The result of a tendon reflex response is an involuntary, rapid contraction of the muscle attached to that tendon. When a healthcare provider taps your tendon with a reflex hammer, the muscle stretches briefly, triggering an automatic contraction that happens in as little as 17 to 21 milliseconds. This is one of the fastest reactions your nervous system can produce, and it happens entirely without input from your brain.
What Happens Inside Your Body
A tendon reflex is powered by one of the simplest circuits in your nervous system, called a monosynaptic reflex arc. “Monosynaptic” means the signal only crosses one connection point, which is why it’s so fast. Here’s the sequence: when the reflex hammer strikes, it briefly stretches the tendon and the muscle it’s attached to. Tiny sensors inside the muscle, called muscle spindles, detect both the speed and magnitude of that stretch. They fire off a signal along a sensory nerve fiber that travels to the spinal cord.
Once the signal reaches the spinal cord, it crosses a single synapse and activates a motor neuron in the front portion of the cord. That motor neuron sends a signal straight back to the same muscle, telling it to contract. The entire loop, from tap to twitch, bypasses your brain completely. Your brain becomes aware of it only after the contraction has already happened.
The Muscle Contracts and Its Opposite Relaxes
The visible result of the reflex is the contraction of the target muscle. In a knee-jerk test, for example, the tap stretches the quadriceps muscle on the front of your thigh, which then contracts and kicks your lower leg forward. But there’s a second, less obvious result happening at the same time: the opposing muscle group relaxes.
This process is called reciprocal inhibition. When the sensory signal arrives in the spinal cord, it branches. One branch activates the motor neuron for the stretched muscle. The other branch activates an inhibitory nerve cell, which suppresses the motor neuron controlling the opposing muscle. In the knee-jerk example, this means the hamstrings on the back of your thigh are actively prevented from contracting. Without this coordinated relaxation, the opposing muscle would fight against the reflex and the leg wouldn’t move smoothly.
Common Tendon Reflexes and What They Test
Different tendon reflexes correspond to specific spinal cord levels, which is why clinicians test several of them during a neurological exam. The biceps reflex (tapping at the inside of your elbow) tests the nerve pathways at the C5 and C6 levels of the cervical spine. The triceps reflex (tapping behind the elbow) tests C7. The patellar reflex at the knee tests the lumbar spine, and the Achilles reflex at the ankle tests the lower lumbar and sacral levels.
Each reflex gives a window into the health of a specific segment of the spinal cord and its associated nerves. If one reflex is absent while others are normal, it helps pinpoint where a problem might be.
How Reflex Responses Are Graded
Clinicians score tendon reflexes on a standardized 0 to 4 scale:
- 0: No response at all. Always abnormal.
- 1+: A slight but detectable response. May or may not be normal.
- 2+: A brisk response. This is considered normal.
- 3+: A very brisk response. May or may not be normal.
- 4+: The tap triggers a repeating, rhythmic contraction called clonus. Always abnormal.
A normal reflex (2+) tells you the sensory nerve, spinal cord segment, and motor nerve are all functioning properly. It’s the absence or exaggeration of the response that carries diagnostic meaning.
What Abnormal Results Indicate
Diminished or absent reflexes (hyporeflexia) point to a problem somewhere along the reflex arc itself, specifically in the peripheral nerves or the lower motor neurons that exit the spinal cord. Conditions that damage these areas, such as herniated discs compressing a nerve root or peripheral neuropathy, produce weak or missing reflexes along with muscle wasting, twitching, and flaccid (floppy) weakness.
Exaggerated reflexes (hyperreflexia) suggest the opposite type of problem. The reflex arc itself is intact, but the brain’s ability to dampen it is impaired. Normally, signals descending from the brain modulate how strongly reflexes fire. When those descending pathways are damaged by conditions like spinal cord injury, stroke, or multiple sclerosis, the reflexes become overactive. This produces very brisk responses, increased muscle tone, and stiffness rather than floppiness. The distinction between these two patterns is one of the most fundamental tools in neurological diagnosis.
Factors That Change the Result
Reflex responses aren’t fixed. Anxiety, muscle tension, and even how relaxed you are during the exam can all affect how strong the reflex appears. If a clinician has trouble getting a response, they may ask you to perform the Jendrassik maneuver: hooking your fingers together and pulling hard, or clenching your teeth, while the reflex is being tested. This technique has been used since the 19th century to reinforce lower limb reflexes.
The maneuver works through at least two mechanisms. Clenching your teeth appears to reduce the spinal cord’s natural suppression of incoming sensory signals, essentially turning up the volume on the stretch signal. Pulling with your hands adds a second effect by slightly increasing the baseline sensitivity of the muscle spindles themselves. Either way, the result is a stronger, more visible reflex response, helping the clinician distinguish between a genuinely absent reflex and one that’s simply hard to elicit in a tense patient.
Temperature, age, and medications can also shift reflex responses. Cold muscles tend to produce sluggish reflexes, while certain drugs that affect the nervous system can either dampen or amplify them. This is why clinicians interpret reflex results in context rather than treating any single score as a definitive diagnosis.