Comparison of voluntary and electrical stimulation contraction torques*

Informed male volunteers completed maximal isometric knee extension efforts under each of three contraction conditions: 1) voluntary, 2) electrical stimulation (ES) only, and 3) superimposed (ES superimposed onto voluntary). Ten subjects completed the three contraction conditions using each of the following current formats: 1) asymmetrical biphasic rectangular wave, 2) asymmetrical biphasic spike wave, and 3) symmetrical monophasic square wave. Under the voluntary and the superimposed contraction conditions no significant differences in mean torque were observed between the three current formats. However, under the ES contraction condition, the torque associated with the symmetrical monophasic square wave was significantly less than that associated with the other two current formats. As well, the torque associated with the asymmetrical biphasic spike wave was significantly less than that associated with the asymmetrical biphasic rectangular wave format. No significant difference was observed between the maximum voluntary contraction (MVC) and the superimposed contraction conditions for the groups receiving the asymmetrical biphasic spike and the symmetrical monophasic square wave current formats, or between all three contraction conditions for the group receiving the asymmetrical biphasic rectangular wave format. However, the ES condition was associated with significantly less torque than were the MVC and the superimposed conditions for the asymmetrical biphasic spike and the symmetrical monophasic square wave formats. In response to a posttest questionnaire, 18 of 30 subjects felt that the ES contraction produced greater torque than did MVC, and 26 subjects selected the superimposed condition as having produced greater torque than the ES condition. The different current formats, resulting in different levels of sensation and preception, and the different output capabilities of each electrical stimulator are considered to have been jointly responsible for the ES only torque discrepancies. It is suggested that ES does not recruit more motor units, resulting in a greater force of contraction, than are recruited under MVC.J Orthop Sports Phys Ther 1984;5(6):324-331.