Modulation in voluntary neural drive in relation to muscle soreness

The aim of this study was to investigate whether (1) spinal modulation would change after non-exhausting eccentric exercise of the plantar flexor muscles that produced muscle soreness and (2) central modulation of the motor command would be linked to the development of muscle soreness. Ten healthy subjects volunteered to perform a single bout of backward downhill walking exercise (duration 30 min, velocity 1 ms−1, negative grade −25%, load 12% of body weight). Neuromuscular test sessions [H-reflex, M-wave, maximal voluntary torque (MVT)] were performed before, immediately after, as well as 1–3 days after the exercise bout. Immediately after exercise there was a −15% decrease in MVT of the plantar flexors partly attributable to an alteration in contractile properties (−23% in electrically evoked mechanical twitch). However, MVT failed to recover before the third day whereas the contractile properties had significantly recovered within the first day. This delayed recovery of MVT was likely related to a decrement in voluntary muscle drive. The decrease in voluntary activation occurred in the absence of any variation in spinal modulation estimated from the H-reflex. Our findings suggest the development of a supraspinal modulation perhaps linked to the presence of muscle soreness

Low-frequency common modulation of soleus motor unit discharge is enhanced during postural control in humans

The maintenance of quiet stance requires the activation of muscles bilaterally. The soleus muscles in each leg share a common function in standing; that is, each muscle acts to control antero-posterior (AP) sway on its own side. We sought to determine the extent to which oscillations in motor unit discharge were related in motor unit pairs of the soleus muscles during postural and voluntary isometric tasks, both within and between legs. Subjects stood quietly for 5 min or performed a voluntary isometric plantarflexion contraction in a seated position. During the postural tasks, the excursions of AP sway between legs were highly correlated (ρ = 0.86 ± 0.06). The strength of common modulation of motor unit discharge rates was assessed using time- and frequency-domain analyses. The time-domain common drive analysis revealed that the strongest correlation in motor unit discharge modulation occurred in the postural task with unilateral pairs (ρ = 0.71 ± 0.13) being more strongly correlated than bilateral pairs (ρ = 0.50 ± 0.16). Common modulation of motor unit discharge was lowest for the voluntary tasks, with ρ = 0.38 ± 0.11 and 0.16 ± 0.08 for unilateral and bilateral pairs, respectively. Similarly, the frequency-domain coherence analysis demonstrated an identical ordering effect, with the largest maximum pooled coherence occurring during standing posture in unilateral (0.070 at 1.6 Hz) and bilateral (0.055 at 1.6 Hz) recordings, whereas minimal coherence was observed in the voluntary task in both unilateral and bilateral recordings within the 0–5 Hz range. These results indicate that in the soleus muscle, common modulation of motor unit discharge is greater during postural tasks than during voluntary isometric tasks and can be observed in both bilateral and unilateral motor unit pairs. Differences in the extent of co-modulation of motor unit discharge between tasks may be attributed to either differences in the descending control or differences in the proprioceptive input between postural and isometric tasks.G. Mochizuki, J. G. Semmler, T. D. Ivanova and S. J. Garlan

A motor unit-based model of muscle fatigue

Muscle fatigue is a temporary decline in the force and power capacity of skeletal muscle resulting from muscle activity. Because control of muscle is realized at the level of the motor unit (MU), it seems important to consider the physiological properties of motor units when attempting to understand and predict muscle fatigue. Therefore, we developed a phenomenological model of motor unit fatigue as a tractable means to predict muscle fatigue for a variety of tasks and to illustrate the individual contractile responses of MUs whose collective action determines the trajectory of changes in muscle force capacity during prolonged activity. An existing MU population model was used to simulate MU firing rates and isometric muscle forces and, to that model, we added fatigue-related changes in MU force, contraction time, and firing rate associated with sustained voluntary contractions. The model accurately estimated endurance times for sustained isometric contractions across a wide range of target levels. In addition, simulations were run for situations that have little experimental precedent to demonstrate the potential utility of the model to predict motor unit fatigue for more complicated, real-world applications. Moreover, the model provided insight into the complex orchestration of MU force contributions during fatigue, that would be unattainable with current experimental approaches