Influence of exercise intensity and joint angle on endurance time prediction of sustained submaximal isometric knee extensions

The purpose of endurance time (T (lim)) prediction is to determine the exertion time of a muscle contraction before it occurs. T (lim) prediction would then allow the evaluation of muscle capacities limiting fatigue and deleterious effects associated with exhaustive exercises. The present study aimed to analyze the influence of exercise intensity and joint angle on T (lim) prediction using changes in surface electromyographic (sEMG) signals recorded during the first moments of the exercise. Fifteen male performed four knee extensions sustained until exhaustion that were different in exercise intensity (20% or 50% of maximal voluntary torque-MVT) and in joint angle (40 or 70A(0), 0A degrees A = full extension). T (lim) prediction was explored using some parameters of the sEMG signals from rectus femoris, vastus medialis and vastus lateralis muscles. Changes in sEMG parameters (root mean square, mean power frequency and frequency banding 6-30 Hz) were expressed using the slope of the linear regression and the area ratio index. Results indicated that relationships between changes in sEMG signal and T (lim) (0.51 < r < 0.83) were greater for experimental conditions associated with higher exercise intensity (50% MVT) and so to lower time duration. Knee joint angle had little influence on T (lim) prediction results. Results also showed higher T (lim) prediction considering spectral parameters and area ratio. This could be in relation to differences in relative contribution of central and peripheral fatigue that seems to change according to the exercise intensity, but also to the influence of psychological factors that increases with the duration of the task.