Y. Masakado et al., MOTOR UNIT FIRING BEHAVIOR IN SLOW AND FAST CONTRACTIONS OF THE FIRSTDORSAL INTEROSSEOUS MUSCLE OF HEALTHY-MEN, ELECTROMYOGRAPHY AND MOTOR CONTROL-ELECTROENCEPHALOGRAPHY AND CLINICAL NEUROPHYSIOLOGY, 97(6), 1995, pp. 290-295
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.