If you haven't experienced it in person, you have likely seen it on television: The doctor in the examining room taps the area just below your kneecap with a small rubber mallet, and your lower leg responds with a "knee-jerk" reaction. Assessing the deep tendon reflex has been a part of medical exams for generations. Understanding the physiology behind it will help you grasp its relevance to your overall health.
Tendon Reflex Tests
Tendon reflex assessments give doctors valuable information about the neuromuscular system. Susan Spinasanta, senior medical editor for SpineUniverse, explains that the absence of reflex responses can indicate damage to the spinal cord, nerve root or muscle. A 2009 study in "Neurology Asia," led by Kheng Seang Lim, set out to establish the normal range of tendon reflex in adults. Using 176 subjects, 90 over age 40 and 86 under age 40, researchers noted a wide range in reflex responses in healthy individuals, and especially noted that asymmetrical responses or absence of response was more prevalent in the elderly. Absent or asymmetrical responses may indicate a partial disconnect between the local muscle and nerve and the central nervous system.
Reflex Stimulus
A series of reactions must occur to elicit a reflex. Muscle spindles are receptors that lie in parallel with muscle fibers. When the muscle spindle is stretched, as it is when tapped by the rubber mallet, the afferent neuron stimulates the muscle spindle, causing a contraction of the muscle. Reflex responses are graded on a scale of zero to four, with zero being no response, two being a brisk normal response, and four being an abnormal repeating reflex. Extreme responses at either end of the scale are considered indicators of a disorder.
Reflex Test Sites
While the typical medical exam only tests reflex response at the patellar tendon, many other sites give useful feedback on damage or injury to correlating nerves that run the length of the spine. The biceps, forearm, triceps, abdomen, quadriceps and ankle are all potential test sites for tendon reflex. The location with an abnormal response may indicate spinal disorders in the area of the associated vertebrae. For example, the biceps tendon corresponds with cervical vertebrae C5 and C6.
Indications
In addition to providing information about injury or deterioration of the spine, the tendon reflex is a useful tool for assessing the progress of a physical therapy patient recovering from injury or surgery, or the improvements made from training by a recovering athlete. While reflex responses have traditionally been assessed by administrator observations, new devices are being developed that measure neural stimulus and corresponding reflex through a surface EMG device, giving a more accurate assessment.
