Characterising sex-related differences in lower- and higher-threshold motor unit behaviour through high-density surface electromyography

Abstract

Emerging questions in neuromuscular physiology revolve around whether males and females share similar neural control in diverse tasks across a broad range of intensities. In order to explore these features, high-density electromyography was used to record the myoelectrical activity of biceps brachii during trapezoidal isometric contractions at 35% and 70% of maximal voluntary force (MVF) on 11 male and 13 female participants. Identified motor units were then classified as lower-threshold (recruited at ≤30%MVF) and higher-threshold (recruited at >30%MVF). The discharge rate, interspike interval variability, recruitment and derecruitment thresholds, and estimates of neural drive to motor neurons were assessed. Female lower-threshold motor units showed higher neural drive (P < 0.001), accompanied by higher discharge rate at recruitment (P = 0.006), plateau (P = 0.001) and derecruitment (P = 0.001). On the other hand, male higher-threshold motor units showed greater neural drive (P = 0.04), accompanied by higher discharge rate at recruitment (P = 0.005), plateau (P = 0.04) and derecruitment (P = 0.01). Motor unit discharge rate normalised by the recruitment threshold was significantly higher in female lower-threshold motor units (P < 0.001), while no differences were observed in higher-threshold motor units. Recruitment and derecruitment thresholds are higher in males across all intensities (P < 0.01). However, males and females have similar activation and deactivation strategies, as evidenced by similar recruitment-to-derecruitment ratios (P > 0.05). This study encompasses a broad intensity range to analyse motor unit sex-related differences, highlighting higher neural drive and discharge rates in female lower-threshold motor units, elevated recruitment and derecruitment thresholds in males, and convergences in activation and deactivation strategies. HIGHLIGHTS: What is the central question of the study? Do male and female motor units behave similarly in low- and high-intensity contractions? What is the main finding and its importance? Female motor units show higher discharge rates in low-intensity tasks and lower discharge rates in high-intensity tasks, with no differences in recruitment behaviour. A broader inter-spike interval variability was also observed in females. These findings underline that there are sex-specific differences concern the firing strategies based on task intensity.

Keywords: HDsEMG; central nervous system; electromyogram; motor control; motor unit; skeletal muscle.

FIGURE 1

Data acquisition from HDsEMG. (a) A HDsEMG grid, long‐axis oriented and placed over the biceps brachii muscle belly. (b, c) The raw signals (b) were filtered, and the single MU action potentials (c) were obtained after the data were processed in the BSS algorithm. Each coloured stripe reflects a motor unit firing activity associated with the force signal (marked as a full black line). (d) Motor units n#1 and n#8 are displayed as examples of the HDsEMG multichannel signals for a lower‐ and higher‐threshold motor unit from the same recording. RT and DT are calculated as the force values at which motor units are activated and deactivated, corresponding to the first and last spike. (e) Estimation of the discharge rate at recruitment (DR_R), plateau (DR_P) and derecruitment (DR_D). DR_MEAN, mean discharge rate; DT, derecruitment threshold; HTMU, higher‐threshold motor unit; LTMU, lower‐threshold motor unit; RT, recruitment threshold. Created with BioRender.com.

FIGURE 2

The relationship between subcutaneous fat, myoelectrical parameters and identified motor unit distribution. (a, b) The average discharge rate (DR) and the interspike variability (ISIv) for each participant, as determined through raw data analysis, are presented in relationship to subcutaneous fat measurements obtained through the assessment of SST in (a) and (b), respectively. (c) The distribution of identified motor units by their recruitment threshold of male and female participants.

FIGURE 3

Discharge rate comparisons. Swarm plots of male‐female comparisons concerning the discharge rate at recruitment (REC), plateau (PLAT), and derecruitment (DER) are displayed for LTMUs (a) and HTMUs (b). Each circle represents a single participant. Data are reported as the mean ± SD. DER, derecruitment; DR, discharge rate; HTMUs, higher‐threshold motor units; LTMUs, lower‐threshold motor units; PLAT, plateau; REC, recruitment.

FIGURE 4

Sex‐related differences in DT, DR and ISIv as a function of the recruitment threshold. (a–c) Scatter plots of lower‐threshold motor unit results for DT–RT (a), DR–RT (b) and ISIv–RT (c). (d–f) Higher‐threshold motor unit results for DT–RT (d), DR–RT (e) and ISIv–RT (f). Each circle represents an identified motor unit. Single slopes per participant are reported for both male and female participants. DT, derecruitment threshold; ISIv, interspike interval variability; RT, recruitment threshold.

FIGURE 5

Sex‐related differences in the DT/RT, DR/RT and ISIv/RT ratios as a function of the recruitment threshold. (a–c) Scatter plots of lower‐threshold motor unit results for DT/RT (a), DR/RT (b) and ISIv/RT (c) ratios. (d–f) Higher‐threshold motor unit results for DT/RT (d), DR/RT (e) and ISIv/RT (f) ratios. Each circle represents an identified motor unit. Single slopes per participant are reported for both male and female participants. DT, derecruitment threshold; ISIv, interspike interval variability; RT, recruitment threshold.

FIGURE 6

Recruitment and derecruitment threshold comparisons. (a,c) Swarm plots of male‐female comparisons in the absolute recruitment and derecruitment threshold in LTMUs (a) and HTMUs (c). (b,d) The relative recruitment and derecruitment threshold for LTMUs (b) and HTMUs (d). Data are reported as the mean ± SD. Each circle represent a single participant. DT,derecruitment threshold; HTMUs,higher‐threshold motor units; LTMUs, lower‐threshold motor units; MVF, maximal voluntary force; RT, recruitment threshold.

FIGURE 7

Input–output slope comparisons of male and female motor units. (a, c) Scatter plots of LTMUs (a) and HTMUs (c) representing the differences in force and discharge rate between the plateau and the recruitment phases during the ramp contraction, reflecting the input–output gain of motoneurons. Each circle represents an identified motor unit. (b, d) The slope differences for LTMUs (b) and HTMUs (d) displayed as aligned dot plots. Data are reported as the mean ± SD. HTMUs, higher‐threshold motor units; LTMUs, lower‐threshold motor units.