The action of the rectus femoris muscle following distal tendon transfer: does it generate knee flexion moment?

Rectus femoris transfer surgery involves detaching the rectus femoris from the patella and reattaching it posterior to the knee. While this procedure is thought to convert the rectus femoris from a knee extensor to a knee flexor, the moments generated by this muscle after transfer have never been measured. We used intramuscular electrodes to stimulate the rectus femoris in four subjects, two after transfer to the semitendinosus and two after transfer to the iliotibial band, while measuring the resultant knee moment. Electromyographic activity was monitored in the quadriceps, hamstrings, and gastrocnemius muscles to verify that the rectus femoris was the only muscle activated by the stimulus. We found that the rectus femoris generated a knee extension moment in all of the subjects tested. This finding suggests that transfer surgery does not convert the rectus femoris to a knee flexor, and that a mechanism exists which may transmit the force generated by the rectus femoris anterior to the knee joint center after distal tendon transfer.