'Muscular wisdom' revisited: Decaying rates of stimulation mitigate torque loss.

Abstract

During a sustained high-intensity isometric maximal voluntary contraction (MVC), declining motor unit firing rates (MUFRs) accompany torque loss. This decline (∼50% over 60 s) helps to maintain torque by preserving peripheral electrical propagation and matching the slowing contractile properties with torque loss (i.e., 'muscular wisdom'). However, it has been suggested that reduced MUFRs contribute to torque loss. Here, we compared torque loss between constant and decaying rates of electrical stimulation to mimic MUFRs reported during MVCs. The dorsiflexors of 8 males and 5 females (21-30 years) underwent three 60 s muscle fatiguing conditions: (1) sustained MVC; (2) constant high-frequency electrical stimulation (40 Hz); and (3) exponentially decaying stimulation rate (from 40 to 20 Hz). The decaying rate demonstrated less torque loss compared with the sustained high-frequency stimulation and the MVC conditions (P < 0.01). Furthermore, torque increased (by ∼17%, P < 0.005) when the constant high-frequency condition was switched to 20 Hz for 2 s at task termination. Conversely, torque loss was accelerated when the decaying stimulation rate was switched from 20 to 40 Hz for 2 s at task termination (by ∼16%, P < 0.001). Following all conditions, evoked twitch responses slowed (by 29%-77%, P < 0.01) but M-wave amplitude was reduced only for the constant high-frequency condition (by ∼23%, P < 0.01). Thus, the reduction in stimulation rates maintained optimal activation by matching the fatigue-induced contractile slowing in combination with preserved peripheral electrical conductance. Therefore, reducing the activation rate preserves torque, rather than contributing to torque loss during high-intensity contractions, thereby supporting the muscle wisdom hypothesis.