Force Fluctuations And Alternating Muscle Activation Patterns Of Synergistic Muscles During A Sustained Submaximal Contraction

Abstract

PURPOSE The purpose of this study was to investigate the muscle activation patterns and force fluctuations of the synergistic heads of the quadriceps muscle during a sustained sub-maximal fatiguing contraction. METHODS Ten healthy subjects (3 men, 7 women) performed a sustained isometric voluntary contraction (20% MVC) of the knee extensor muscles until exhaustion. Co-activation and alternating patterns of EMG activity between pairs of the superficial quadriceps muscles (vastus medialis and vastus lateralis, vastus medialis and rectus femoris, vastus lateralis and rectus femoris) were calculated with linear regression analyses over consecutive 1 s intervals throughout the fatigue task. Positive and negative correlations represented co-activation or alternating patterns of EMG activity and were counted over the first-, mid-, and last-third of endurance time. The average EMG amplitudes (%MVC) of each muscle and the coefficient of variation of the applied knee extension force were calculated over the first-, mid- and last-5% of endurance time. Two-way repeated measures ANOVA with Tukey post-hoc analyses was used to contrast the dependent variables. RESULTS The fatigue task on average lasted for 707 s (range 360 to 1435 s) and induced a 25% drop in the MVC (p<.05). Average EMG amplitudes and the magnitude of the force fluctuations were greater during the last-5% compared to the first-5% of endurance time (p<.05). The number of alternations between each pair of synergistic muscles was greater during the first-third compared to the last-third of endurance time (p<.05) whereas this pattern was reversed for the co-activations (p<.05). CONCLUSIONS It is postulated that increasing rates of synergistic co-activation, rather than alternation, parallel progressively increasing magnitudes of the force fluctuations over the course of a sustained sub-maximal fatiguing contraction.