HAUTIER, C. A., L. M. ARSAC, K. DEGHDEGH, J. SOUQUET, A. BELLI, and J.-R. LACOUR. Influence of fatigue on EMG/force ratio and cocontraction in cycling. Med. Sci. Sports Exerc., Vol. 32, No. 4, pp. 839–843, 2000.
Purpose: The purpose of the present study was to observe force and power losses and electromyographic manifestations of fatigue during repeated sprints performed on a friction-loaded cycle ergometer.
Methods: Ten subjects performed 15 maximal 5-s sprints with 25-s rests between them. Power, velocity, and torque were measured during sprints 1 and 13 and during two submaximal constant-velocity (50 rpm) periods of cycling performed before and after the sprints. The EMG signals of five leg muscles were stored to determine the EMG/force ratio of power producer muscles and the coactivation of antagonist muscles. The power producer muscles were activated to the same level during sprints 1 and 13, despite a loss of force, whereas the vastus lateralis muscle was recruited more during the submaximal cycling period under fatigue conditions.
Results: This led to an increased EMG/force ratio for the power producer muscles, indicating the peripheral fatigue status of these muscles. Antagonist muscles were less activated during the sprints after fatigue; whereas they stayed unchanged during the last submaximal cycling period.
Conclusions: This suggests that there is a decrease in coactivation as agonist force is lost. This decrease in coactivation under fatigue conditions has not been previously reported and is probably due to the training status of the subjects. Subjects may have learned to better use their antagonist muscles to efficiently transfer force and power to the rotating pedal. This coordination can be adapted to cope with fatigue of the power producer muscles.
Muscle fatigue can be defined as the failure to maintain a required or expected power output (10). Because the type of effort required for many sprint sports such as soccer, basketball, and hockey is maximal-intensity intermittent exercise, many recent studies have focused on the influence of recovery time on performance during repeated sprints (2,3). They showed that a series of 6-s bouts of work separated by a 30-s rest caused muscular fatigue. Colliander et al. (8) used a similar protocol to show that subjects with a high percentage of fast twitch fibers were more sensitive to fatigue than those with more slow ones. But fatigue during repeated sprints has never been seen as a change in EMG. It is thus important to know whether a decrease in force and power that accompanies fatigue is completely attributable to contractile loss or whether muscle activation accounts for a part of this loss.
Several authors have studied the influence of fatigue on EMG parameters during isometric contractions (4,5,25), isokinetic contractions (16), and stretch-shortening cycles (11,20,26). Häkkinen et al. (14) reported a significant increase in IEMG during sustained submaximal isometric contractions; whereas Krogh-Lund and Jørgensen (21,22) and Öberg et al. (27) obtained the same result with the root mean squared (RMS) EMG signal. Whereas single muscle fatigue has been well documented, little attention has been paid to intermuscular contractions in pluriarticular movement. Lombard (23) was the first to observe antagonistic contraction during knee extension movement in cycling, which demonstrated the complexity of intermuscular coordination patterns. Others have studied intermuscular coordination patterns in complex movements such as jumping (6), running (19), or cycling (12,17,18). All of these studies indicated that monoarticular and biarticular muscles have different functions. The first can be considered to be power producers, whereas the second type are modulated to transfer power between articulations. These functions are involved in the optimization of intermuscular coordination for the best efficiency. Psek and Cafarelli (28) examined the activation of antagonist muscles under fatigue conditions and found that fatigue of the vastus lateralis muscle increased the activation of the biceps femoris muscle, which acts as an antagonist in knee extension movement. This suggests that fatigue of a muscle group decreases the global movement overall efficiency by disorganizing muscular coordination.
The present study was carried out to observe the electromyographic changes that occur during fatigue produced by repeated maximal sprints on a friction-loaded cycle ergometer. The fatigue of each muscle group was determined by EMG analysis together with the possible influence of fatigue status on antagonist muscles activation. These results will help clarify the extent to which force and power losses are the result of muscle contractile loss or changes in intermuscular coordination.