The force level of a voluntary muscle contraction is controlled through two mechanisms: recruitment and firing rate modulation of motor units. According to the common drive theory, the central nervous system regulates the net excitation to the motor units, while their hierarchical organization and their intrinsic properties result in a highly ordered recruitment and firing rate response. The basis of this theory comes from studies of short duration isometric contractions. In this study we investigated whether the control properties remained invariant during fatiguing contractions.; Five young (21.4 ± 0.9 years) volunteers performed a series of isometric knee extensions until exhaustion. Electromyographic signals were recorded from the vastus lateralis muscle via a quadrifilar fine wire electrode and subsequently decomposed into the individual motor unit action potentials. Additionally, whole-muscle mechanical properties were measured during the fatigue protocol using electrical stimulation. A monotonic decrease in the recruitment threshold of all motor units and a progressive recruitment of new units, all without a change of the recruitment order, was observed. The decrease in recruitment threshold was linearly related to the decline in the elicited torque response of the muscle. The firing rate hierarchy, characterized by an inverse relationship between motor unit firing rate and recruitment threshold, was maintained throughout the contraction series. In all but one subject, the firing rate first decreased and then increased with time. In a complimentary fashion, the elicited torque response of the muscle first increased and then decreased.; A computer model of motor unit firing and force output was constructed to simulate the firing patterns that are required to maintain a constant muscle output when the force capacity of the muscle is changing. Results were in agreement with the experimentally observed motor unit firing behavior.; The common firing rate and recruitment adaptations complemented the mechanical changes of the muscle which suggests that the firing behavior of motor units adapts to counteract the change in the force capacity of the muscle. The observed firing patterns are consistent with a compensatory regulation via the common drive to all motor units. We therefore conclude that motor unit control remains invariant during fatigue.
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