The purpose of this study was to investigate which of 4 positions produced the highest action potential in the rectus femoris muscle of normal adult subjects. Testing was performed in supine with the right leg performing a simple straight leg raise with the knee fully extended. The left leg, however, was placed in 4 different positions: 1. Full support with $0^{circ}$ flexion. 2. Flexed on the plinth with $60^{circ}$ knee flexion and foot flat. 3. Same as N0.2 but with $90^{circ}$ knee flexion. 4. Left leg hanging over the end of the plinth with $90^{circ}$ knee flexion, $0^{circ}$ hip flexion and no foot support. This study was designed to compare the level of electromyographic activity of the rectus femoris under 4 positions. Fourty-three healthy young adults performed three trials of each exercise condition in random order in the supine position. Electromyographic activity was recorded from surface electrodes. Rectus femoris action potentials in all 4 positions were significantly different. The highest action potential at the end of movement of the right leg occurred with the left leg hanging over the end of the plinth with $90^{circ}$ knee flexion. It is therefore recommended the straight leg raising be performed with the contralateral leg flexed at $90^{circ}$ over the end of the supporting surface to obtain a maximum rectus femoris isometric contraction.