MOTOR UNIT FIRING BEHAVIOR IN SLOW AND FAST CONTRACTIONS OF THE FIRSTDORSAL INTEROSSEOUS MUSCLE OF HEALTHY-MEN

The motor unit recruitment threshold and firing rate were evaluated du ring slow and fast contraction of the first dorsal interosseous (FDI) muscle by healthy young men. Using a special quadrifilar electrode myo electric activity was recorded during voluntary isometric contraction. Motor unit action potentials (MUAPs) were decomposed into individual MUAP trains by the electromyography (EMG) signal decomposition techniq ue. Recruitment thresholds of the motor units decreased with the incre ase in the speed of contraction, and there was no recruitment reversal despite the increase. In terms of rate coding, the firing rates of th e motor units increased as the speed of contraction increased; however , a high threshold motor unit always had a lower firing rate than a lo w threshold motor unit regardless of the contraction speed. At all con traction speeds, recruitment and rate coding may act through the same mechanism. If excitation of the motoneuron pool occurs rather than exc itation of an individual motoneuron, a low threshold motor unit is eas ier to recruit and fire repetitively than a high threshold one. The mo tor unit firing behavior during fast contraction basically may be the same as during slow contraction.