Content area
Full text
Aim. The purpose of the study was to compare the influence of two intervals between consecutive intermittent contractions with the elbow flexors on time to failure and the accompanying changes in electromyographic (EMG) activity and excitationcontraction coupling.
Methods. Subjects performed 6-s intermittent isometric contractions at 50% of the maximal voluntary contraction (MVC) force to task failure in two sessions. The recovery period between consecutive contractions was 4-s (short recovery protocol, SRP) in one and 14-s (long recovery protocol, LRP) in the other session.
Results. The number of contractions performed to task failure was greater for the LRP (99±24) compared with the SRP (46±18; P<0.05). The amplitude and duration of the maximal compound action potential (M-wave) did not change during either protocol. EMG activity (% peak MVC) for the elbow flexors and the force fluctuations increased more rapidly during the SRP compared with the LRP (P<0.05) and was greater at task failure for the SRP protocol (P<0.05). These changes were accompanied by a faster decline in twitch force.
Conclusion. The results indicate that a briefer interval between consecutive contractions was associated with a more rapid impairment of the excitation-contraction coupling and a faster increase in muscle activation, despite a similar net muscle force for the two protocols.
Key words: Electromyography - Muscle contraction - Muscle fatigue - Elbow.
Intermittent muscle contractions are common in daily tasks and athletic activities.1 Muscle fatigue develops gradually during intermittent submaximal isometric contractions,2-5 and has been attributed to both muscular and neural impairments. Previous studies have reported that impairment of excitation-contraction coupling 2,5 is likely due to metabolic changes within muscle,6 whereas others have reported that repeated intermittent contractions are accompanied by decreases in spinal and supraspinal excitability,7 and modulation of motor unit recruitment and discharge rate.4 These differences can be related to a large variety of factors, including the muscle group involved, contraction intensity, and the prescribed duty cycle.810 Thus, it appears unlikely to isolate a single main factor responsible for muscle fatigue. An alternative would be to compare the performance of the same task under different conditions of fatigue and to identify the potential physiological adjustments that limit performance during the more difficult (i.e., fatiguing) task.8 In the present study, this was accomplished by examining how varying the...