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Stretch Reflex

Exams and Signs

Historical Overview

  • In the late 1800s, Wilhelm Erb, Joseph Babinski, William Gowers, and others helped develop the neurologic examination as we know it today. Erb was one of the first to emphasize a detailed and systematic neurologic exam and was co-discoverer of the muscle stretch reflex.1
  • Silas Weir Mitchell percussed muscles with a hammer and described the response in 1859, but he did not percuss tendons to elicit the motor response or use the term “reflex” at that time. Jean-Martin Charcot was also partially responsible for recognizing the muscle stretch reflex early, but it was Erb and Westphal that independently and simultaneously brought it to the attention of the medical world in 1875.1
  • In the early 1950s, John Eccles and his colleagues used the stretch reflex as the model to study the physiology of synaptic transmission in the peripheral nervous system.2

Description

  • The stretch reflex—or myotatic reflex—is the contraction of a muscle in response to its passive stretching, which is tested for to evaluate for the presence or severity of certain neurological disorders.

Pathophysiology

  • When a muscle is stretched, the stretch reflex regulates the length of the muscle automatically by increasing its contractility as long as the stretch is within the physiological limits. When a muscle lengthens, the muscle spindle located inside the muscle is stretched, and the rate of neural firing of muscle spindle afferents increases. This augments alpha motor neuron activity in the anterior horn cell pool, causing the muscle fibers to contract and therefore resist the stretching. Another subset of neurons then direct the antagonistic muscles to relax by the mechanism of reciprocal inhibition and in this way the entire reflex process functions to maintain the muscle at a constant length. Gamma motor neurons regulate how sensitive the stretch reflex is by tightening or relaxing the fibers within the spindle.2

Instructions

  1. Obtain an accurate and complete patient history.
  2. Strike the muscle or tendon respective to the reflex being evaluated (eg, for the extensor digitorum reflex, strike the extensor digitorum muscle when the fingers are flexed or half flexed)
  3. Observe the patient’s reaction to the stimulus.

Related Signs and Tests

  • Extensor digitorum reflex
  • Tendon jerk reflex
  • Knee jerk reflex
  • Jaw jerk reflex
  • H-reflex
  • Biceps reflex
  • Brachioradialis reflex
  • Patellar reflex
  • Ankle jerk reflex
  • Triceps reflex

Diagnostic Performance Characteristics

  • Findings from several studies indicate that the magnitude of the stretch reflex increases when subjects are asked to maintain a constant limb position in the presence of a destabilizing load.3

Additional Information

Definition of Positive Result
  • A positive result occurs when the stimulus elicits the appropriate musculoskeletal response (eg, for the extensor digitorum reflex, the patient extends the fingers after the muscle is struck).
Definition of Negative Result
  • A negative result occurs when the stimulus does not elicit the appropriate musculoskeletal response (eg, for the extensor digitorum reflex, the patient does not extend the fingers after the muscle is struck).
Comments and Pearls
  • The stretch reflex is commonly used as a paradigm for assessing spasticity following stroke. During these assessments, a muscle is stretched in a single joint, and the response is measured at that same joint. While these measurements provide valuable information about stretch reflex activity, activation of nonstretched muscles at other joints and, conversely, the influence of proprioceptive feedback from muscles at other joints, have not been extensively studied.4
References
  1. Boes, CJ. The history of examination of reflexes. J Neurol 2014;261(12):2264-74. PMID: 24695995
  2. Bhattacharyya, KB. The stretch reflex and the contributions of C David Marsden. Ann Indian Acad Neurol 2017;20(1):1-4. PMID: 28298835
  3. Maluf, KS, Barry, BK, Riley, ZA, et al. Reflex responsiveness of a human hand muscle when controlling isometric force and joint position. Clin Neurophysiol 2007;118(9):2063-71. PMID: 17646129
  4. Hoffmann, G, Kamper, DG, Kahn, JH, et al. Modulation of stretch reflexes of the finger flexors by sensory feedback from the proximal upper limb poststroke. J Neurophysiol 2009;102(3):1420-9. PMID: 19571191
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