Strength training by electrostimulation conditions for efficacy.

The overload imposed on the neuromuscular system under Electrostimulation (ES) can be expressed by applied current intensity or by Electrically Evoked Torque (EET). The aim of this study was to discern which of these two parameters is the one which is determinant for the efficacy of training by ES. Test and training involved isometric contraction of the flexion maintained at a joint angle of 25 degrees (0 degree = extension). The 16 trained subjects received 15 sessions of 25 electrically evoked contractions, using a monophasic rectangular waveform current at a frequency of 2500 Hz modulated at 90 Hz. Each stimulation lasted for 5 sec at the maximal tolerable current. Contrary to the control group (n = 16), the trained group significantly increased their MVIC (15.6%). The individual strength gains ranged from -5% to 49%. No correlation was found between current level and strength modifications. A direct relation was established between the EET and the strength gains (a minimum threshold of EET must be reached during at least 8 sessions to induce strength increases). The "overload principle", previously described for voluntary contraction strengthening, seems to be suitable for electrical stimulation and concerns the EET shown on the ergometer as the effect of the contraction of agonist and antagonist muscles.