Effects Of Obesity On Motor Unit Properties For A Non-weight Bearing Muscle

Despite increased obesity rates, there is minimal research regarding the effects of excessive fat on neuromuscular function. Different motor unit (MU) properties were previously reported for the first dorsal interosseous between normal weight (NW) and overweight (OW) children. However, little is known regarding MU properties in obese adults. PURPOSE: This study examined MU properties of the abductor digit minimi (ADM) in NW and OW adults. METHODS: Participants were categorized by body fat percentages using a Dual-energy X-ray Absorptiometry full body scan (BF < 20% = NW, BF > 20% = OW). Eight NW (mean ± SD, age = 27.14 ± 5.58 yrs., ht. = 178.57 ± 3.78 cm., wt. = 84.77 ± 12.96 kg, BF = 16.41% ± 2.54%) and 8 OW males (age = 25.88 ± 2.53 yrs., ht. = 173.94 ± 3.59 cm., wt. = 93.74 ± 16.47 kg, BF% = 26.5% ± 5.50) performed three maximal voluntary contractions (MVCs) for the ADM on a finger ergometer, followed by a submaximal isometric trapezoidal muscle action at 50% MVC. Surface electromyographic (sEMG) signals recorded from the ADM were decomposed. Recruitment thresholds (RT), MU action potential amplitudes (MUAPAMPS), mean firing rates (MFR) and normalized EMG amplitude (N-EMGRMS) at steady force were analyzed. The y-intercepts (y-ints) and slopes were calculated for the MUAPAMP and MFR vs. RT relationships. Five independent samples t-tests compared N-EMGRMS and the y-ints and slopes for the MUAPAMP and MFR vs. RT relationships between groups. RESULTS: There were no significant differences for the slopes (p = 0.190) or y-ints (p = 0.540) for the MFR vs. RT relationships, or the slopes (p = 0.547) for MUAPAMP vs. RT relationships. However, there were significant differences for the y-ints from the MUAPAMP vs. RT relationships (p = 0.015; NW = 0.260 ± 0.183 mV, OW = -0.010 ± 0.189 mV) and N-EMGRMS (p = 0.031; NW = 52.21 ± 21.24%, OW = 84.82 ± 29.63%). CONCLUSION: The OW required greater N-EMGRMS to produce the same relative force, whereas the NW had larger MU sizes regardless of recruitment thresholds. The greater N-EMGRMS required by the OW may be the result of smaller force twitches for the recruited MUs. In addition, a non-weight bearing muscle differentiated MU properties between groups. Future research should compare muscle morphology and MU MFRs relative to MUAPAMPS to better elucidate differences in MU behavior between OW and NW adults.