Have you ever wondered why your leg will kick instantly once your doctor or therapist hit your patella tendon with a reflex hammer?
The phenomenon of leg kicking or knee extension is called a patellar reflex, also known as a deep tendon reflex. This reflex is a protection mechanism of our muscles. Within our muscles and tendons, there are many sensory receptors that help protect the muscle by sensing the muscle tension or length, which prevents our muscles from being torn easily. The one that helps sense the muscle length is known as the muscle spindle. When there is a sudden change in muscle length such as a sudden stretch of the muscle, the muscle spindle will send a signal to the spinal cord. The spinal cord will then send a signal to the same muscle to cause contraction to stop the muscle from being overstretched and inhibit the opposite muscle. This is a classic example of a monosynaptic reflex arc.
Striking the patellar tendon with a reflex hammer stretches the muscle spindle of the quadriceps muscles, hence a signal is sent to the spinal cord and causes the quadriceps to contract.
This test is used to test for any lower motor neuron lesion because no kicking will be observed if there is damage to the sensory or motor nerves of that specific spinal cord level.
Other deep tendon reflexes that are commonly being tested are your Achilles tendon, bicep tendon, triceps tendon, and your brachioradialis tendon.