Adiposity Does Not Influence Motor Unit Behavior During Sustained Contractions In Sedentary Males

PURPOSE: Increased adiposity resulting in poorer muscle composition may influence motor unit (MU) behavior. The purpose is to examine MU behavior during fatiguing contractions between normal fat (NF) and overfat (OF) males. METHODS: Twenty-seven sedentary males were placed in NF (n = 14, age = 21.9 ± 4 y) or OF (age = 25.9 ± 6 y) groups. OF were classified as body fat % (BF%) >24%. Dual-energy X-ray absorptiometry (DXA) measured BF%, leg lean mass (LM), and leg Fat%. Maximal voluntary isometric contractions (MVCs) were performed Pre and Post isometric trapezoidal contractions of the knee extensors at 50% MVC via dynamometer. The plateau was held for 20s. Contractions (rep) were repeated until failure. Surface electromyographic (EMGRMS) signals were recorded from the vastus lateralis (VL) and decomposed for the First and Last completed rep. EMGRMS was normalized (N-EMGRMS) to Pre MVC EMGRMS. Recruitment thresholds (RT), MU action potential amplitudes (MUAPAMP), mean firing rates (MFRs), and AP duration (DUR) were calculated for each MU. Linear regression was used to calculate the slopes and y-intercepts (y-int) of the MUAPAMP and MFR vs RT relationships. Independent t-tests were used for body composition and # of reps. Repeated measures ANOVAs (2 x 2) were used for MVC and MU variables. RESULTS: OF had greater body mass (p = 0.004), BF% (p < 0.001), leg LM (p = 0.003), and leg Fat% (p < 0.001) than NF. There was no group difference in # of reps (p > 0.05). No ANOVA had an interaction (p > 0.05). MVC had a time (Pre > Post; p < 0.001) and group (OF > NF; p = 0.049) effect. N-EMGRMS had a time (First < Last; p < 0.001) but no group effect (p > 0.05). MFR vs RT slopes had a time (First < Last; p = 0.019) but no group effect (p > 0.05). MFR vs RT y-int had no time (p > 0.05) nor group (p > 0.05) effects. There were no time effects for MUAPAMP vs RT slopes (p > 0.05) nor y-int (p > 0.05). The larger leg Fat% for OF likely acted as a filter resulting in MUAPAMP vs RT group effects for slope (NF > OF; p < 0.001) and y-int (NF < OF; p = 0.012) since AP DUR was greater for OF than NF (p = 0.008) with no time effect (p > 0.05). CONCLUSIONS: Despite adiposity differences, OF and NF had similar neuromuscular responses to fatiguing contractions. To maintain force output, both groups increased the relative excitation to the VL from First to Last leading to greater firing rates of the higher threshold MUs.