We all know the patellar (knee Jerk) reflex as the thing the doctor checks during our physical. Sitting on the table with our legs crossed, he hammers softly just below our knee-cap - bump, kick; bump, kick -- he's through and we're fine. The question is 'what does this reflex do? What function does it serve?'

To answer this we need to quickly review what the reflex is. The knee jerk reflex is the simplest of reflexes involving only two neurons - one sensory ('feeling') and one motor ('movement'). When the stretch receptors in the patellar tendon detect that the tendon has been stretched, a sensory neuron in the quadriceps muscle carries an impulse to the spinal cord, where it synapses directly with a motor neuron. If the signal is strong enough, an action potential is generated in the motor neuron, causing a contraction of the quadriceps muscle which extends the leg. This is how the reflex works; now on to its function.

As you begin to lose your balance from an upright position the patellar tendon which is carrying the load of your weight, is suddenly slack, this causes the patellar reflex to be triggered. The quadriceps violently contract, which launches the body up into the air. This hop gives your body time to get your feet back underneath you.

The knee-jerk reflex is a falling reflex that, when we are off balanced, 'kicks' us into the air to help us recover our footing.