Neuromuscular performance in boxers is closely related with muscle power, speed of movement, rate of force development and possibility to maintain force during fixed time period. It has been shown that muscle group fatigue and neuromuscular pathology are often caused by maximal dynamic exercise. Exercise-induced fatigue leads to decreased force output accompanied by decreases in electromyographic activity measured from the muscles in maximal voluntary contraction conditions (Komi and Rusko, 1974). The aim of the present study was to compare changes in contractile properties of the knee extensor (KE) muscles in boxers and untrained men after repeated maximal isokinetic knee extensions. Methods Ten male boxers with mean duration of sport training of 4 years (training load 10 hours per week) and ten untrained men participated in the study (mean age 22.3±0.4 and 22.7±0.6 years, respectively). The isokinetic exercise consisted of 60 maximal knee extensions at angular velocity of 90°/s using modernized Cybex II isokinetic dynamometer. Maximal power decrease (MPD) of isokinetic contractions of KE as the ratio of data at the end of exercise to data at the beginning of exercise was calculated. Surface electromyogram (EMG) of rectus femoris muscle was recorded during the test for analyses of mean frequency of EMG power spectrum (MPF) at the beginning and at the end of test. Isometric maximal voluntary contraction (MVC) force, rate of force development (RFD) and half-relaxation time (HRT) of KE were recorded using quadriceps bench (Pääsuke, 1999) before the exercise and 1 min, 5 min, 10 min and 15 min after it. Post-activity potentiation index (PAPES) of electrically evoked twitch contraction, rate of force development (RFDES), half-relaxation time (HRTES) and contraction time (CTES) of KE were estimated using direct electrical stimulation of rectus femoris muscle. One electrode was placed 10 cm proximally from patellae and another - 10 cm proximally from the first. The dimensions of electrodes (Compex) were 10 x 5 cm. Standard one-way analysis of variance (ANOVA) with Bonferroni correction was performed. Significance level was set at p<0.05. Results Boxers demonstrated a lower percent of decrease of maximal power of isokinetic exercise in comparison with untrained men (61 and 70 %, respectively). Results of the study showed no significant differences of KE isometric MVC in boxers and untrained men. Decrease of KE isometric MVC after repeated isokinetic exercise was lower in boxers than in untrained men (16.0 and 26.8%, respectively). No significant differences in isometric MVC 15 min after repeated isokinetic exercise were noted. Boxers had significantly shorter HRT of KE at isometric MVC (p<0.01) before the isokinetic exercise and during 15-min period of recovery as compared with untrained men. RFD of KE isometric MVC was significantly greater in boxers immediately after isokinetic exercise and 15 min after it. Twitch PAPES was greater in boxers before repeated isokinetic exercise and 15 min after it as compared with untrained men. RFDES of twitch was significantly greater in boxers before the test and 10 and 15 min during recovery period in comparison with untrained men. The slope of MPF of rectus femoris muscle EMG recorded before and immediately after repeated isokinetic exercise did not differ significantly between boxers and untrained men. Discussion/Conclusion Frequencies of the EMG power spectrum are related to the average conduction velocity of the muscle active fibers and muscle fiber conduction velocity is higher for fast twitch fibers (Arendt-Nielsen and Mills, 1985). No significant differences in slope of EMG power spectrum after repeated isokinetic knee extensions between boxers and untrained men in this study were noted. Data of Linamo et al (2000) showed that explosive exercise seemed to have facilitated the function of the neuromuscular system rather than producing fatigue.
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