Neuromuscular recruitment strategies of the vastus lateralis according to sex

Abstract

Aim: Despite males typically exhibiting greater muscle strength and fatigability than females, it remains unclear if there are sex-based differences in neuromuscular recruitment strategies e.g. recruitment and modulation of motor unit firing rate (MU FR) at normalized forces and during progressive increases in force.

Methods: The study includes 29 healthy male and 31 healthy female participants (18-35 years). Intramuscular electromyography (iEMG) was used to record individual motor unit potentials (MUPs) and near-fibre MUPs from the vastus lateralis (VL) during 10% and 25% maximum isometric voluntary contractions (MVC), and spike-triggered averaging was used to obtain motor unit number estimates (MUNE) of the VL.

Results: Males exhibited greater muscle strength (P < .001) and size (P < .001) than females, with no difference in force steadiness at 10% or 25% MVC. Females had 8.4% and 6.5% higher FR at 10% and 25% MVC, respectively (both P < .03), while the MUP area was 33% smaller in females at 10% MVC (P < .02) and 26% smaller at 25% MVC (P = .062). However, both sexes showed similar increases in MU size and FR when moving from low- to mid-level contractions. There were no sex differences in any near-fibre MUP parameters or in MUNE.

Conclusion: In the vastus lateralis, females produce muscle force via different neuromuscular recruitment strategies to males which is characterized by smaller MUs discharging at higher rates. However, similar strategies are employed to increase force production from low- to mid-level contractions. These findings of similar proportional increases between sexes support the use of mixed sex cohorts in studies of this nature.

Keywords: motor unit; neuromuscular junction; sex differences; vastus lateralis.

FIGURE 1

Box‐and‐jitter plots of the individual participant means, and group means (black dot) of (A) peak torque, (B) vastus lateralis cross‐sectional area, (C) motor unit number estimates (MUNE), and (D) force steadiness at 10% and 25% maximum voluntary contraction (MVC), in males (blue) and females (red). CoV, coefficient of variation

FIGURE 2

Box‐and‐jitter plots of the individual participant means, and group means (black dot) of (A) motor unit (MU) firing rate and (B) firing rate variability in males (blue) and females (red) at 10% and 25% maximum voluntary contraction (MVC)

FIGURE 3

Box‐and‐jitter plots of the individual participant means, and group means (black dot) of motor unit potential (MUP) (A) Area; (B) Phases; (C) Amplitude; (D) Turns; (E) Duration in males (blue) and females (red) at 10% and 25% maximum voluntary contraction (MVC)

FIGURE 4

Box‐and‐jitter plots of the individual participant means, and group means (black dot) of near fibre motor unit potential (NFM) (A) Jiggle; (B) Duration; (C) Segment (Seg) Jitter; (D) Dispersion; (E) Area in males (blue) and females (red) at 10% and 25% maximum voluntary contraction (MVC)

FIGURE 5

Forest plots of the standardized regression coefficient estimate for associations between motor unit characteristics and contraction levels in males and females models. Beta value and 95% confidence intervals (CI) represent the standardized model predicted change per unit from 10% to 25% maximum voluntary contraction (MVC). All statistical analysis was based on multilevel mixed effect linear regression models with each subject as an independent cluster. Standardized values of each parameter make the comparisons executable between men and women. ^‡ P < .001

FIGURE 6

Representative data from a male (A) and a female (B) participant. Top panels show knee extensor force traces at 10% and 25% of MVC, and corresponding intramuscular electromyography (iEMG) raw data recorded from the vastus lateralis. A representative MUP template and corresponding NFM shimmer plot isolated from each contraction are shown below each iEMG signal. Vertical orange lines on MUPs and NFMs indicate the start and end time of the NFM. Dur, duration; kV, kilovolt; ms, millisecond; mV, millivolt; MVC, maximum voluntary contraction; MUP; motor unit potential; N, Newtons; NF, near fibre; NFM, near fibre MUP; μV, microvolt