The purpose of the study was to assess the influence of short, intermediate, and long muscle lengths on dorsiflexor force steadiness and the discharge characteristics of motor units in tibialis anterior during submaximal isometric contractions. Steady contractions were performed at 5 target forces (5, 10, 20, 40, and 60% maximal voluntary contraction, MVC) for 3 ankle angles (75 °, 90 °, and 105°). MVC force was less (p=0.043) at the smallest joint angle compared with the other two angles. The absolute (standard deviation) and normalized amplitudes (coefficient of variation) of the force fluctuations were similar for all 3 ankle angles at each target force. The coefficient of variation for force decreased progressively from 5% to 20% MVC force and then it plateaued at 40% and 60% MVC force. At all target forces, the mean discharge rate (MDR) of the motor units at 75o was greater than at 90o (p=0.006) and 105o (p=0.034). Moreover, the MDR was similar for 5% and 10% MVC forces and then increased gradually until 60% MVC force (p<0.005). The variability in discharge times (coefficient of variation for interspike interval) and variability in neural drive (coefficient of variation of filtered cumulative spike train) were similar at all ankle angles. Variability in neural drive had a greater influence on force steadiness than did the variability in discharge times. Changes in ankle-joint angle did not influence either the normalized amplitude force fluctuations during steady submaximal contractions or the underlying modulation of the discharge characteristics of motor units in tibialis anterior.
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