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1 Laboratory of Applied Biology, Universite Libre de Bruxelles, Brussels, Belgium
* To whom correspondence should be addressed. E-mail: jduchat{at}ulb.ac.be.
The influence of repetitive dynamic fatiguing contractions on the neuromuscular characteristics of the human triceps surae was investigated in 10 subjects. The load was 50% of the torque produced during a maximal voluntary contraction (MVC) and the exercise ended when the ankle range of motion (ROM) declined to 50% of control. The maximal torque of the triceps surae and the electromyographic (EMG) activities of the soleus (Sol) and medial gastrocnemius (MG) were studied in response to voluntary and electrically-induced contractions before and after the fatiguing task and following 5 min of recovery. Reflex activities were also tested by recording the Hoffmann reflex (H reflex) and tendon reflex (T reflex) in the Sol muscle. The results indicated that while the MVC torque, tested in isometric conditions, was reduced to a greater extent (P<0.05) at 20°of plantar flexion (-33%) compared with the neutral position (-23%) of the ankle joint, the EMG activity of both muscles was not significantly reduced after fatigue. Muscle activation, tested by the interpolated-twitch method or the ratio of the voluntary EMG to the amplitude of the muscle action potential (M-wave), as well as the neuromuscular transmission and sarcolemmal excitation, tested by the M-wave amplitude, did not change significantly after the fatiguing exercise. Although the H and T reflexes declined slightly (10-13%; P<0.05) after fatigue, these adjustments did not appear to have a direct deleterious effect on muscle activation. In contrast, alterations in the mechanical twitch time course and post-activation potentiation (PAP) indicated that intracellular Ca2+- controlled excitation-contraction (E-C) coupling processes most likely played a major role in the force decrease after dynamic fatiguing contractions performed for short duration.
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