The knee jerk test, also known as the patellar reflex test, is a common, non-invasive procedure used by healthcare professionals. It offers a quick way to assess neurological function, helping evaluate the nervous system’s health.
The Science Behind the Reflex
The knee jerk reflex is an involuntary muscle stretch reflex. This rapid response involves a straightforward neural pathway known as a reflex arc. When the patellar tendon, located just below the kneecap, is tapped, it stretches the quadriceps muscle. This stretch is detected by specialized sensory receptors called muscle spindles within the quadriceps.
These muscle spindles then send a signal along sensory neurons to the spinal cord, typically at the L2, L3, and L4 segments. Within the spinal cord, the sensory neuron directly connects with a motor neuron without involving the brain. This direct connection, known as a monosynaptic reflex arc, allows for a very fast response. The motor neuron then sends an impulse back to the quadriceps muscle, causing it to contract and the lower leg to kick forward.
How the Test is Performed
To perform the knee jerk test, a patient sits on an examination table with their legs dangling freely, allowing the knees to be bent at approximately a 90-degree angle. This position ensures the quadriceps muscle is relaxed and the leg can swing without obstruction.
The healthcare professional uses a reflex hammer to gently tap the patellar tendon, which is located in the slight gap directly below the kneecap. A normal response involves a quick, slight forward kick of the lower leg. Observing this movement helps confirm the proper functioning of the reflex pathway.
What the Results Indicate
A normal knee jerk reflex indicates healthy nerve function along the reflex arc involving the L2, L3, and L4 spinal cord segments. A brisk and symmetrical response on both sides is expected. This suggests the sensory and motor neurons, as well as the spinal cord connections, are functioning as they should.
However, abnormal responses can provide important clues about underlying issues. An absent or significantly diminished reflex, known as areflexia or hyporeflexia, suggests damage to the peripheral nerves, motor neuron disease, or muscle disorders like muscular dystrophy. This type of response points to a problem within the reflex arc itself, affecting the conduction of impulses.
Conversely, an exaggerated or overactive reflex, termed hyperreflexia, indicates issues with the central nervous system, particularly involving upper motor neurons. Conditions such as spinal cord injuries, stroke, or multiple sclerosis can lead to hyperreflexia by disrupting the normal inhibitory pathways. An asymmetrical response, where one leg reacts differently than the other, also signals localized nerve damage or the early stages of certain progressive neurological diseases. The knee jerk test serves as one component of a comprehensive neurological examination, guiding further diagnostic steps.