Eight weeks of resistance training increases strength, muscle cross-sectional area and motor unit size, but does not alter firing rates in the vastus lateralis.

Abstract

Previous investigations analyzing resistance training's influence on motor unit (MU) firing rates have yielded mixed results. These mixed results may be clarified by concurrently measuring changes in MU size. Thus, this study analyzed whether post-training strength gains were due to increases in MU firing rates and/or sizes as measured indirectly via action potential amplitudes. Sixteen males (age = 20.7 ± 1.9years) completed 8 weeks of resistance training, while eight males (age = 19.4 ± 2.5years) served as controls. Vastus lateralis surface electromyography signals collected during submaximal isometric knee extensions were decomposed to yield an action potential amplitude (MUAPAMP), mean firing rate (MFR), and recruitment threshold (RT) for each MU. Each contraction's average MFR and MUAPAMP, and coefficients of the linear (y-intercept and slope) MUAPAMP-RT, linear MFR-RT and exponential (A and B terms) MFR-MUAPAMP relationships were analyzed. Firing instances and action potentials were validated via reconstruct-and-test and spike-triggered averaging procedures. Vastus lateralis cross-sectional area (CSA) was analyzed with ultrasonography. Resistance training increased isometric strength from 204.6 ± 34.9 to 239.8 ± 36.3 Nm and vastus lateralis CSA from 28.7 ± 4.7 to 34.0 ± 5.0 cm2. Resistance training did not affect MFR-RT relationship parameters or average MFRs but did increase the slopes of the MUAPAMP-RT relationships (0.0067 ± 0.0041 to 0.0097 ± 0.0045mV/%MVC) and average MUAPAMPs from 0.189 ± 0.093 to 0.249 ± 0.127mV. MU hypertrophy altered the MFR-MUAPAMP relationships (B terms: -3.63 ± 1.40 to -2.66 ± 1.38 pps/mV). Resistance training induced MU and muscle hypertrophy, but did not alter firing rates. Greater MU twitch forces resulting from larger MUs firing at pre-training rates likely explain resistance training strength gains.