Low frequency fatigue of quadriceps muscle after sustained maximum voluntary contractions.

Immediately after sustained maximum voluntary contractions for 60 s, greater quadriceps muscle fatigue and, especially, low frequency fatigue is observed. The results of our study have shown that immediately after the exercise there was a significant (P<0.05) decrease in muscle force induced by low (20 Hz) and high (50 Hz) stimulation frequencies and maximum voluntary contractions (it is not muscle length-dependent) and it did not recover to its initial (pre-exercise) level 15 min after the end of exercise. These observations suggest the observed low frequency fatigue may consist of 2 phases: an early, rapid recovery phase (to 3 min), likely related to muscle potentiation and metabolite build-up, and a slow recovery phase, which is not dependent on metabolite levels, but is especially dependent on muscle mechanical damage. There is an increase in low frequency fatigue during a slow recovery phase and it is more pronounced when low frequency fatigue is registered at short muscle length.