Voluntary activation of human quadriceps during and after isokinetic exercise.

The extent of voluntary activation in fresh and fatigued quadriceps muscles was investigated during isometric and isokinetic voluntary contractions at 20 and 150 degrees/s in 23 normal human subjects. The muscles were fatigued by a total of 4 min of maximal knee extension at an angular velocity of 85 degrees/s. Voluntary activation was determined by the superimposition of tetanic electrical stimulation at 100 Hz for 250 ms, initiated at a constant knee angle. The relationship between voluntary and stimulated force was similar to that found with the established twitch superimposition technique used on isometric contractions. In fresh muscle all the subjects showed full voluntary activation during isometric contractions. Some activation failure was seen in five subjects at 20 degrees/s [2.0 +/- 0.9 degrees (SE)] and in two subjects at 150 degrees/s (0.7 +/- 0.5). After fatigue all subjects showed some activation failure at 0 and 20 degrees/s (36.4 +/- 3.1 and 28.8 +/- 4.1 degrees, respectively), but only two showed any at 150 degrees/s (1.4 +/- 5.7). We conclude that brief high-intensity dynamic exercise can cause a considerable failure of voluntary activation. This failure was most marked during isometric and the lower-velocity isokinetic contractions. Thus a failure of voluntary activation may have greater functional significance than previous studies of isometric contractions have indicated.