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RELIABILITY OF MOTOR UNIT FIRING RATE VERSUS RECRUITMENT THRESHOLD RELATIONSHIPS OF THE FIRST DORSAL INTEROSSEOUS

Abstract

Mandy E. Wray1, Jonathan D. Miller1, Adam J. Sterczala2, Trent J. Herda1

1University of Kansas, Lawrence, KS; 2University of Pittsburg, Pittsburgh, PA

Traditionally, motor unit (MU) mean firing rates (MFRs) have been regressed against their recruitment thresholds (RT), to provide insight on changes in MU behavior following intervention. However, the reliability of these parameters via decomposition methods has yet to be assessed. PURPOSE: To examine the reliability of MFR vs. RT relationships of the first dorsal interosseous (FDI) during a low and moderate intensity contraction. METHODS: Twenty-three healthy subjects (male=10 female=13, age=20.26 ± 2.05 years, weight=76.54 ± 20.13 kg, height=174.07 ± 10.30 cm) volunteered for this investigation. Participants visited the laboratory for one familiarization trial and two experimental trials separated by 24 hours. Participants performed isometric muscle actions of the FDI which included 3 maximal voluntary contractions (MVCs). The highest MVC force was used to determine the target force for the subsequent, randomized contractions at 20% and 50% MVC. Surface EMG signals were recoded from the FDI using a 5-pin surface array sensor. Action potentials were extracted into the firing events of single MUs via decomposition. Slopes and y-intercepts were calculated for the linear MFR vs. RT relationships and used for statistical analysis. Intraclass correlation coefficients (ICC) model “2,1”, standard error of the measurements (SEM), and repeated measures ANOVAs were calculated to assess reliability between the first and second experimental trials. P level was set to 0.05 RESULTS: The means ± SDs (collapsed across trials), ICCs, SEMs, and ANOVA P values are presented in Table 1. For the MFR vs. RT relationships there were no differences between trials 1 and 2. The ICC values of the slopes (20% MVC=0.443, 50% MVC=0.343) indicated poor reliability, and the ICC values of the y-intercepts (20% MVC=0.637, 50% MVC=0.494) indicated moderate reliability. CONCLUSIONS: There is a lack of reliability overall between trials when RT range is not controlled for. The lack of reliability between trials may be due to inconsistency in MUs observed between trials. Future studies should determine if controlling for RT range of recorded MUs between trials effects reliability.

Additional Files
Wray Table 1.docx (13 kB)
Wray Table 1

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