Hektoen International

A Journal of Medical Humanities

Origins of the knee jerk

JMS Pearce 
East Yorks, England


Wilhelm Heinrich Erb who researched the knee jerk reflex
Wilhelm Heinrich Erb. By F. Langbein & Cie. Heidelberg. 1897. Universitätsbibliothek Heidelberg. Via Wikimedia. CC BY-SA 4.0

Reflex hammers are the icon or hallmark of every neurologist. How important are the reflexes they elicit? What is their mechanism?

The advent of modern technology has made it easy to forget how important the skills and means in eliciting physical signs were to clinicians of the nineteenth and early twentieth centuries. Though clinical history is the most important part of neurological diagnosis, an essential part of physical examination is tendon reflex testing,1,2 which distinguishes reliably upper from lower motor neuron lesions. It provides localizing diagnostic information that may not be obtained by any other method, including clinical neurophysiological and neuroradiological investigations. More specifically, they “can give information about patients’ brain functioning that may not be revealed by interviews, imaging, or through performance of deliberate tasks.”


The nature of reflexes

Tendon reflexes depend on a reflex contraction of a muscle in response to stretching. As long ago as 1763 Robert Whytt, noting that isolated muscle would twitch after stimulation, demonstrated the salient physiology of stretching muscle fibers. Contraction varied with the strength of stimulus:

Numberless experiments and observations show pricking a muscle causes it to contract, . . . but whatever stretches the fibers of any muscle so far as to extend them beyond their usual length, excites them into contraction in about the same manner, as if they had been irritated by a sharp instrument, or acrid liquor. The motion of stretching the fibers of any muscle will be greater or less, as the muscle is more or less stretched; unless it be so extended as quite to lose its tone and become paralytic.4

Over a century later when the eminent German neurologists Erb and Westphal in 1875 described the clinical value of the tendon or muscle stretch reflexes they unwittingly initiated the history of reflex hammers and the testing of a wide range of tendon reflexes.

Erb studied reflexes using his finger or a percussion hammer:

Slightly flexed at the hip and knee, the leg is held fast while all its muscles are relaxed and, with a finger or percussion hammer, a very light and elastic tap is delivered (exactly as in very light and elastic percussion of the chest or in testing for fluid in the abdomen) to the region of the ligamentum patellae. Each tap is followed by a contraction of the quadriceps, immediate like lightning, unmistakable, visible, palpable, and apparently reflex: manifestly and often quite strongly is the shank set in motion. It is extraordinarily difficult to suppress this reflex by a voluntary effort . . .

Carl Westphal who researched the knee jerk reflex
Carl Westphal. By G. Engelbach (Lithograph). circa 1860. Via Wikimedia

The separate structure and functions of sensory and motor nerves were the result of the nineteenth century critical experiments of Sir Charles Bell 5 and Magendie.6 In the same era, Marshall Hall’s experiments executed in his own home led to his discovery of the reflex arc. It was based on what he called an “excito-motory system”7,8 of the spinal cord and nerves. Marshall Hall introduced the term diastaltic, referring to the reflex contraction operating through the spinal cord. The later extension of these principles developed into the clinically elicited reflexes;9 but physiologically they were vital to the understanding of the muscle stretch (myotatic) reflex shown in the 1920s by Liddell and Sherrington.10,11 These theories about reflexes developed into the clinical observations of both tendon and cutaneous reflexes.

Erb and Westphal’s papers in 1875 on the tendon or muscle stretch reflexes started the vogue for reflex hammers. Heinrich Erb (1840-1921) had started to use percussion in about 1870.12 He submitted his paper “Patellarsehnenreflex” (patellar tendon reflex) to the German Archives of Psychiatry and Nervous Diseases.13 But he quickly discovered that its editor, Carl Friedrich Westphal, (1833-90) was preparing a paper14 on the same subject. Whereas Erb believed the knee jerk was a simple reflex caused by excitation of sensory nerves within the tendon, Westphal thought the knee phenomenon resulted from local excitation of the quadriceps muscle. Fortunately, prudence superseded rivalry. They collaborated so that both papers were published consecutively in the same issue, Erb’s preceding Westphal’s. Erb advised eliciting the patellar tendon reflex by striking the tendon with the finger or a percussion hammer. Westphal’s technique was similar—by percussing the patellar ligament:

. . . Lightly but . . . quickly with a finger—best when the index finger is put on top of the middle finger and the former is accelerated downwards—or even more effectively with a percussion hammer.

Interestingly, in terms of priority Sir William Gowers stated:

[The knee jerk] was first systematically studied by Erb and Westphal, but previously partially recognized and employed in diagnosis by Charcot.15

Gowers was quick to take up this technique, describing in detail the knee and ankle jerks, clonus, and their physiology. In the 1880’s he coined and applied the phrase “myotatic reflex” for the knee-jerk and recommended striking the patellar tendon with either the side of the hand or the rubber edge of the end of a stethoscope.

There soon followed many such tools with variations of weight, shape, and size, with their inventors’ names appended for posterity.


Read more on tendon reflex hammers. 



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  3. Sanders RD, Gillig PM. Reflexes in psychiatry. Innov Clin Neurosci. 2011;8(4):24-29.
  4. Whytt R. On the vital and involuntary movements of animals. Edinburgh. 1763. cited by Jefferson G. In: Selected papers of Sir Geoffrey Jefferson. pp. 73-93. London: Pitman. 1960
  5. Bell C. Idea of a new anatomy of the brain submitted for the observations of his friends. London. Straham and Preston 1811 (private circulation). Facsimile reprinted, Dawsons of Pall Mall, London 1966.
  6. Magendie F. Expériences sur les fonctiones des racines des nerfs rachidienes. J Physiol Exp Pathol 1822; 2:276.
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  8. Hall M. Memoirs on the nervous system. Memoir II. On the true spinal marrow and the excito-motory system. London Sherwood, Gilbert and Piper, 1837.
  9. Pearce JMS. Marshall Hall and the concepts of reflex action. J Neurol Neurosurg Psychiatry. 1997; 62(3): 228.
  10. Liddell EGT, Sherrington CS. Reflexes in response to Stretch. Proceedings of the Royal Society 1924;96 B:212-5.
  11. Sherrington CS. Integrative action of the nervous system. New Haven, Yale 1906. Revised 1947.
  12. Tyler KL, C. McHenry LC. Fragments of Neurological History, The Knee Jerk and Other Tendon Reflexes. Neurology 1983; 33 (5): 609.
  13. Erb WH. Uber Sehnenreflexe bei Gesunden und bei Ruckenmarkskranken. Arch Psychiat Nervenkr 1875;5:792-802.
  14. Westphal CF. Uber einige durch mechanische Einwirkung auf Sehnen und Muskeln hervogebrachte Bewegungs-Erscheinungen. Arch Psychiatry Nervenkr 1875;5:803-34.
  15. Gowers WR. Diagnosis of diseases of the brain and spinal cord. New York, Wm. Wood & Co, 1885 p. 222-3.



JMS PEARCE, MD, FRCP, is a retired neurologist and author with a particular interest in the history of science and medicine.


Summer 2020  |  Sections  |  Neurology

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