Sex differences in the modulation of the motor unit discharge rate leads to reduced force steadiness

Applied Physiology Nutrition and Metabolism ◽

10.1139/apnm-2020-0953 ◽

2021 ◽

Author(s):

J Greig Inglis ◽

David A. Gabriel

Keyword(s):

Sex Differences ◽

Motor Unit ◽

Coefficient Of Variation ◽

Discharge Rate ◽

Maximum Voluntary Contraction ◽

Motor Unit Action Potential ◽

Pulse Interval ◽

Unit Discharge ◽

Force Steadiness ◽

Motor Unit Discharge

The purpose of this study was to evaluate the relationship between the variability in the motor unit inter-pulse interval and force steadiness at submaximal and maximal force outputs between the sexes. Twenty-four male and twenty-four female participants were recruited to perform isometric dorsiflexion contractions at 20, 40, 60, 80, and 100% maximum voluntary contraction (MVC). Tibialis anterior myoelectric signal was recorded by an intramuscular electrode. Females had lower force steadiness (coefficient of variation of force (CoV-Force), 27.3%, p<0.01) and a greater coefficient of variation of motor unit action potential inter-pulse interval (CoV-IPI), compared to males (9.6%, p<0.01). There was no significant correlation between the normalized CoV-IPI and CoV-Force (r=0.19, p>0.01), but there was a significant repeated measures correlation between the raw scores for root-mean-square force error and the standard deviation of motor unit discharge rate (r=0.65, p<0.01). Females also had a greater incidence of doublet discharges on average across force levels (p<0.01). The sex differences may result from motor unit behaviors (ie, doublet and rapid discharges, synchronization, rate coding or recruitment), leading to lower force steadiness and greater CoV-IPI in females. Novelty Bullets: • Sex differences in force steadiness may be due to neural strategies • Females have lower force steadiness compared to males • Greater incidence of doublet discharges in females may result in lesser force steadiness

Download Full-text

Sex differences in motor unit discharge rates at maximal and submaximal levels of force output

Applied Physiology Nutrition and Metabolism ◽

10.1139/apnm-2019-0958 ◽

2020 ◽

Vol 45 (11) ◽

pp. 1197-1207

Author(s):

J. Greig Inglis ◽

David A. Gabriel

Keyword(s):

Sex Differences ◽

Motor Unit ◽

Discharge Rate ◽

Maximum Voluntary Contraction ◽

Voluntary Contraction ◽

Force Output ◽

Neural Drive ◽

Discharge Rates ◽

Intramuscular Electrodes ◽

Motor Unit Discharge

This study evaluated potential sex differences in motor unit (MU) behaviour at maximal and submaximal force outputs. Forty-eight participants, 24 females and 24 males, performed isometric dorsiflexion contractions at 20%, 40%, 60%, 80%, and 100% of a maximum voluntary contraction (MVC). Tibialis anterior electromyography was recorded both by surface and intramuscular electrodes. Compared with males, females had a greater MU discharge rate (MUDR) averaged across all submaximal intensities (Δ 0.45 pps, 2.56%). Males exhibited greater increases in MUDR above 40% MVC, surpassing females at 100% MVC (p’s < 0.01). Averaged across all force outputs, females had a greater incidence of doublet and rapid discharges and a greater percentage of MU trains with doublet and rapid (5–10 ms) discharges (Δ 75.55% and 61.48%, respectively; p’s < 0.01). A subset of males (n = 8) and females (n = 8), matched for maximum force output, revealed that females had even greater MUDR (Δ 1.38 pps, 7.47%) and percentage of MU trains with doublet and rapid discharges (Δ 51.62%, 56.68%, respectively; p’s < 0.01) compared with males at each force output, including 100% MVC. Analysis of the subset of strength-matched males and females suggest that sex differences in MU behaviour may be a result of females needing to generate greater neural drive to achieve fused tetanus. Novelty Females had higher MUDRs and greater percentage of MU trains with doublets across submaximal force outputs (20%–80% MVC). Differences were even greater for a strength matched subset. Differences in motor unit behaviour may arise from musculoskeletal differences, requiring greater neural drive in females.

Download Full-text

Frequency Modulation of Motor Unit Discharge Has Task-Dependent Effects on Fluctuations in Motor Output

Journal of Neurophysiology ◽

10.1152/jn.00390.2005 ◽

2005 ◽

Vol 94 (4) ◽

pp. 2878-2887 ◽

Cited By ~ 28

Author(s):

Carol J. Mottram ◽

Evangelos A. Christou ◽

François G. Meyer ◽

Roger M. Enoka

Keyword(s):

Motor Unit ◽

Frequency Modulation ◽

Discharge Rate ◽

Motor Output ◽

Unit Discharge ◽

Rate Of Increase ◽

Output Force ◽

Position Task ◽

Motor Unit Discharge ◽

Target Force

The rate of change in the fluctuations in motor output differs during the performance of fatiguing contractions that involve different types of loads. The purpose of this study was to examine the contribution of frequency modulation of motor unit discharge to the fluctuations in the motor output during sustained contractions with the force and position tasks. In separate tests with the upper arm vertical and the elbow flexed to 1.57 rad, the seated subjects maintained either a constant upward force at the wrist (force task) or a constant elbow angle (position task). The force and position tasks were performed in random order at a target force equal to 3.6 ± 2.1% (mean ± SD) of the maximal voluntary contraction (MVC) force above the recruitment threshold of an isolated motor unit from the biceps brachii. Each subject maintained the two tasks for an identical duration (161 ± 93 s) at a mean target force of 22.4 ± 13.6% MVC. As expected, the rate of increase in the fluctuations in motor output (force task: SD for detrended force; position task: SD for vertical acceleration) was greater for the position task than the force task ( P < 0.001). The amplitude of the coefficient of variation (CV) and the power spectra for motor unit discharge were similar between tasks ( P > 0.1) and did not change with time ( P > 0.1), and could not explain the different rates of increase in motor output fluctuations for the two tasks. Nonetheless, frequency modulation of motor unit discharge differed during the two tasks and predicted ( P < 0.001) both the CV for discharge rate (force task: 1–3, 12–13, and 14–15 Hz; position task: 0–1, and 1–2 Hz) and the fluctuations in motor output (force task: 5–6, 9–10, 12–13, and 14–15 Hz; position task: 6–7, 14–15, 17–19, 20–21, and 23–24 Hz). Frequency modulation of motor unit discharge rate differed for the force and position tasks and influenced the ability to sustain steady contractions.

Download Full-text

Blockade of 5-HT2 receptors suppresses rate of torque development and motor unit discharge rate during rapid contractions

Journal of Neurophysiology ◽

10.1152/jn.00470.2021 ◽

2021 ◽

Author(s):

Benjamin Ian Goodlich ◽

Sean A Horan ◽

Justin J Kavanagh

Keyword(s):

Motor Unit ◽

Unit Activity ◽

Repeated Measures ◽

Discharge Rate ◽

Voluntary Contraction ◽

Unit Discharge ◽

Motor Unit Activity ◽

Rate Of Torque Development ◽

Motor Unit Discharge ◽

Torque Development

Serotonin (5-HT) is a neuromodulator that is critical for regulating the excitability of spinal motoneurons and the generation of muscle torque. However, the role of 5-HT in modulating human motor unit activity during rapid contractions has yet to be assessed. Nine healthy participants (23.7 ± 2.2 yr) ingested 8 mg of the competitive 5-HT2 antagonist cyproheptadine in a double-blinded, placebo-controlled, repeated-measures experiment. Rapid dorsiflexion contractions were performed at 30%, 50% and 70% of maximal voluntary contraction (MVC), where motor unit activity was assessed by high-density surface electromyographic decomposition. A second protocol was performed where a sustained, fatigue-inducing dorsiflexion contraction was completed prior to undertaking the same 30%, 50% and 70% MVC rapid contractions and motor unit analysis. Motor unit discharge rate (p < 0.001) and rate of torque development (RTD; p = 0.019) for the unfatigued muscle were both significantly lower for the cyproheptadine condition. Following the fatigue inducing contraction, cyproheptadine reduced motor unit discharge rate (p < 0.001) and RTD (p = 0.024), where the effects of cyproheptadine on motor unit discharge rate and RTD increased with increasing contraction intensity. Overall, these results support the viewpoint that serotonergic effects in the central nervous system occur fast enough to regulate motor unit discharge rate during rapid powerful contractions.

Download Full-text