Fatigue during repeated eccentric-concentric and pure concentric muscle actions of the plantar flexors

Objective. To compare the fatigue development during pure concentric actions with eccentric-concentric actions in standardized isokinetic, open-chain conditions. Design. The study was performed with measurement of muscular strength and EMG of the plantar flexors on an isokinetic dynamometer. Background. Fatigue development on a dynamometer has mostly been studied with repeated pure concentric actions. Eccentric-concentric actions are, however, often used in normal activities. An eccentric action has an immediate effect on a following concentric action and therefore it is of interest to follow the fatigue process in repeated combined actions. Methods. Ten healthy young women participated. The pure concentric test was performed with the same number of cycles as in the eccentric-concentric test, which was performed until exhaustion. Development of MPF and RMS of the EMG in the triceps surae, as well as work, were evaluated. Results. The total amount of work was significantly higher in the eccentric-concentric test than in the pure concentric test. The work decreased similarly (32-39%) with no significant difference between the two tests. A significant decrease (12-16%) in MPF was only seen in the eccentric-concentric test. No changes in RMS were seen. Conclusion. The similar reductions of work seen in both tests should primarily be sought in the contractile process as such. Changes in MPF may then be viewed as a phenomenon dependent on the timing of the duty cycle.