The knee-jerk test

It’s the stuff of slapstick comedy: the doctor hits the knee with a hammer, and the lower leg bounces up. When my doctor struck my left knee, the leg jerked robustly, but a rap on the right knee brought no response. She glared at the hammer accusingly, but she knew the tool wasn’t the problem.

It was my nerves.

The so-called “knee-jerk reflex” happens when the patellar tendon, which runs over the kneecap from quadriceps to the lower leg, is struck just below the knee. The impact sends a signal from the muscle to the spinal cord and back, setting off an electrical pulse that contracts the thigh muscle, yanking the tendon and pulling the lower leg upward.

If that doesn’t happen, it’s a sign that the circuit is broken. In my case, it turned out that a herniated disc between two lower vertebrae was squeezing down on the sciatic nerve root and disrupting the signal. This hefty nerve runs across the buttocks and down the leg, and pain, tingling, and weakness caused by its irritation is called sciatica.