Intermittent low-intensity contractions cause a dissociation between maximal motor unit firing rates and changes in torque and contractile speed.

The purpose was to test: (1) whether the decline in maximal motor unit (MU) firing rate is an invariable response when maximal voluntary contraction (MVC) torque is reduced following a submaximal muscle fatiguing task; and (2) whether fatigue-induced slowing of contractile speed and firing rate decline are co-dependent. Ten participants (three females) underwent a protocol (duty cycle, 0.6 s) of knee extensions at 30% MVC for 30 min, which has been shown previously to reduce MVC torque with negligible muscle metabolic disturbances. Electrically evoked quadriceps torque (1, 50 and 100 Hz), MVC torque and maximal firing rates were assessed intermittently throughout the protocol. Intramuscular microelectrodes were inserted into the vastus lateralis and medialis to record MU firing rates (n = 1557). By 30 min, MVC and electrically evoked quadriceps torques declined (30%-50%, all P ≤ 0.02); however, measures of contractile speed increased (∼30%, all P < 0.001). Grouped mean maximal firing rates were not significantly changed (24.1 ± 8.9 Hz) from pre-task (24.7 ± 9.2 Hz, P = 0.16). Therefore, reductions in maximal torque generation, contractile slowing and decreased maximal MU firing rates are not invariable correlates of a fatiguing task. Instead, reduced MVC torque can be dissociated from the usual slowing of maximal firing rates and contractile speed when fatigue is induced with, presumably, negligible metabolic disturbances. The constancy of maximal firing rates despite large reductions in MVC torque supports that firing rate moderation with fatiguing activation is probably attributable to reflexive inhibition from muscle metabolic alterations and thus not directly related to changes in contractile speed.