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Biceps reflex

The biceps reflex is a monosynaptic deep tendon reflex elicited by tapping the biceps brachii tendon in the antecubital fossa, which causes a brief contraction of the biceps muscle and flexion of the forearm at the elbow joint.^[1] This reflex arc involves sensory input from muscle spindles via Ia afferent fibers and motor output through alpha motoneurons, primarily testing the integrity of the C5 and C6 spinal nerve roots supplied by the musculocutaneous nerve.^[1] It serves as a fundamental component of the neurological examination to assess upper motor neuron and lower motor neuron function in the upper limb.^[2] In anatomical terms, the biceps brachii muscle, located on the anterior aspect of the upper arm, originates from the coracoid process of the scapula and the supraglenoid tubercle, inserting via its tendon into the radial tuberosity and bicipital aponeurosis.^[1] The reflex is potentiated by slight muscle stretch and can be enhanced using the Jendrassik maneuver, where the patient clenches their fists to increase gamma motoneuron activity and overall reflex excitability.^[3] Physiologically, the response is rapid due to the direct synaptic connection in the spinal cord, bypassing higher brain centers, and it helps maintain muscle tone and posture against sudden stretches.^[2] Clinically, the biceps reflex is tested by supporting the patient's forearm in a partially flexed position and delivering a brisk tap to the tendon with a reflex hammer, observing for symmetric forearm flexion.^[2] Responses are graded on a 0 to 4+ scale, where 2+ indicates a normal brisk response, 0 signifies absence (suggesting lower motor neuron lesions like peripheral neuropathy or C5-C6 radiculopathy), and 4+ denotes hyperreflexia with clonus (indicating upper motor neuron pathology such as stroke or multiple sclerosis).^[1] Abnormalities in this reflex can also correlate with conditions like anxiety, hyperthyroidism, or muscular dystrophies, often appearing before overt weakness.^[3]

Anatomy

Biceps brachii muscle and tendon

The biceps brachii is a two-headed muscle located in the anterior compartment of the upper arm, consisting of the long head and the short head, which together form its fusiform belly. The long head originates from the supraglenoid tubercle of the scapula and the superior glenoid labrum, while the short head arises from the coracoid process of the scapula. These origins allow the muscle to span both the shoulder and elbow joints, contributing to its role in upper limb flexion. The muscle is innervated by the musculocutaneous nerve. The converged muscle bellies insert distally via the biceps tendon onto the radial tuberosity of the radius, with the short head attaching more distally at the apex and the long head proximally. An additional insertion occurs through the bicipital aponeurosis, a broad, flat tendon that extends medially to blend with the fascia overlying the forearm flexors. The distal biceps tendon, situated in the cubital fossa, represents the key anatomical site involved in the reflex response due to its superficial position and accessibility. In terms of gross anatomy, the biceps brachii features parallel-arranged muscle fibers within its fusiform structure, enabling substantial excursion and force production without significant pennation. This fiber orientation optimizes the muscle's length-tension relationship for elbow flexion and forearm supination. The associated tendon is composed of dense regular connective tissue, predominantly type I collagen bundles organized in a hierarchical fashion to withstand high tensile loads during contraction.

Innervation and spinal segments

The biceps brachii muscle receives its primary motor innervation from the musculocutaneous nerve, a terminal branch of the lateral cord of the brachial plexus that originates from the ventral rami of spinal nerves C5, C6, and C7.^[4] The musculocutaneous nerve pierces the coracobrachialis muscle and travels distally between the biceps brachii and brachialis muscles, providing motor fibers directly to the biceps brachii to facilitate elbow flexion and supination.^[5] While the C5 and C6 roots contribute the majority of fibers to this innervation, there is a minor contribution from C7, particularly through the lateral cord's formation.^[6] The sensory afferents for the biceps reflex arise from Ia proprioceptive fibers within the muscle spindles of the biceps brachii muscle, which detect stretch and transmit signals via the dorsal roots of spinal nerves C5 and C6.^[1] These Ia fibers, characterized by their large diameter and rapid conduction velocity, enter the spinal cord at the C5-C6 segments to synapse directly with alpha motor neurons in the reflex arc.^[7] The primary spinal segments involved in the biceps reflex arc are thus C5 and C6, with any C7 involvement limited to supportive motor outflow rather than core sensory or reflex mediation.^[8] The brachial plexus, relevant to biceps innervation, is structured from the anterior divisions of the C5-C7 roots, which unite to form the lateral cord and subsequently the musculocutaneous nerve, ensuring coordinated neural supply to the anterior arm compartment.^[9] This segmental organization underscores the reflex's reliance on upper cervical spinal integrity for normal function.^[10]

Physiology

Muscle stretch reflex

The muscle stretch reflex, also known as the myotatic reflex, is an automatic, involuntary response that causes contraction of a muscle in reaction to its sudden lengthening, helping to resist the stretch and maintain muscle length.^[7] This reflex operates at the spinal cord level without requiring input from higher brain centers, ensuring rapid protection against excessive muscle extension.^[11] Central to this reflex are muscle spindles, specialized sensory organs embedded within the skeletal muscle that detect changes in muscle length and the rate of lengthening.^[12] These spindles consist of intrafusal muscle fibers surrounded by a connective tissue capsule, with sensory endings that monitor stretch.^[13] When the muscle is stretched, the intrafusal fibers are deformed, activating primary (Ia) afferent fibers from annulospiral endings, which are highly sensitive to both the velocity and magnitude of stretch, and secondary (group II) afferent fibers from flower-spray endings, which primarily respond to sustained length changes.^[12] These afferents transmit signals via the dorsal root to the spinal cord. The reflex arc is monosynaptic, characterized by a direct synaptic connection between the Ia afferent fibers and alpha motor neurons in the ventral horn of the spinal cord, bypassing interneurons for a swift response.^[7] This direct pathway results in excitation of the agonist muscle's alpha motor neurons, leading to its contraction, while Ia afferents also inhibit antagonist muscles via polysynaptic connections involving inhibitory interneurons.^[12] The muscle stretch reflex plays a crucial role in maintaining muscle tone, posture, and smooth movement by providing continuous feedback on muscle length during daily activities.^[11] Its sensitivity is modulated by gamma motor neurons, which innervate the intrafusal fibers and adjust the spindles' responsiveness to stretch, allowing adaptation to varying loads and voluntary movements.^[14] For instance, in the biceps brachii muscle, this mechanism contributes to arm stability during extension.^[7]

Neural pathway of the biceps reflex

The neural pathway of the biceps reflex is a classic example of a monosynaptic stretch reflex arc, involving a direct connection between sensory and motor neurons in the spinal cord.^[7] The afferent limb begins with the activation of muscle spindles in the biceps brachii muscle, which detect rapid stretch when the tendon is tapped. These spindles contain Ia afferent fibers that respond to the change in muscle length, generating action potentials that travel peripherally from the muscle spindles via the musculocutaneous nerve and centrally through the dorsal roots to enter the spinal cord at segments C5 and C6. The cell bodies of these Ia afferents are located in the dorsal root ganglia, and the central processes of the fibers enter the spinal cord via the dorsal roots, projecting to the ventral horn where they synapse directly with alpha motor neurons.^[5]^[7] Within the spinal cord, the Ia afferents form a direct monosynaptic connection with alpha motor neurons located in the ventral horn at the same C5-C6 levels, bypassing any interneurons in the core reflex arc. This synapse occurs in lamina IX of the ventral horn, where excitatory neurotransmitters, primarily glutamate, are released to depolarize the motor neurons and initiate an action potential.^[5]^[7] The efferent limb consists of the alpha motor neurons projecting axons out through the ventral roots of C5 and C6, rejoining the musculocutaneous nerve to innervate the biceps brachii muscle fibers. This results in a rapid contraction of the biceps, producing elbow flexion and counteracting the initial stretch. The entire reflex is ipsilateral, occurring on the same side of the body as the stimulus.^[5]^[7] In addition to the direct excitatory pathway, a branch of the Ia afferent activates an inhibitory interneuron that synapses with alpha motor neurons innervating the antagonist triceps brachii muscle (via C7-C8 segments), facilitating reciprocal inhibition to prevent co-contraction and ensure smooth movement.^[5]^[7]

Clinical Examination

Eliciting the reflex

The biceps reflex test is performed in a clinical setting to assess the integrity of the C5 and C6 spinal nerve roots and associated neural pathways. The patient is positioned seated on the edge of an examination table or bed, or supine if preferred, with the arm relaxed and the elbow slightly flexed to approximately 90 degrees; the forearm is supported by resting it on the patient's thigh or the examiner's arm to maintain a midway position between full flexion and extension.^[2]^[1]^[8] To elicit the reflex, the examiner locates the biceps tendon in the cubital fossa by palpating the antecubital fossa; the examiner's thumb is placed firmly over the tendon with fingers curling around the elbow for stabilization, and a brisk tap is delivered directly to the tendon using a reflex hammer, employing a quick wrist flick rather than arm motion to generate the appropriate stretch stimulus.^[2]^[1] Standard equipment includes a Taylor, Tomahawk, or Queen Square reflex hammer, which delivers a precise percussive force of 80-140 grams to avoid excessive impact.^[1] A normal response involves visible or palpable contraction of the biceps brachii muscle, resulting in brief flexion of the forearm at the elbow.^[2] Precautions are essential to ensure accurate assessment: the patient must be fully relaxed, which can be facilitated by engaging them in conversation, having them count backward, or performing a distracting task to minimize voluntary muscle contraction or conscious inhibition that could dampen the reflex.^[2]^[8] If the reflex is absent or weak, reinforcement techniques such as the Jendrassik maneuver—where the patient interlocks their fingers and pulls against resistance—may be employed to enhance the response.^[2]^[1] The reflex is always tested bilaterally for comparison, starting with the unaffected side if asymmetry is suspected, to detect subtle differences in reactivity.^[2]^[8]

Grading the response

The biceps reflex, as a deep tendon reflex, is typically graded using a standardized scale from 0 to 4+, which assesses the response's presence, amplitude, and any associated clonus following elicitation by tapping the biceps tendon.^[2] This scale, widely adopted in clinical neurology, provides a qualitative measure of reflex integrity without requiring specialized equipment.^[8] Grade 0 indicates an absent reflex, where no muscle contraction occurs despite adequate stimulation.^[2] Grade 1+ denotes a trace or diminished response, often detectable only with reinforcement techniques such as the Jendrassik maneuver.^[8] Grade 2+ represents a normal, brisk contraction that is symmetric and appropriate in amplitude.^[2] Grade 3+ signifies an increased or very brisk response, exceeding normal amplitude but without clonus.^[8] Grade 4+ indicates a very brisk response with clonus (repeating reflex contractions).^[2] Several factors can influence the grading of the biceps reflex, including patient age, which often leads to slightly diminished responses due to age-related declines in muscle fiber number and neural efficiency; baseline muscle tone, where heightened tone may amplify the reflex while hypotonia dampens it; and examiner experience, as variations in tap force or observation can alter perceived amplitude.^[15]^[16] Inter-rater variability in grading deep tendon reflexes, including the biceps reflex, is a recognized challenge, with studies showing moderate to limited agreement among examiners due to subjective interpretation of response strength and duration.^[17] Standardization efforts in neurological exams emphasize consistent use of the 0-4+ scale, training protocols for uniform elicitation, and adjunctive tools like reinforcement maneuvers to improve reliability across practitioners.^[18]

Clinical Significance

Normal response

In healthy individuals, the biceps reflex manifests as a brisk contraction of the biceps brachii muscle upon tapping the biceps tendon, producing a visible or palpable twitch and slight flexion of the elbow joint. This monosynaptic stretch reflex involves activation of muscle spindles, leading to an immediate alpha motor neuron discharge via the C5-C6 spinal segments. The response is typically brisk and proportionate, reflecting intact neural pathways without exaggeration or diminution.^[2]^[5] Symmetry between the left and right sides is a hallmark of the normal biceps reflex in most adults, with bilateral responses generally equal in amplitude and speed. Slight asymmetry may occur naturally due to minor variations in muscle tone or positioning, but significant differences warrant further evaluation. This bilateral equality helps clinicians assess overall neuromuscular integrity.^[5]^[8] Age-related changes influence the biceps reflex, with responses often brisker and more pronounced in young adults compared to the elderly, where diminution is common. Starting in the fifth decade, reflex amplitude progressively declines due to degenerative changes in muscle spindle afferents and intrafusal fibers, with studies showing reductions approaching approximately 50% of young adult levels in deep tendon reflexes by age 65. These alterations stem from reduced density of sensory neurons and functional impairments in the proprioceptive arc.^[19] Normal variability in the biceps reflex can arise from physiological factors such as fatigue, which reduces response amplitude by impairing motor neuron excitability, or anxiety, which may enhance the reflex through heightened sympathetic activity and adrenaline release. Reinforcement techniques, like clenching the fists, can temporarily enhance these responses in fatigued states, restoring typical briskness. A normal response aligns with a 2+ grade on standard reflex scales.^[5]^[20]

Abnormal responses and associated conditions

Abnormal responses in the biceps reflex deviate from the typical brisk contraction observed in healthy individuals (graded 2+). Hyperreflexia, characterized by an exaggerated response graded 3+ or 4+, often with clonus, indicates disruption of upper motor neuron pathways, such as those from lesions in the brain, brainstem, or spinal cord above the C5-C6 segments.^[1] Common associated conditions include stroke, where infarction interrupts corticospinal tracts leading to increased reflex excitability; multiple sclerosis, involving demyelination that impairs inhibitory signals; and cervical spinal cord injury above C5, which severs descending motor control and results in spastic hyperreflexia.^[1]^[1]^[5] In contrast, hyporeflexia (graded 1+) or areflexia (graded 0), marked by diminished or absent contraction, suggests lower motor neuron involvement at or below the C5-C6 level, affecting the afferent or efferent arcs of the reflex.^[5] This can arise from C5-C6 radiculopathy due to compression from herniated discs or spondylosis, leading to weakened biceps response alongside arm weakness and sensory loss.^[21] Brachial plexopathy, often from trauma or inflammation, impairs the musculocutaneous nerve pathway, resulting in reduced or absent biceps reflex with associated shoulder and elbow dysfunction.^[1] Peripheral neuropathies, such as those from diabetes or alcoholism, further contribute by damaging sensory afferents, causing bilateral hyporeflexia.^[1] Asymmetry in biceps reflex responses, where one side shows hyper- or hyporeflexia compared to the other, points to focal unilateral lesions rather than systemic issues.^[5] For instance, a herniated cervical disc at C5-C6 may unilaterally diminish the reflex on the affected side due to nerve root impingement, often accompanied by radicular pain.^[21] Traumatic injuries, such as those causing brachial plexus damage, can produce asymmetric areflexia with localized weakness and sensory deficits.^[1] In broader clinical contexts, these abnormalities aid in diagnosing progressive neurodegenerative or inflammatory conditions. Amyotrophic lateral sclerosis (ALS) frequently presents with hyperreflexia in the biceps due to upper motor neuron degeneration, though lower motor neuron involvement may later cause mixed findings.^[22] Guillain-Barré syndrome typically features areflexia, including in the biceps, from acute peripheral demyelination leading to widespread weakness.^[23] Hypothyroidism can induce hyporeflexia with delayed relaxation phase in deep tendon reflexes like the biceps, stemming from metabolic effects on muscle and nerve function.^[1]

References

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Physiology, Deep Tendon Reflexes - StatPearls - NCBI Bookshelf
Aug 28, 2023 · Biceps Reflex. Muscle involved: biceps brachii. Nerve supply: musculocutaneous. Segmental innervation: C5-C6 ; Brachioradialis Reflex. Muscle ...
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Brachioradialis reflex: flexion and supination of the forearm. Biceps reflex: flexion of the forearm. You will feel the biceps tendon contract if the biceps ...
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Biceps reflex increased with lesions at or above C3/C4. Other causes: Anxiety ... Whole body vibration: Reduces H-reflex; High amplitude stimulus.
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Aug 11, 2025 · The musculocutaneous nerve is the terminal branch of the lateral cord of the brachial plexus (C5, C6 and C7) and emerges at the inferior ...
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Anatomy and Physiology · Biceps: C5-C6, musculocutaneous, just anterior to the elbow · Triceps: C7-C8 (predominantly 7), radial, just posterior to the elbow.Missing: definition | Show results with:definition
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Overview, Gross Anatomy, Blood Supply of the Brachial Plexus
Oct 28, 2024 · The musculocutaneous nerve originates from the lateral cord of the brachial plexus, typically arising from spinal nerves C5 to C7. The ...<|control11|><|separator|>
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Biceps reflex C5/C6: This is performed by striking the biceps tendon within the cubital fossa. This will cause elbow flexion. Brachioradialis reflex C6: ...Microanatomy · Mechanism · Clinical Testing Of Reflexes
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Gamma motor neurons activate the intrafusal muscle fibres, changing the resting firing rate and stretch-sensitivity of the afferents. Consist of several ...Missing: monosynaptic | Show results with:monosynaptic
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By tuning both the dynamic and static sensitivity of muscle spindle afferents to stretch, γ motor neurons play an important role in motor control, locomotion, ...
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Mar 8, 2024 · Individual factors: There are individual differences in DTR, with some healthy individuals having hyperreflexia and others having hyporeflexia.
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Age: Reflexes tend to change with age, often becoming slightly less brisk. Patient Preparation: Anxiety or muscle tension can affect the reflex response.<|control11|><|separator|>
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Limitations of DTRs are: qualitative nature of the assessments based upon subjective grading, and limited inter-rater reliability. This preliminary study was ...
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To assess the reliability of DTR measurements, we examined inter-rater reliability for the patellar tendon reflex of the knee joint, one of the most widely ...
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Aging affects deep tendon reflexes at different levels of the proprioceptive reflex arc, including degenerative changes in muscle spindle receptors, functional ...
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Aug 13, 2025 · Enhancing muscle spindle sensitivity during periods of stress or uncertainty may represent some distinctive protective mechanism to maintain or ...
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Guillain-Barré syndrome (GBS) is characterized by rapidly evolving ascending weakness, mild sensory loss and hypo- or areflexia, progressing to a nadir over up ...