Non-linear dynamics of lower leg muscle surface electromyogram during repeated plantar flexion

This study investigated non-linear behaviour of gastrocnemius medialis (calf muscle) surface electromyogram (EMG) collected during repeated plantar flexion in both fatigue and non-fatigue conditions. Data were collected in the beginning and immediately after the participant reported fatigue. The study used the phase space reconstruction method to estimate the largest Lyapunov exponents and Kaplan-Yorke dimension of EMG signals. Positive Lyapunov exponent and fractional Kaplan-Yorke dimension values confirmed the presence of chaotic behaviour in the EMG readings. A paired sample t-test showed that the mean largest Lyapunov exponent following fatigue onset was significantly higher than before fatigue (p < 0.05) for right, left and combined right and left gastrocnemius muscles. A logistic regression model revealed the largest Lyapunov exponent as a significant predictor of fatigue onset with a high odds ratio. However, these measures were not found to be good predictors of postural stability as measured by the centre of pressure data.