Mean Power Frequency of Boys and Men during a Progressive Isometric Contractions Protocol to Exhaustion

Abstract

Background: The mean power frequency (MPF) of an electromyographic (EMG) signal is affected by contraction intensity and muscular fatigue but is also a potential indicator of motor unit (MU) recruitment. Methods: This study is a secondary analysis (Woods et al. 2019) in which participants (17 boys, 20 men) completed a progressive isometric contraction protocol while EMG was recorded from the vastus lateralis (VL), using tripolar surface electrodes. MPF and EMG threshold (EMGTh) were calculated for each completed intensity. The latter reflects the onset of accelerated increased in higher-threshold MU recruitment. Independent t-tests were used to assess differences between groups in demographic variables, mean MPF (MPFmn), peak MPF (MPFPK), force (%1RM) at MPFPK, and MPF range. An ANOVA for repeated measures was used to assess differences between groups in MPF pattern, interpolated over ten stages. A correlation analysis was used to assess the relationship between %1RM at MPFPK and %1RM at EMGTh. Results: Both, MPFmn and MPFPK were higher in the men, but only reached statistical significance when %body fat was used as a covariate in the statistical analysis. 65% of participants displayed an expected (inverted-U shape) MPF pattern. Within this subset, the %1RM at which MPFPK occurred was significantly higher (i.e., occurred later) in the boys compared with the men. Additionally, a moderate correlation was observed between the %1RM at MPFPK and the %1RM at EMGTh (r = 0.51). Discussion: Overall, the findings of the current analysis provide support for the hypothesis of lower type-II MU activation in children. The high variability in MPF patterns may be a result of the interaction between confounding factors that affect MPF (intensity and fatigue). Future research should use an exercise protocol that examines MPF under the influence of each factor separately.