Fatigue and EMG of Repeated Fast Voluntary Contractions in Man

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Abstract
A fatigue test consisting of repeated fast maximal contractions of the left quadriceps muscle in an isokinetic apparatus was performed by 12 healthy male subjects (19-25 yr). EMG [electromyography] signals were recorded from the surface of the left vastus lateralis muscle, from which also biopsies were obtained for muscle fiber classification. Only minor changes were observed in the EMG variables despite a decrease in muscle strength performance, in terms of peak torque, work and power to about 50% of initial values after 100 contractions. The concomitantly obtained positive correlation between the increase in EMG/torque ratio and the individual percentage of fast twitch (FT) muscle fibers indicated that local factors in the muscle, primarily in the FT fibers, were causing the development of fatigue during repeated dynamic contractions with high power outputs.

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