the other 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 force by recruitment of more slow-twitch units than do other subjects for comparable levels of force and type of task.

Neuromuscular adaptations of the plantar flexor muscles were assessed before and subsequent to short-term electromyostimulation (EMS) training. Eight subjects underwent 16 sessions of isometric EMS training over 4 wk. Surface electromyographic (EMG) activity and torque obtained under maximal voluntary and electrically evoked contractions were analyzed to distinguish neural adaptations from contractile changes. After training, plantar flexor voluntary torque significantly increased under isometric conditions at the training angle (+8.1%, P < 0.05) and at the two eccentric velocities considered (+10.8 and +13.1%, P < 0.05). Torque gains were accompanied by higher normalized soleus EMG activity and, in the case of eccentric contractions, also by higher gastrocnemii EMG (P < 0.05). There was an 11.9% significant increase in both plantar flexor maximal voluntary activation (P < 0.01) and postactivation potentiation (P < 0.05), whereas contractile properties did not change after training. In the absence of a change in the control group, it was concluded that an increase in neural activation likely mediates the voluntary torque gains observed after short-term EMS training.

This study was designed to investigate the effect of ageing on the mechanical and electromyographic (EMG) characteristics of the soleus motor units (MUs) activated by the maximal Hoffmann reflex (Hmax) and by the direct muscle compound action potential (Mmax). Eleven young (mean age 25 +/- 4 years) and ten elderly (mean age 73 +/- 5 years) males took part in this investigation. The senior group presented lower amplitudes of Mmax (-57 %, P < 0.001) and Hmax (-68 %, P < 0.001) waves compared to the younger population. These were associated with a depression of relative twitch torque of the plantar flexors. The average values of the Hmax/Mmax ratio did not statistically differ between the two populations, despite a tendency for lower values (~23 %) in the senior group. However, the older adults showed a greater relative amplitude of the sub-maximal M wave evoked at Hmax (MatHmax) than did the younger males (young 5 % vs. elderly 29 % of the Mmax, P < 0.01). This finding suggests an increased homogeneity between the excitability threshold of sensory and motor axons. The twitch torque at Hmax (PtH-M) was subsequently calculated by subtraction from the total twitch torque of the mechanical contamination associated with MatHmax. The resulting PtH-M was significantly lower in the elderly (-59 %, P < 0.001). Despite a discrepancy of 20 % between the two groups, the mechanical ratio (PtH-M/PtM; PtM, twitch tension related to the Mmax compound action potential), like the EMG ratio, did not statistically differ between the young and older individuals. Nevertheless, the senior subjects exhibited a higher twitch/EMG ratio for the reflexively activated MUs (PtH-M/Hmax) than the younger individuals (+40 %, P < 0.05). This finding suggests an on-going neuromuscular remodelling, resulting in an increased innervation ratio. The neural rearrangement may be viewed as a compensatory adaptation of the motor system to preserve the mechanical efficiency of the surviving MUs, despite the age-related impairment of the segmental reflex system. This phenomenon is confirmed by the maintenance, with senescence, of the approximately constant values of the twitch/EMG ratio for the entire motor pool (PtM/Mmax).

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