Discharge characteristics of motor units during long‐duration contractions

Abstract

New Findings r What is the central question of this study?How long can humans sustain motor unit discharge during a voluntary contraction? r What is the main finding and its importance?Human motor units can discharge action potentials for a longer time during a voluntary contraction than can be achieved by intracellular or extracellular current injection of motor neurons with in vivo and in vitro animal preparations. These recordings comprise a previously unexplored range of durations for human motor unit recordings during a continuously sustained isometric recording.The purpose of the study was to determine how long humans could sustain the discharge of single motor units during a voluntary contraction. The discharge of motor units in first dorsal interosseus of subjects (27.8 ± 8.1 years old) was recorded for as long as possible. The task was terminated when the isolated motor unit stopped discharging action potentials, despite the ability of the individual to sustain the abduction force. Twenty-three single motor units were recorded. Task duration was 21.4 ± 17.8 min. When analysed across discharge duration, mean discharge rate (10.6 ± 1.8 pulses s −1 ) and mean abduction force (5.5 ± 2.8% maximum) did not change significantly (discharge rate, P = 0.119; and abduction force, P = 0.235). In contrast, the coefficient of variation for interspike interval during the initial 30 s of the task was 22.2 ± 6.0% and increased to 31.9 ± 7.0% during the final 30 s (P < 0.001). All motor units were recruited again after 60 s of rest. Although subjects were able to sustain a relatively constant discharge rate, the cessation of discharge was preceded by a gradual increase in discharge variability. The findings also showed that the maximal duration of human motor unit discharge exceeds that previously reported for the discharge elicited in motor neurons by intracellular current injection in vitro.

A reduction in compliance or activation level reduces residual force depression in human tibialis anterior

“…Naturally, the above assumptions imply that there is a specific set of motor units responsible for producing a certain force and this is supported for the human TA as its motor unit recruitment threshold has been shown to range from 1%‐88% MVC . If the same number of motor units are active after a reduction in activation and their average discharge rate is reduced, which has been commonly observed at motor unit derecruitment compared with recruitment, this would imply that torque should also be reduced, but this is not always observed . The similar force and EMG levels achieved after motor unit derecruitment in our study also raises the possibility that various mechanisms may be at play to reduce the predicted level of rFD following a reduction in activation.…”

Modulation of motor unit activity in biceps brachii by neuromuscular electrical stimulation applied to the contralateral arm

“…Interspike intervals Ͼ250 ms (Ͻ4 pulses/s) or Ͻ20 ms (Ͼ50 pulses/s) were excluded from the calculations of discharge rate (26). Once discriminated, the discharge times over periods of ϳ5 s were determined before, during, and after the application of NMES to the right elbow flexors.…”

Neural Contributions to Muscle Fatigue

“…Additionally during long endurance lower intensity tasks, new MUs are likely recruited and active MUs may drop-out (derecruited) for a period of time and then become activated again (67). This process is referred to as MU rotation or substitution (41).…”

“…No change, increases in rates, decreases in rates, or variable changes among MUs within the same experiment have been reported (reviewed in 37). At intensities of 20% or less of MVC during long duration contractions, units may show no change in rates but increases in variability in inter-discharge intervals as task failure approaches (67). Indeed, many factors including whether sustained or intermittent tasks, muscles of upper limb or lower limb, proximal or distal muscles, the composition and architecture of the muscle in terms of muscle fiber types and MU numbers, and training status among others can all be reasonably speculated to variably influence rate changes with submaximal fatiguing contractions.…”