Muscle phenotype is related to motor unit behavior of the vastus lateralis during maximal isometric contractions

Abstract

Previous investigations have reported a relationship between skeletal muscle phenotype and motor unit (MU) firing parameters during submaximal contractions. The purpose of the current investigation, however, was to examine the relationships between motor unit firing behavior during a maximal voluntary contraction, Myosin Heavy Chain (MHC) isoform content, and various molecular neuromuscular targets of the vastus lateralis (VL) muscle in resistance‐trained men. Ten resistance‐trained males completed a trapezoidal ramp contraction up to 100% of their maximal voluntary isometric strength (MVIC). Surface electromyography was recorded from the VL using a multichannel electrode array and decomposed to examine the firing characteristics of individual MUs. A skeletal muscle biopsy of the VL was also collected from each subject. Regression analyses were performed to identify relationships between type II fiber area and the slopes and/or intercepts of the mean firing rate (FRMEAN) versus recruitment threshold (RT), max firing rate (FRMAX) versus RT, and RT versus MU action potential amplitude (MUAPPP) relationships. There were significant inverse relationships between type II fiber area and the y‐intercept of the FR versus RT relationship (P < 0.05). Additionally, strong relationships (r > 0.5) were found between type II fiber area and FRMEAN versus RT slope and RT versus MUAPPP slope and intercept. These data further support the hypothesis that skeletal muscle phenotype is related to MU behavior during isometric contraction. However, our data, in concert with previous investigations, may suggest that these relationships are influenced by the intensity of the contraction.

Sex‐related differences in muscle size explained by amplitudes of higher‐threshold motor unit action potentials and muscle fibre typing

“…18,36 Despite similar activity levels, males displayed greater contractile tissue (larger mCSA, lower mEI), which might help explain the greater age-related decline in muscle thickness for males than females. 22 In line with previous studies, 8,10,11,37 MUAP waveforms discriminated from the surface EMG signal demonstrated a systematic pattern for all subjects, with higher-threshold MUs exhibiting larger peak-to-peak amplitudes for the VL. Initially, these findings would seem to be in disagreement with Del Vecchio et al 38 who reported weak or no relationships between the root mean square (RMS) of MUAPs via a spike trigger averaging (STA) technique and RTs.…”

Genetic variant in the β2‐adrenergic receptor (Arg16Gly) influences fat‐free mass, muscle strength and motor unit behaviour in young men

“…Interestingly, in the present study, we observed a more negative slope in the GlyGly than the ArgArg group during muscle actions performed at 50% MVIC, but a less negative slope in the GlyGly and ArgGly groups than in the ArgArg group during muscle actions performed at 70% MVIC (Figure ). Thus, our results, in combination with these previous studies (Colquhoun et al., ; Trevino et al., ), might suggest skeletal muscle fibre‐type differences for the β 2 ‐AR genotypes.…”

“…We observed not only that muscle strength was significantly greater in the GlyGly than the ArgArg group (Figure a), but also that MU behaviour was dependent on β 2 ‐AR genotype. Previous studies (Colquhoun et al., ; Trevino et al., ) have observed relationships between type I and type II fibre area and the slope of the MFR versus RT relationship. These studies appear to demonstrate that individuals with greater type II fibre content demonstrate a MFR versus RT relationship characterized by a more negative slope during low‐ to moderate‐force contractions, but a less negative slope during high‐force contractions.…”

“…LabVIEW program (LabVIEW 2015; National Instruments, Austin, TX, USA) was then used to analyse the MUs that met the inclusion criteria described above. The MU properties calculated by the LabVIEW program from each muscle action included the RT, which was defined as the relative force (% MVIC) at which the MU first discharged, and the mean firing rate (MFR), which was calculated of as the average firing rate (in pulses per second) during the plateau in the firing curve of each individual MU (Colquhoun et al., ; Muddle et al., ).…”

The influence of input excitation on the inter- and intra-day reliability of the motor unit firing rate versus recruitment threshold relationship

“…1. The MU properties calculated in the present investigation were RT, defined as the relative torque (%MVIC) at which the motor unit first discharged, and MFR, calculated as the average firing rate (pulses/s, pps) during the plateau in each individual MU's firing curve (Colquhoun et al 2018;Muddle et al 2018). Finally, the synaptic inputs received by the motoneuron pool were compared between torque levels.…”