EFFECT OF FREQUENCY ON ELECTROSTIMULATED MUSCULAR STRENGTH

The aim of this work is to determine the effect of low frequency curre nt (L) versus medium frequency current (M) in order to generate a diff erent force level on soleus and gastrocnemius muscles. The experiment was conducted on 10 healthy, male volunteer subjects aged from 19 to 2 1 who provided informed consent to participate in the experiment. None of the subjects had experienced electrostimulation (ES) procedures be fore. The subjects were seated with vertical legs and horizontal thigh s. Isometric force of the gastrocnemius and soleus muscles was measure d using a strain gage dynamometer (100 daN). The transducer was adjust ed on the knee, along the vertical axis of the leg. For the conditions of ES, a bi-phasic, symmetrical square wave signal was applied at eac h of five low frequencies : 20, 40, 60, 80 and 100 Hz and at each of s ix medium frequencies (2222 Hz) modulated or not, to yield 20, 40, 60, 80, 100 bursts of pulses. Maximum voluntary contraction (MVC) was als o recorded. The phase duration was constant throughout 180 mu s. The c urrent level was set individually for each subject at the maximum that could be tolerated and applied during 10 s. Each contractile conditio n was randomly chosen. The forces produced by the different regimen of ES used are significantly different (P<0.001). For each type of stimu lation we got tetanic contractions. After an initial ramp, a nearly fl at curve corresponding to tetanus was reached. For a 60 hz low frequen cy current, the force production reached 76% of MVC for a mean intensi ty of 58 mA, corresponding to a charge of 1253 mu C. For medium freque ncy with a 80 Hz modulation, the maximum force values (64% of MVC) wer e reached after 3 s stimulation. The mean intensity used was 62 mA wit h a charge nearly 23000 mu C. In order to produce a maximal muscular t ension, low frequencies (about 60 hz) seem more suitable. For infra ma ximal contraction medium modulation frequencies can be used.