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

Abstract

Purpose: 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.

Methods: 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.

Results: %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).

Conclusion: 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.

Keywords

Abductor digiti minimi / Body fat / Electromyography / Motor unit / Ultrasonography

CRediT authorship contribution statement

Lyric D. Richardson: Writing - review & editing, Writing - original draft, Supervision, Software, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Alex A. Olmos: Writing - review & editing, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Allen L. Redinger: Writing - review & editing, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Stephanie A. Sontag: Writing - review & editing, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Sunggun Jeon: Writing - review & editing, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Brenden L. Roth: Writing - review & editing, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Emma G. High: Writing - review & editing, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Breanne S. Baker: Writing - review & editing, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Jerome Hauselle: Writing - review & editing, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Michael A. Trevino: Writing - review & editing, Writing - original draft, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.

Ethical approval statement

This study was performed in line with the policy statement regarding the use of human subjects by the Helsinki Declaration of 1975, as revised in 2013. Written informed consent and a health and exercise status questionnaire were obtained for all participants. Approval of all experimental procedures was granted by The University Institutional Review Board (protocol number: IRB-20-464-STW).

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thank all the participants who took time out of their schedules to help with these projects.

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