Effects of a 60~second maximum voluntary isometric contraction on torque production and EMG output of the quadriceps muscle group

Abstract

Background: Fatigue is thought to impact performance and may be a factor in injury mechanism. However, the full impact of a potentially fatiguing task on performance is unclear. Objective: To explore changes in maximum voluntary torque and electromyographic (EMG) activity following a sixty second isometric maximum voluntary contraction (MVC) of the quadriceps muscles and to follow the time course of any changes observed. Method: 21 subjects (14 males and 7 females) were recruited. Torque during brief maximum voluntary isometric contractions of the quadriceps muscle group was recorded in right and left legs using a Cybex Norm Isokinetic Dynamometer before, during and at intervals after a 60 second isometric fatigue task. EMG activity of the vastus medialis, rectus femoris and vastus lat eralis was also recorded synchronously. Results: Mean peak torque was significantly ( p < 0.05) lower in both legs at the end of the fatigue task. However, 10 minutes following the cessation of the task, it had increased and was significantly higher than the pre-fatigue level. It remaine d higher until the end of the protocol (30 mins post fatigue). The EMG amplitudes decreased slightly compared to pre-fatigue values during the fatigue task, a few of which were significantly low er than those of the pre-fatigue task. Following the fatigue task, EMG amplitudes increased gradually and were significantly h igher than those of the pre-fatigue task in all the muscles at the final time point. Median frequencies decreased significantly dur ing the fatigue task and were similar to pre-fatigue values by 5mins post-fatigue. Conclusion: This study has revealed that both torque and EMG rise following a sustained isometric 60 second hold to higher levels than pre-hold and that this rise is maintained for up t o 30 minutes. This rise however, was not associated with increased muscle firing frequency. The mechanisms behind this rise rem ain to be elucidated.