Fatigue-related modulation of low-frequency common drive to motor units

Purpose This study investigated fatigue-related modulation of common neural inputs to motor units (MUs) under 5 Hz, which determines force precision control. Methods Twenty-seven adults performed a sequence of fatiguing contractions. The participants were assessed with a static isometric index abduction at 20% maximal voluntary contraction in the pre-test and post-test. Discharge characteristics of MUs of the first dorsal interosseous muscle were analyzed with decomposed EMG signals. Results Along with increases in the mean (58.40 +/- 11.76 ms -> 62.55 +/- 10.83 ms, P = 0.029) and coefficient of variation (0.204 +/- .014 -> 0.215 +/- 0.017, P = 0.002) in inter-spike intervals, the fatiguing contraction caused reductions in the mean frequency (16.84 +/- 3.31 Hz -> 15.59 +/- 3.21 Hz, P = 0.027) and spectral dispersions (67.54 +/- 4.49 -> 62.64 +/- 6.76 Hz, P = 0.007) of common neural drive, as estimated with smoothed cumulative motor unit spike trains (SCMUSTs). Stabilogram diffusion analysis of SCMUSTs revealed significant fatigue-related reductions in the long-term effective diffusion coefficient (1.91 +/- 0.77 Hz(2)/s -> 1.61 +/- 0.61 Hz(2)/s, P = 0.020) and long-term scaling exponent (0.480 +/- 0.013 Hz(2)/s -> 0.471 +/- 0.017 Hz(2)/s, P = 0.014). After fatiguing contraction, mutual information of force fluctuations and SCMUSTs was augmented roughly by 12.95% (P = 0.041). Conclusions Muscular fatigue could compress and shift the low-frequency common drive to MUs toward lower spectral bands, thereby enhancing transmission of twitch forces through the muscle-tendon complex with a low-pass filter property. The fatigue-induced changes involve increased closed-loop control of the common modulation of MU discharge rates.