Electromyographic analysis of selected muscles during the lateral step-up exercise.

The purpose of this study was to determine electromyographically if significant differences in level of electrical activity in the vastus medialis, rectus femoris, biceps femoris, and semimembranosus/semitendinosus muscles occurred during the concentric and eccentric phases of a 4- and 8-in lateral step-up exercise. Twenty-nine subjects with no pathological condition of the knees participated in the study. The surface electromyograms obtained were processed by linear envelope detectors and normalized, in percentage, to a maximum voluntary contraction of each subject. Within-muscle differences, step height, and contraction type were tested with a two-way analysis of variance. Results revealed that the average values for muscular activity, depending on the height and direction, ranged from 24 percent to 60 percent for vastus medialis, 8 percent to 23 percent for rectus femoris, 3 percent to 9 percent for biceps femoris, and 4 percent to 9 percent for semimembranosus/semitendinosus muscles. All muscles were significantly more active at the 8-in height than at the 4-in height. The concentric phase produced significantly greater activity (p less than .001) than the eccentric phase at each height except for semimembranosus/semitendinosus muscles, where no significant difference existed. The results of this study are discussed with respect to the limited role that the tested hamstrings and quadriceps femoris muscles play in preventing anterior shear of the tibia during the lateral step-up exercise.