EFFECTS OF CONTRACTION DURATION ON LOW-FREQUENCY FATIGUE IN VOLUNTARYAND ELECTRICALLY-INDUCED EXERCISE OF QUADRICEPS MUSCLE IN HUMANS

The aims of this study were to investigate if low-frequency fatigue (L FF) dependent on the duration of repeated muscle contractions and to c ompare LFF in voluntary and electrically induced exercise, Male subjec ts performed three 9-min periods of repeated isometric knee extensions at 40% maximal voluntary contraction with contraction plus relaxation periods of 30 plus 60 s, 15 plus 30 s and 5 plus 10 s in protocols 1, 2 and 3, respectively, The same exercise protocols were repeated usin g feedback-controlled electrical stimulation at 40% maximal tetanic to rque. Before and 15 mill after each exercise period, knee extension to rque at 1, 7, 10, 15, 20, 50 and 100 Hz was assessed, During voluntary exercise, electromyogram root mean square (EMG(rms)) of the vastus la teralis muscle was evaluated. The 20-Hz torque:100-Hz torque (20:100 H z torque) ratio was reduced more after electrically induced than after voluntary exercise (P < 0.05). During electrically induced exercise, the decrease in 20:100 Hz torque ratio was gradually (P < 0.05) reduce d as the individual contractions shortened, During voluntary exercise, the decrease in 30:100 Hz torque ratio and the increase in EMG(rms) w ere greater in protocol 1 (P < 0.01) than in protocols 2 and 3, which did not differ from each other. In conclusion, our results showed that LFF is dependent on the duration of individual muscle contractions du ring repetitive isometric exercise and that the electrically induced e xercise produced a more pronounced LFF compared to voluntary exercise of submaximal intensity. It is suggested that compensatory recruitment of faster-contracting motor units is an additional factor affecting t he severity of LFF during voluntary exercise.