A reflex is an involuntary and nearly instantaneous movement in response to a stimulus. The reflex that the doctor checks when he taps on your knee during a routine checkup is called the patellar, or DTR (deep tendon reflex). Striking the patellar tendon just below the patella stretches the quadriceps muscles in the thigh, which activates stretch sensory receptors which trigger a muscle contraction of the femoris muscle (this causes the leg to kick). The absence or decrease of this reflex is known as Westphal’s sign. On the other hand, multiple oscillation of the leg following the tap may be a sign of a cerebellar disease.

This reflex is important in helping you keep your balance. When we stand up, gravity might cause a slight bend of the knee, which could make us fall if we didn’t have this reflex to straighten the knee and keep us standing upright. Moreover, a doctor often checks for DTRs to make sure that the nervous system is in order; aside from the knee, they can also be checked along the outside of the elbows, in the crooks of the arms, and at the wrists and ankles.

So next time you are at the doctor’s office, you can rest assured that your valuable insurance dollars are being well spent on all of those little rubber hammers….

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