Nerve Function Test! Patellar Reflex! Knee Jerk Reflex! A stretch reflex or knee jerk, is a stretch reflex which tests the L2, L2 and L4 segments of the spinal cord. Striking the patellar tendon with a reflex hammer just below the patella stretches the muscle spindle in the quadriceps muscle, producing a signal that travels back to the spinal cord & an impulse back to the quadriceps femoris muscle triggering a contraction causing the leg to kick. Healthcare and Medicine!