Contribution of rectus femoris and vasti to knee extension. An electromyographic study

Electromyographic (EMG) activity was recorded from the five components of the quadriceps during maximum knee extension with the limb in six combinations of hip and knee flexion: the hip at 0 degrees, 40 degrees, and 80 degrees with the knee at 15 degrees and 60 degrees. None of the six positions could isolate the vasti from the rectus femoris. The highest EMG activity of all five muscles was recorded with the limb in 40 degrees hip flexion and 15 degrees knee flexion. The lowest EMG activity was recorded with the hip at 0 degrees and the knee at 60 degrees flexion. Overall, the knee extension torques were 40% higher with the knee at 60 degrees flexion than at 15 degrees flexion. The lowest torques were generated with the limb positioned at 80 degrees hip flexion and 15 degrees knee flexion. During maximum knee extension, the vasti muscles do not work in isolation of the rectus femoris, regardless of the amount of hip flexion. All portions of the quadriceps worked in all positions, which suggests these muscles may be strengthened effectively in many different positions to accommodate the patient's comfort and interest.