Effect of electrical stimulation superimposed with isokinetic contractions

Previous studies have considered the effects of the superimposition of electrical stimulation (ES) upon maximal voluntary contraction (MVC) under isometric conditions. This investigation considered these effects using isokinetic muscle action. Eleven males, non athletes, volunteered for the experiment. Isokinetic torque for extension of the right knee was measured by means of a Cybex 340 using two speeds of movement, 12 degrees/s and 30 degrees/s. Torque was measured at 5 degrees increments (from 100 degrees flexion to full extension) under conditions of MVC alone and MVC+ES. Stimulation was provided by means of a bi-phasic, symmetrical, square wave with a pulse width of 600 mu. Frequency of stimulation was either 30 Hz or 80 Hz. It was found that the pattern of torque production was unaffected by the application of ES. Peak torque values at both speeds were significantly lower when ES was applied at both frequencies compared to MVC alone. This inhibitory effect was found to extend throughout the middle range of movement. This tended to be more pronounced with the 80 Hz frequency. In general these findings were in agreement with those reported previously for isometric conditions. Possible explanations for these results include the inability of ES to recruit more motor units than MVC alone; the limitations of the subjects to tolerate a current of a sufficiently high intensity to elicit a stronger contraction, possibly due to lack of familiarization with these forms of muscle action; and the characteristics, especially pulse width, of the stimulating current.