In order to quantify the preferred direction of the individual heads of quadriceps femoris muscle, we focused on the interaction between exerted force fluctuations and muscle activity. Six subjects performed seven force-matching tasks for 30-s in 7 different directions on the horizontal plane. Surface electromyograms were recorded from 5 muscles: rectus femoris (RF), vastus intermedius (VI), vastus lateralis (VL), vastus medialis obliquus (VMO), and vastus medialis longus (VML). The preferred direction in each muscle was calculated by electromyogram-weighted averaging (EWA) method, which is based on the cross-correlation function between filtered force fluctuations and rectified EMG. The EWA revealed that the preferred direction of RF, VL, VML, and VI corresponded to 93.6°, 54.7°, 94.8°, and 68.4°, respectively. However, VMO had no preferred direction. These results indicate that the individual muscles composing quadriceps femoris muscle have different preferred directions on the horizontal plane although they have a common role of knee extension. Therefore, it is suggested that preferred direction on the horizontal plane is determined by integrated results from the anatomical, biomechanical, physiological and neurophysiological features of the muscles.

DESCENTE SPORTS SCIENCE Vol.34/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE