Mean power frequency and integrated electromyogram of repeated isokinetic plantar flexions.

Peak torque (PT), contraction work (CW), mean power (CP), and time for contraction (t) were recorded for each of 200 full range-of-motion repetitive isokinetic (maximum effort) plantar flexions at the preset angular velocity 60 degrees s-1 (degrees X s-1). The subjects were 12 healthy males (42-46 years). Simultaneous recordings of electromyographic activity of the three parts of the triceps surae muscle were made. The electromyograms were truly integrated and the mean power frequency was also calculated for each of the contractions. The mechanical variables decreased steeply during approximately the first 70 contractions (the fatigue phase) followed by a steady-state level (the endurance level) with no further significant decrease. The fatigue phase was composed of two parts. The first was characterized by decreases in CW, MPF and in the ratio CW/iEMG, while iEMG and iEMG/t were unaltered. During the second part decreases in iEMG and iEMG/t occurred while MPF and CW/iEMG had stabilized. During the endurance level all the variables were stable but lowered when compared with the initial contractions of the fatigue phase. The results indicated that the mechanical fatigue went through two parts before the steady-state level was established.