The purpose of this study is to make clear the function of the quadriceps muscles by using EMG. Two separate experiments were conducted. In the first experiment, electromyographic records were obtained from the long fibers of the vastus medialis, the oblique fibers of the vastus medialis, the vastus lateralis, the vastus intermedius and the recus femoris muscles of ten normal subjects during isometric extension (knee joint angle: 90°, 60°, 30°, 15°, 0°) with no load and 5 kg load, and isotonic extension (knee joint angle: between 90° to 0°) with 5 kg load. Those measurements were made in sitting and supine positions. In the second experiment, EMG were recorded from the vastus medialis and other three heads of the quadriceps of six normal subjects before and after infiltration of local anesthesia into the vastus medialis. The measurements were made during isometric extension with no load and with maximum effort, and during isotonic extension (knee joint angle: between 90° to 0°) with no load in the sitting position. The same measurements were also taken while the subjects were walking, standing-up and sitting-down on a chair, and ascending and descending stairs. In order to assess the muscular force, torque was measured as well during isometric extension of the knee joint with maximum effort. The results were obtained as follows: (1) EMG activities of the oblique fibers and the long fibers of the vastus medialis were similar. (2) The activities of the vastus medialis were not different at the terminal extension of the knee joint from the other three heads of quadriceps muscles. (3) Rectus femoris showed greater activity in the supine than in the sitting position.
