LOSS OF POWER DURING FATIGUE OF HUMAN LEG MUSCLES

1. We have investigated the loss of power seen during high-intensity e xercise of human leg muscles such as might occur during sprinting. Sub jects exercised the quadriceps and hamstring muscle groups using a Cyb ex dynamometer at an angular velocity of 90 deg s(-1) once a second fo r 6 min. At 1 min intervals the quadriceps were electrically stimulate d via the femoral nerve to produce an isometric contraction which was then released into an isokinetic shortening contraction at 90 deg s(-1 ). 2. The extent of central fatigue was assessed by comparing the forc e of a voluntary isokinetic contraction with that elicited by electric al stimulation during isokinetic releases. Two subjects were repeatedl y tested. In the first series of experiments, exercising the quadricep s of one leg, the instantaneous power fell to about 50% over the cours e of 2 min and remained constant for the rest of the exercise. For one subject the voluntary and electrically stimulated forces declined in parallel while for the second subject the voluntary force was 10% less than the stimulated force at the end of the exercise. These results s how that central fatigue represented a minor factor contributing no mo re than one-fifth of the total loss of power in these circumstances. 3 . In a second series of experiments the subjects alternately contracte d the quadriceps and the hamstrings of both legs in an exercise which had a high rating of perceived exertion and entailed considerable resp iratory and cardiovascular effort. The time course and proportionate l oss of power mere very similar to those seen with the one-leg exercise and neither subject showed evidence of significant central fatigue. T he pattern of force loss was very similar for the hamstrings. We concl ude that, for determined subjects, afferent feedback from muscles, ten dons and joints or from the respiratory and cardiovascular systems doe s not have a major role in inhibiting voluntary activation of the quad riceps during heavy exercise. 4. In both series of experiments the pow er output during electrically stimulated isokinetic contractions was r educed to 50% of the initial value after 2 min of exercise while the i sometric force, measured immediately before the release, fell to only 75%. This suggests that fatigue affects isometric and shortening contr actions to different extents and the loss of power may be due to a com bination of factors, only one of which is evident in the loss isometri c force.