Reflex testing is one of the quickest and most revealing parts of a neurological exam, giving clinicians a real-time readout of how well the nervous system is communicating between the brain, spinal cord, and muscles. The basic idea is straightforward: a healthcare provider taps a tendon with a small hammer, and the speed, strength, and symmetry of the muscle’s response tell a surprisingly detailed story about what is happening along that nerve pathway. But reflexes go well beyond the familiar knee jerk. Understanding which reflexes get tested, what the responses actually mean, and why your results might vary from visit to visit can help you make sense of what your doctor is looking for and what the findings suggest.
What Happens When a Tendon Is Tapped
When a reflex hammer strikes a tendon, it briefly stretches the muscle attached to it. Inside that muscle are specialized sensory fibers called muscle spindles, which detect the stretch and fire a rapid signal along a sensory nerve to the spinal cord. At the spinal cord, that signal connects almost directly to a motor nerve, which fires back to the same muscle and causes it to contract. The whole loop takes a fraction of a second and does not require any input from the brain, which is why it is called a reflex. The formation of this circuit depends on the development of those internal sensory fibers during early life; animal research has shown that disrupting their growth essentially eliminates the stretch reflex entirely.1Europe PMC. The role of muscle spindles in the development of the monosynaptic stretch reflex
This simplicity is exactly what makes the test so useful. Because the reflex arc bypasses the brain, testing it isolates specific segments of the spinal cord and the nerves running to and from them. A sluggish or absent response at one joint, paired with a normal response at another, can point to a problem at a specific spinal level. An exaggerated response can suggest the opposite: that the brain’s normal dampening influence on the spinal cord has been lost, often because of damage higher up in the nervous system.
The Major Reflexes and Where to Tap
A standard neurological exam checks several deep tendon reflexes, each corresponding to a different nerve root level in the spinal cord. The most commonly tested ones in the arms and legs cover a broad stretch of the spine from the neck down to the lower back.
- Biceps reflex: The examiner places a thumb over the biceps tendon at the inside of the elbow and strikes it. This tests the nerve roots at the C5 and C6 levels in the neck.
- Brachioradialis reflex: A tap on the forearm just above the wrist tests C5 through C7. The forearm should flex slightly.
- Triceps reflex: A tap on the triceps tendon just above the elbow, with the arm relaxed, tests C7.
- Patellar (knee jerk) reflex: The classic knee tap tests L3 and L4 nerve roots in the lower back. The lower leg should kick forward.
- Achilles (ankle jerk) reflex: Tapping the Achilles tendon above the heel tests the S1 nerve root. The foot should point downward briefly.
When testing the upper extremity reflexes for signs of cervical nerve root compression, research has found these tests to be highly specific. A scoping review of neurological exams for cervical radiculopathy found that biceps, brachioradialis, and triceps reflex testing each had specificities above 90%, and when compared against any cervical root level, specificity reached 99% for the biceps and brachioradialis reflexes.2BioMed Central. Neurological examination for cervical radiculopathy: a scoping review In practice, that means a diminished biceps reflex is unlikely to be a false alarm; if it is truly absent, there is a strong chance something is affecting that particular nerve root.
How Reflexes Are Graded
Clinicians use a simple scale, typically from 0 to 4+, to record what they find. The system is not perfectly standardized across every clinic, but the general framework looks like this:
- 0: No response at all, even with reinforcement techniques.
- 1+: A diminished or trace response. The muscle contracts, but barely.
- 2+: A normal response. This is the baseline most people fall into.
- 3+: A brisker-than-average response, which can be normal in some people but warrants attention.
- 4+: A very brisk, exaggerated response, often accompanied by clonus (rhythmic bouncing of the joint). This is almost always considered abnormal.
The single most important thing a clinician looks for is symmetry. If both knees produce a 3+ response, that may simply be your baseline. If the left knee is 3+ and the right is 1+, that asymmetry is far more clinically significant than the absolute number on either side. Side-to-side differences suggest a localized problem rather than a systemic one.
Why Technique and Tools Matter
Reflex testing looks deceptively simple, but the quality of the result depends heavily on how it is performed. The patient’s positioning, the angle of the strike, the type of hammer used, and whether the patient is truly relaxed all influence the response. Research has emphasized that improper technique can alter the findings enough to cloud the true picture of how the nervous system is functioning.3Europe PMC. Deep Tendon Reflex: The Tools and Techniques. What Surgical Neurology Residents Should Know A patient who tenses up in anticipation of the tap, for instance, can produce a dampened or inconsistent result. That is why examiners often distract the patient with conversation or ask them to look away.
When a reflex seems absent or very weak, clinicians turn to a technique called the Jendrassik maneuver. The patient hooks their fingers together and pulls outward while the examiner taps a lower-extremity tendon. This remote muscle contraction can boost reflex responses enough to reveal a reflex that appeared absent. The mechanism behind why this works has been debated for decades. One study found that physically contracting remote muscles facilitated the patellar reflex, while purely mental activity did not, supporting the idea that the reinforcement works through a reduction in inhibitory signals at the spinal cord level rather than through general arousal.4Europe PMC. Anatomically remote muscle contraction facilitates patellar tendon reflex reinforcement while mental activity does not: a within-participants experimental trial Other research using electrophysiological recordings confirmed that both presynaptic disinhibition and other facilitation mechanisms play a role during similar reinforcement maneuvers.5Elsevier. Standardization of the Jendrassik maneuver in Achilles tendon tap reflex The practical takeaway is simple: if your reflexes seem absent on the first try, a good examiner will ask you to clench before concluding the reflex is truly gone.
What Absent or Diminished Reflexes Suggest
A reflex that is weak or missing (graded 0 or 1+) points toward a problem somewhere in the lower motor neuron pathway, meaning the nerve running from the spinal cord out to the muscle. Common causes include a herniated disc pressing on a nerve root, peripheral neuropathy from conditions like diabetes, or direct nerve injuries. The Achilles reflex at the ankle is a well-known example: it is frequently the first deep tendon reflex to fade in diabetic neuropathy because the longest nerves in the body are the first to be affected. A sluggish or “hung-up” ankle reflex, where the foot points down but returns to its starting position unusually slowly, has also been described as a classic finding in hypothyroidism.6Europe PMC. A classic sign of hypothyroidism: a video demonstration
The pattern of which reflexes are diminished helps narrow the diagnosis. A missing biceps reflex with a normal triceps reflex, for instance, localizes the problem to the C5-C6 nerve root region. A missing ankle jerk with a preserved knee jerk suggests S1 involvement. These patterns are part of why the neurological exam remains such a powerful first-line diagnostic tool, even in an era of advanced imaging.
What Exaggerated Reflexes Mean
Reflexes that are overactive (3+ to 4+) suggest a different kind of problem: damage to the upper motor neuron pathway, the nerve connections running from the brain down through the spinal cord. When those descending pathways are injured, the spinal cord loses the inhibitory signals that normally keep reflexes in check, and the reflexes become exaggerated. Conditions like spinal cord compression, stroke, multiple sclerosis, and amyotrophic lateral sclerosis can all produce hyperreflexia.
At the extreme end, exaggerated reflexes can produce clonus, a rhythmic, involuntary oscillation of a joint when it is stretched. Ankle clonus is the most commonly tested version: the examiner quickly pushes the foot upward and holds it, and the foot bounces repeatedly if clonus is present. Research modeling the mechanics behind clonus has found that it results from a combination of increased reflex gain in the motor neuron pool and changes in the muscle tissue’s stiffness and elasticity, with tissue stiffness playing a particularly strong role in whether clonus appears and persists.7PubMed Central. Clonus is explained from increased reflex gain and enlarged tissue viscoelasticity A few beats of clonus at the ankle can occasionally be normal, especially in anxious or cold patients, but sustained clonus lasting more than a few beats is a red flag for an upper motor neuron lesion.
Pathological Reflexes Beyond the Tendon Tap
Several reflexes that clinicians check are not deep tendon reflexes at all but rather pathological reflexes, meaning responses that should not be present in healthy adults. The most famous is the Babinski sign. To test it, the examiner firmly strokes the sole of the foot from heel toward the toes with a blunt instrument. In a normal adult, the toes curl downward. In someone with damage to the corticospinal tract, the big toe extends upward while the other toes fan out. Joseph Babinski first described this response in 1896, noting it was a consistent finding in patients with lesions of the pyramidal tract and absent in cases of hysterical weakness, making it one of the first reliable ways to distinguish organic neurological disease from functional symptoms.8PubMed Central. History of the extensor plantar response: Babinski and Chaddock signs
In the upper extremities, clinicians check for the Hoffmann sign and the Trömner sign, both of which test for hyperexcitability of the finger flexors. For the Hoffmann sign, the examiner flicks the nail of the patient’s middle finger downward; a positive result is an involuntary flexion of the thumb and index finger. The Trömner sign uses a similar approach, flicking the fingertip from below. Both signs can indicate cervical myelopathy, a condition where the spinal cord in the neck is being compressed. In a study of surgically treated cervical myelopathy patients, a Hoffmann sign was present in about two-thirds of cases, compared to a Babinski sign in only a third, making the Hoffmann sign more sensitive for early detection.9PubMed Central. Clinical correlations of cervical myelopathy and the Hoffmann sign In patients with milder disability, the Hoffmann sign was still present in nearly half, whereas the Babinski sign appeared in only about one in ten. Electrophysiological measurement of the Trömner sign has shown it to be an even more sensitive marker, and the amplitude of the response correlates with the severity of spinal cord compression.10Europe PMC. Quantification of the Trömner signs: a sensitive marker for cervical spondylotic myelopathy
That said, a positive Hoffmann sign on its own is not proof of disease. A systematic review found that the Hoffmann sign has a positive likelihood ratio of about 2.2, meaning it roughly doubles the estimated probability of cervical myelopathy but is not sufficient by itself to confirm or rule out the diagnosis.11PubMed Central. A Systematic Review of the Utility of the Hoffmann Sign for the Diagnosis of Degenerative Cervical Myelopathy About 12% of patients who presented exclusively with lower back complaints also tested positive for the Hoffmann sign, some of whom turned out to have asymptomatic spinal cord compression visible on imaging.9PubMed Central. Clinical correlations of cervical myelopathy and the Hoffmann sign This is why clinicians never rely on a single reflex finding in isolation; they combine it with other exam findings and imaging when needed.
How Age and Fatigue Change Your Reflexes
Reflexes are not static throughout your life. Research measuring the mechanical and electrical properties of tendon reflexes across age groups has found that aging brings a measurable decrease in reflex gain, along with slower muscle contraction rates and longer delays between the stimulus and the response.12Elsevier / PubMed Central. Aging-related neuromuscular changes characterized by tendon reflex system properties This means that mildly diminished reflexes in an 80-year-old may be entirely age-appropriate, while the same finding in a 25-year-old would warrant further investigation. A clinician who sees you for the first time has to interpret your results in the context of your age, your overall health, and the symmetry of your responses.
Muscle fatigue can also temporarily change reflex behavior. Research on elbow extensor fatigue has shown that after sustained exertion, the stretch reflex gain at higher force levels drops significantly.13American Physiological Society. Reflex and intrinsic changes induced by fatigue of human elbow extensor muscles If you have just come from a workout or a physically demanding job, your reflexes may test differently than they would at rest. It is not a concern, but it is one of many reasons why a single reflex test is a snapshot, not a definitive ruling.
Primitive Reflexes and What Their Return Means
Babies are born with a set of reflexes that disappear during normal brain development: the rooting reflex, the grasp reflex, the snout reflex, and others. These primitive reflexes are suppressed as the cortex matures and gains control over lower brain centers. Their persistence in a child beyond the expected age can signal developmental delays or intellectual disability.
At the other end of life, the reappearance of these same reflexes carries a very different clinical meaning. These are sometimes called frontal release signs because they reflect the loss of cortical inhibition over brainstem activity. Their reemergence is frequently observed in older patients with dementia and other neurodegenerative conditions.14CrossRef. Primitive reflexes in developing and adult brain – from intellectual disability to dementia Recent research has even shown that the presence of frontal release signs in cognitively normal older adults may predict future decline, making them a potential early warning sign before memory loss or confusion becomes obvious.15JAMA Network Open. Frontal Release Signs and Future Decline in Research Participants With Intact Cognition Checking for a palmomental reflex (scratching the base of the thumb and watching for a chin twitch) or a grasp reflex during a neurological exam in an older patient is therefore not routine but can provide meaningful information when cognitive decline is a concern.
Reflexes Beyond the Exam Room
The pupillary light reflex is another reflex test most people have experienced without thinking of it as part of the same family. When a clinician shines a light into your eye, the pupil constricts rapidly. This involuntary response tests the integrity of the brainstem pathways and autonomic nervous system. The pupillary light reflex is increasingly recognized as an objective marker of brainstem and autonomic function and is used in everything from concussion assessment to monitoring patients in intensive care.16CrossRef. Video-Based Measurement of the Pupillary Light Reflex for Baseline Neurological Analysis Asymmetry between the two eyes or a sluggish response to light can indicate conditions ranging from optic nerve damage to increased pressure inside the skull.
In more specialized settings, clinicians go beyond what a reflex hammer and flashlight can provide. Electrodiagnostic studies can measure the H reflex, which is the electrical equivalent of the Achilles tendon reflex. By electrically stimulating a nerve and recording the muscle’s response, the H reflex tests the entire length of the nerve, including the segments closest to the spinal cord that standard nerve conduction studies miss. This makes it useful for detecting problems in conditions like radiculopathy and peripheral neuropathy where the site of damage is hard to localize with surface testing alone.17Elsevier / PubMed Central. F wave, A wave, H reflex, and blink reflex F waves, another late electrical response, complement the H reflex by testing motor nerve function along the same full-length pathway. These studies are ordered when the clinical reflex exam raises questions that imaging alone cannot resolve.
A Brief History of the Knee Jerk
The deep tendon reflex exam is so embedded in medicine that it is easy to assume it has been around forever. In fact, the knee jerk was formally introduced to the medical literature in 1875, when two German physicians, Wilhelm Erb and Carl Otto Westphal, independently described it in the same issue of the same journal.18PubMed Central. Erb and Westphal: simultaneous discovery of the deep tendon reflexes There is evidence that the knee jerk response was already well known to non-physicians before that, but Erb and Westphal were the first to recognize its diagnostic value and incorporate it into formal neurological examination. Within a few decades, deep tendon reflex testing had become one of the central pillars of the neurological exam, a status it still holds today. The durability of such a low-tech test, in an era of MRI scanners and nerve conduction machines, says something about how much information a small hammer and a trained eye can actually extract.