Evidence of bilinearity in the relationship between rate of neuromuscular excitation and rate of force development

Abstract

The purpose was to examine the relationship between the rate of neural excitation (rate of rise in the electromyogram, EMG) and the rate of isometric force development (RFD) to determine whether surface EMG measures can detect nonlinearity that is expected due to underlying motor unit discharge behavior and the summation of progressively larger motor unit potentials throughout recruitment. Due to interest in obtaining a change point, a bilinear model was hypothesized to provide the best fit of the EMG-RFD relationship compared to a linear model, exponential model and log-transformed data. 21 young adult participants performed isometric dorsiflexion contractions to 40% of their maximal voluntary contraction (MVC) force. Contractions were performed in RFD conditions ranging from slow (20 %MVC/s) to fast (peak volitional rate). The Akaike Information Criterion supported nonlinear best fit models in 16 of the 21 participants with the greatest overall support for the bilinear model (n = 13). The bilinear models indicated a mean change point at 204%MVC/s. The present data do not identify the specific motor unit control mechanisms at play and the influence of amplitude cancellation on the electromyogram must be carefully considered.