Introduction: The ankle is one of the most loaded parts of the human body. For optimal dynamic stability of the ankle, fast responses of foot evertor muscles are important to protect the joint from unexpected inversion injuries. Since most sports injuries occur at the onset of muscle fatigue, it is assumed that the latter affects the neuromuscular function of the ankle. Purpose: The purpose of this master thesis was to examine the effects of local muscle fatigue of the foot evertors on reaction time to a visual stimulus and to determine how neuromuscular activation of peroneus longus and tibialis anterior muscles changes during isotonic fatiguing. We also studied the correlation between the fatigue-induced change in the MVIC torque of the foot evertors and the change in their reaction time to a visual stimulus. Methods: The study was conducted on a sample of 24 healthy individuals. We measured the reaction time of foot evertors to a visual stimulus at 25% and 50% of the target force on an isokinetic dynamometer. The measurements were performed before and after an isotonic fatigue protocol performed in a semi-sitting position on the isokinetic dynamometer. Neuromuscular activation of peroneus longus and tibialis anterior muscles during the fatiguing protocol was also measured using surface EMG. The reaction time, amplitude and frequency of the EMG signal before and after the fatiguing task were compared using Student's paired t-test. In order to estimate the connection between the change in reaction time and the change in MVIC, Spearman's correlation coefficient was calculated. Results: After an isotonic fatigue protocol the foot evertor reaction time to a visual stimulus increased significantly at both 25 % MVIC (+9,4 %; p < 0,05) and 50 % MVIC (+12,2 %; p < 0,05). The mean EMG amplitude was significantly higher during the last five repetitions of the fatigue protocol compared to the first five repetitions, while the median frequency decreased. There was no statistically significant association between the change in reaction time and the change in MVIC after fatigue. Discussion and conclusion: Isotonic fatiguing of the foot evertors causes a delay in reaction time to a visual stimulus and decreases MVIC torque, but the changes in these parameters are not correlated. Fatigue of the foot evertors is reflected by an increase in the amplitude and a decrease in the frequency of EMG activity of the peroneus longus and tibialis anterior muscles, indicating increased recruitment of motor units necessary to maintain force. For decisive conclusions, standardization of research methodology and a larger number of subjects is required.
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