The test can be performed by tapping the triceps tendon[3][note 1] with the sharp end of a reflex hammer while the forearm is hanging loose at a right angle to the arm. A sudden contraction of the triceps muscle causes extension,[note 2] and indicates a normal reflex.[4][5]
Reflex arc
The arc involves the stretch receptors in the triceps tendon, from where the information travels along the radial nerve, through the C7/C8 nerve root to the spinal cord, and the motor signal for contraction returns through the radial nerve.[note 3]
Test indicators
Absence of a reflex (areflexia): If no reflex is elicited then it is essential to try again with reinforcement, with the patient clenching his or her teeth just as the reflex hammer strikes. If true, it can indicate a lower motor neuron lesion or a problem in the neuromuscular junction.
Hyperreflexia (a response far larger than considered normal): Indicates a potential upper motor neuron lesion.
Other medical problems that may cause irregular reflexes include Hyperthyroidism.
Notes
^A tendon is a strip or sheet of connective tissue that transmits the force generated by the contraction of muscle to the bone by attaching with it. Thus, in simple words, a tendon attaches a muscle to a bone
^A straightening at the elbow joint) of the forearm
^A reflex arc is the path in the nervous system through which the sensory information (stimulation—sudden stretching of the triceps tendon in this case) travels to the spinal cord in form of an action potential through a sensory neuron, and comes in contact with a motor neuron at a synapse, which also carries an action potential back to the muscle of origin and results in its (triceps's) contraction. Note that the contact between the sensory and motor neurons does not require immediate involvement of the brain making it involuntary, though the sensation of stretch (as the hammer hits the skin and tendon beneath) and contraction of the muscle are both sent to the brain making the subject aware of the entire testing procedure.
