EMG, Reflex and Reaction Time Components, Muscle Structure, and Fatigue During Intermittent Isometric Contractions in Man

Abstract

Effects of fatigue produced in 100 isometric one-leg knee extensions on maximal isometric force, reflex and reaction time components, and EMG parameters were studied in 29 male students. Maximal force, total reflex time, reflex motor time, total reaction time, reaction premotor time, and mean power frequency of EMG spectrum decreased significantly during fatigue. The change of maximal force and that of integrated EMG during the first nine contractions were significantly (r = 0.36-0.45, P < 0.05) related to muscle fiber distribution of m. vastus lateralis so that the decrease was greater in subjects with a high percentage of fast twitch (FT) fibers. This was thought to be due to fatiguing and dropping out of FT motor units during the early phase of fatigue. Reaction motor time (electromechanical delay) did not change significantly during fatigue. Two different mechanisms in addition to changes in recruitment pattern are suggested to be connected with the effects of fatigue: increase of intramuscular temperature and accumulation of metabolic by-products in the muscle.

Key Words

fatigue - EMG - reaction time - reflex time - fiber types