Comparisons of mechanical and electromyographical muscular utilization ratios

Abstract

The physical loading of a muscle during functional activities can be estimated by the muscular utilization ratio. This ratio is defined as the percentage of muscular involvement relative to the maximal capacity. Either mechanical or electromyographical approaches can be used to obtain the muscle utilization ratio. However, the non-linear relationship between electromyographical activity and muscle force, as well as the non-equivalence between agonist muscles, may create differences between the mechanical muscle utilization ratio calculated from joint moments and the electromyographical muscle utilization ratio calculated from electromyographical data. The aim of this study was to compare, during a squat test, the mechanical muscle utilization ratio and the electromyographical muscle utilization ratio estimated by three different methods; direct linear approximation, second order polynomial regression and linear interpolation. The knee extensor moment and electromyographical data of rectus femoris and vastus medialis of 11 subjects were recorded during both knee extension and squat. Both tests were performed with the knee maintained at 90° of flexion. The results showed that: a) the electromyographical muscle utilization ratio, calculated from the average of vastus medialis and rectus femoris, significantly underestimates the mechanical muscle utilization ratio (ANOVA, p < 0.01), b) the differences between the mechanical muscle utilization ratio and the electromyographical muscle utilization ratio are larger for the direct linear approximation method than for the second order polynomial regression (ANOVA, p < 0.01) or the linear interpolation method (ANOVA, p < 0.01), and c) independent of the method utilized, there is no difference between the electromyographical muscle utilization ratio predicted by the vastus medialis as compared with the rectus femoris (ANOVA, p > 0.01). These results indicate that the muscle utilization ratio calculated from electromyographical data underestimates the mechanical muscle utilization ratio even after correction for the non-linearity between moment and electromyographical activity.