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.

Examination of muscle morphology and neuromuscular function in normal weight and overfat children aged 7‐10 years

“…In addition, there was a significant difference in the range of MFRs at the relative targeted torque which was primarily the result of lower MFRs of the higher‐threshold MUs for the NW (Figure ). The greater decay of MFRs in relation of RT has been reported for individuals with greater area of fast‐twitch fibers . The present study contradicted the findings of Trevino et al as the children that could potentially have greater area of fast‐twitch fibers (OF) possessed less negative slopes.…”

“…The greater decay of MFRs in relation of RT has been reported for individuals with greater area of fast‐twitch fibers . The present study contradicted the findings of Trevino et al as the children that could potentially have greater area of fast‐twitch fibers (OF) possessed less negative slopes. It is possible that another mechanism other than fast‐twitch fiber area could be dictating differences in MFRs between NW and OF.…”

“…Although a lower relative torque (50% MVC), Stock et al reported greater y ‐intercepts (29.4 ± 6.0 pps) and more negative slopes (−0.420 0.165 pps/%MVC) from the MFR vs RT relationships in comparison with the children ( y ‐intercepts = 23.6 ± 6.0 pps; slopes = −0.266 ± 0.130 pps/%MVC) in the present study. Previously, Trevino et al reported that adults with greater area of fast‐twitch fibers (type II myosin heavy‐chain isoform content) possessed more negative slopes from the MFR vs RT relationships of the VL. The observation that the adults would have more negative slopes in comparison with the children in the present study fits well with the evidence that adults possess greater area of fast‐twitch fibers than young children .…”

“…For each MU, three parameters were extracted from the firing rate data: (a) the recruitment threshold (RT) expressed relative to %MVC, (b) the mean firing rate (MFR) at the targeted contraction level (pulses per second [pps]), and (c) the duration of time from peak‐to‐peak amplitude of the MU action potential waveform . The RT was determined as the average of a 0.01‐second epoch of torque that began at the first firing instance of the MU during the increasing segment of the trapezoid . The MFR was calculated as the average value of the mean firing rate trajectory during a steady 0.25‐second epoch at the targeted torque (Figure ).…”

“…To our knowledge, there are no studies that have extensively examined MU firing rates in relation to RT in children (MU firing rate vs RT relationship). The y ‐intercept from the firing rates vs RT relationship indicates the firing rates of the earlier recruited (lower‐threshold) MUs, whereas the negative slope from the firing rates vs RT relationship indicates that MFRs of the later recruited (higher‐threshold) MUs are lower than earlier recruited MUs at the targeted torque . The slopes from the firing rates vs RT relationships have distinguished differences in individuals as a function of neuromuscular diseases, exercise training, and the myosin heavy chain isoform content ( muscle fiber typing ) of the muscle .…”