The effect of knee and foot position on the electromyographical activity of the superficial quadriceps

Isometric quadriceps exercises are used early in knee rehabilitation. Varying knee or foot position is hypothesized to selectively activate specific quadriceps muscles. This study examined the activities of the vastus medialis oblique, vastus lateralis, and rectus femoris during isometric contractions at 90°, 150°, and 175° knee angles with internally rotated, neutral, and externally rotated foot positions. Subjects performed three perceived maximal isometric contractions at each knee angle/foot position while electromyographic activity (EMG) was collected. Statistical analysis consisted of a three-way repeated measures multiple analysis of variance, with post hoc analysis as was appropriate. Although no significant differences were detected among foot positions for the rectus femoris at 90°, this knee angle produced significantly greater EMG activity for the neutral position compared with 150° or 175°. The 90° angle was also superior to 175° for the externally rotated position. The results for both the vastus medialis and vastus lateralis were similar, with the 90° angle producing greater EMG activity than 175° in the externally rotated position. In addition, the neutral position produced significantly more activity than the internally rotated position at 90°. At 150°, the neutral position was superior to the externally rotated position. Finally, at the 175° knee angle, the highest level of EMG activity was with the foot internally rotated. Considering the combination of effects for EMG activity across all muscles tested, the 90° knee angle with a neutral foot position may provide the most effective condition for rehabilitation of aft muscles tested. However, during quadriceps setting (175°), the vastus lateralis and vastus medialis oblique show the greatest activity with the foot internally rotated.