Comparison of maximal strength, muscle morphology, and motor unit recruitment and firing rate patterns of the abductor digiti minimi in normal-fat and over-fat males

This study examined potential differences in strength, muscle morphology, and motor unit (MU) behavior of the abductor digiti minimi (ADM) between normal-fat (NF) and over-fat (OF) males.

Dual-energy X-ray absorptiometry assessed percent body fat (%BF). Ultrasonography determined muscle cross-sectional area (CSA), echo intensity (EI), and subcutaneous fat (sFAT). MU behavior was assessed during isometric muscle actions at 50% of maximal voluntary contraction (MVC) by analyzing the y-intercepts and slopes for the MU action potential amplitude (MUAP_AMP) vs. recruitment threshold (RT) relationships, the A and B terms for the mean firing rate (MFR) vs. RT relationships, and normalized electromyographic amplitude (N-EMG_RMS). MU firing times and waveforms were validated with reconstruct-and-test and spike trigger average procedures.

%BF was greater for OF (25.70% ​± ​5.40%) than NF (16.50% ​± ​2.20%; p < 0.001). MVC was greater for NF (27.13 ​± ​7.16) N than OF ([19.89 ​± ​4.96] N; p ​= ​0.014). CSA was greater for NF (2.48 ​± ​0.39) cm² than OF ([1.95 ​± ​0.47] cm²; p ​= ​0.011). The y-intercepts for the MUAP_AMP vs. RT relationships were greater for NF (0.283 ​± ​0.254) mV than OF ([-0.221 ​± ​0.659] mV; p ​= ​0.004). The B terms for the MFR vs. RT relationships were greater for NF (−0.024 ​± ​0.003) pps/%MVC than OF ([-0.031 ​± ​0.009] pps/%MVC; p ​= ​0.038). N-EMG_RMS was similar between groups (p ​= ​0.463).

Maximal strength, muscle size, and MU recruitment and firing rate patterns for a non-weight bearing muscle differed between normal-fat and over-fat males.