Acute Effects of Static Stretching in the Manifestation of Neuromuscular Fatigue in Submaximal Isometric Contraction

Abstract

PURPOSE: This study aimed to examine the acute effects of a static stretching protocol on the muscle's response to fatigue during an isometric handgrip exercise by means of electromyography (EMG). METHODS: A group of 17 healthy men (mean age (± SD) = 27.1 (2.9) yrs; height = 181 (9.1) cm; and body mass = 78.9 (9.0) kg) performed the fatigue tests applied to the right hand before (NS) and after stretching (ST), which was considered as the intervention group (IG). In addition, two tests were applied to the left hand without stretching and used as the control group (CG). The stretching exercise comprised of three repetitions of static stretching for the wrist flexors lasting 50s each until the discomfort point. The fatigue test consisted of 30s of a handgrip isometric muscle action with a digital dynamometer, and was maintained within a range between 50% and 60% of the MVC, which was monitored by the subject and the tester on the screen of the computer. The tests of IG were performed in a counterbalanced mode with 10 minutes between one test and the other, while the CG only performed the test twice with 10 minutes between each other. During all tests, the subjects had the flexor carpi radialis (FC) and flexor digitorum (FD) muscles monitored by surface EMG. From the EMG data, the root mean square (RMS) values and the mean frequency (MF) were obtained for each consecutives 2s windows of the test, forming time series. The EMG data were normalized in respect to the values obtained during the MVC. From each of the time series, it was determined the coefficient of slope of linear regression, which was used in the comparison of the tests NS and ST and the CG tests by means of paired t tests. RESULTS: The results showed a higher rate of increase in EMG on ST than NS test, both for the FC (p = 0.02) and the FD (p = 0.04) muscles. The same pattern of increased activity was detected in control tests. However, the average of both tests was similar and did not show any statistical difference in CG. A decrease of MF during the tests was observed in the frequency domain, and the FC showed an average decrease rate greater in ST than in the NS test (p = 0.02). Likewise, the average decrease of MF in CG was similar in both tests and no statistical difference was observed. CONCLUSION: It is concluded that static stretching may enhance the manifestation of fatigue and mainly related to their neuromuscular responses, when performed immediately before an isometric contraction. PRACTICAL APPLICATION: It is not recommended to use static stretching before activities that require the application of isometric force over moderate periods of time, as is common in sports that require the task of isometric handgrip muscle actions such as judo and rowing.