Influence of the contractile properties of muscle on motor unit firing rates during a moderate-intensity contraction in vivo

Abstract

It is suggested that firing rate characteristics of motor units (MUs) are influenced by the physical properties of the muscle. However, no study has correlated MU firing rates at recruitment, targeted force, or derecruitment with the contractile properties of the muscle in vivo. Twelve participants (age = 20.67 ± 2.35 yr) performed a 40% isometric maximal voluntary contraction of the leg extensors that included linearly increasing, steady force, and decreasing segments. Muscle biopsies were collected with myosin heavy chain (MHC) content quantified, and surface electromyography (EMG) was recorded from the vastus lateralis. The EMG signal was decomposed into the firing events of single MUs. Slopes and y-intercepts were calculated for 1) firing rates at recruitment vs. recruitment threshold, 2) mean firing rates at steady force vs. recruitment threshold, and 3) firing rates at derecruitment vs. derecruitment threshold relationships for each subject. Correlations among type I %MHC isoform content and the slopes and y-intercepts from the three relationships were examined. Type I %MHC isoform content was correlated with MU firing rates at recruitment (y-intercepts: r = -0.577; slopes: r = 0.741) and targeted force (slopes: r = 0.853) vs. recruitment threshold and MU firing rates at derecruitment (y-intercept: r = -0.597; slopes: r = 0.701) vs. derecruitment threshold relationships. However, the majority of the individual MU firing rates vs. recruitment and derecruitment relationships were not significant (P > 0.05) and, thus, revealed no systematic pattern. In contrast, MU firing rates during the steady force demonstrated a systematic pattern with higher firing rates for the lower- than higher-threshold MUs and were correlated with the physical properties of MUs in vivo.

Keywords: motor unit firing rates; myosin heavy chain; onion skin control scheme; surface EMG decomposition; vastus lateralis.

Fig. 1.

An example of the average firing rate plots of detected motor units recorded from the 5-pin surface array sensor for the vastus lateralis during isometric trapezoidal contraction at 40% maximal voluntary contraction (MVC) for 1 participant. The black line shows the force signal as it appeared to the participant during the trial. The gray curves represent the average firing rates in pulses per second (pps) across time for each motor unit.

Fig. 2.

Results of the type I myosin heavy chain (MHC) isoform content analysis for subjects 2 (S2) and 5 (S5). Percent type I MHC isoform content was 65.3% and 30.7% for subjects 2 and 5, respectively.

Fig. 3.

Plotted relationships between type I % myosin heavy chain (%MHC) isoform content and the slopes (A) and y-intercepts (B) for the firing rate at recruitment vs. recruitment threshold relationships.

Fig. 4.

Plotted motor unit firing rates at recruitment (FR-REC) [pulses per second (pps)] vs. recruitment [% maximal voluntary contraction (%MVC)] (top), mean firing rate (pps) vs. recruitment (middle), and firing rates at derecruitment (FR-DEREC) (pps) vs. derecruitment threshold relationships (bottom) with linear regressions applied to the significant relationships for subjects 2 [65% type I myosin heavy chain (MHC)] and 11 (30% type I MHC) (left) and all subjects (black lines) (right).

Fig. 5.

Plotted relationships between type I % myosin heavy chain (%MHC) isoform content and the slopes (A) and y-intercepts (B) for the mean firing rate vs. recruitment threshold relationships.

Fig. 6.

Bottom left: plotted relationships between the recruitment threshold [expressed as percent maximal voluntary contraction (%MVC)] and the coefficient of determination for the predicted firing rate [pulses per second (pps)] vs. type I % myosin heavy chain isoform content (%MHC) relationships. Top: predicted firing rates vs. type I %MHC relationships for MUs recruited at 5% (left) and 35% (right) MVC. Bottom right: plotted predicted mean firing rate vs. recruitment threshold relationships for subjects 2 (65% type I MHC), 7 (43% type I MHC), and 11 (31% type I MHC). *Significant relationship between the predicted firing rate and type I %MHC for the respective recruitment threshold (P ≤ 0.05).

Fig. 7.

Plotted relationships between type I % myosin heavy chain (%MHC) isoform content and the slopes (A) and y-intercepts (B) for the firing rate at derecruitment vs. derecruitment threshold relationships.

Fig. 8.

Plotted motor unit (MU) firing rates at derecruitment (FR-DEREC) vs. firing rates at recruitment (FR-REC) for all subjects. The dashed line represents a perfect relationship (y-intercept = 0, slope = 1). MUs above the dashed line had a greater FR-DEREC than FR-REC, while MUs below the dashed line exhibited the opposite behavior.