Issue: November 2008
November 10, 2008
3 min read

How percussion hammers evolved into reflex hammers

The popular phrase ‘knee-jerk reaction’ has its roots in medicine.

Issue: November 2008
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Before the days of diagnostic imaging, physicians were left to use clues on the outside of the body to guess what was happening on the inside of the body. One of the first diagnostic breakthroughs to aid in this guessing game was the stethoscope. Several decades later another tool was added to the medical arsenal: the reflex hammer. What the stethoscope did for diseases of the chest, the reflex hammer was able to do for the neurological disorders.

In the late 19th century, the idea of reflexes was not necessarily novel. Research in the early 1800s by Marshall Hall had established blinking, sneezing and vomiting as reflexes all controlled by the human nervous system. Hall also came up with the concept of the reflex arc, a neural pathway that mediates all of the body’s reflexes. However, what caused reflexes and what they meant medically had yet to be fully explored.

In 1875, two physicians published papers on the diagnostic utility of a new set of reflexes: deep tendon reflexes. Heinrich Erb and Carl Friedrich Otto Westphal agreed on simultaneous publication of their scientific papers that spoke specifically about the patellar or “knee-jerk” reflex. These papers were the catalyst for widespread research into tendon reflexes. Even today, deep tendon reflexes — or smooth muscle reflexes — are used as a vital part of neurological exams. Eliciting a reflex from tendons in the biceps, triceps, knee, and ankle all convey different meaning.

Shortly after the discovery of these reflexes came debate over the best methods to elicit these responses. Many physicians began to use a tool that they already carried with them: a percussion hammer.

A new use for percussion hammers

Thoracic percussion was developed by Leopold Auenbrugger in 1761 as a way to examine the chest. The technique was adapted from a method his father had used to gauge the level of beer or wine in a barrel. The technique took decades to catch on among the medical community. A hammer was thought to be a more effective tool than a physician’s finger when using thoracic percussion. In 1841, a German physician, Max A. Wintrich, designed the first popularly used percussion hammer. In the following years, many designs were introduced incorporating ebony, whalebone, rubber, brass and many other materials.

Once deep tendon reflexes were discovered, physicians began to use percussion hammers to conduct the test. However, they soon realized that percussion hammers were not ideally suited for the task.

Silas Weir Mitchell, MD, at his clinic in the Infirmary for Nervous Diseases, Philadelphia, with a reflex hammer in hand
Silas Weir Mitchell, MD, at his clinic in the Infirmary for Nervous Diseases, Philadelphia, with a reflex hammer in hand.

Source: The National Library of Medicine

Birth of reflex hammers

The first reflex hammer, as physicians know it today, was developed by John Madison Taylor in 1888. Madison was working as the personal assistant to Silas Weir Mitchell at the Philadelphia Orthopedic Hospital and Infirmary for Nervous Disease. Taylor’s design featured the triangular rubber head that is frequently seen today. The new design was exhibited at the 1888 meeting of the Philadelphia Neurological Society. The minutes from the meeting described the invention as “a cone flattened on the opposite side, with apex and base carefully beveled or rounded, of about the thickness throughout of the human index finger. … The special feature of this hammer is that the shape of the striking surface is like the outer surface of the extended hand, palm downward, which is more often used in obtaining tendon jerk.”

Another popular model of reflex hammer was designed by Williams Christopher Krauss. His hammer had two rounded rubber pieces — one large for the patellar reflexes, one small for biceps reflexes — attached to a metal and rubber handle. Krauss unveiled his design, which incorporated other tools to test sensations as well as reflexes, at the American Neurological Association meeting in 1894. When using this new design “the handle being of hard rubber becomes warm on friction, while the head being of metal remains cold, thus offering the means of examining the sense of heat and the sense of cold.” Beneath the small rubber cap was a triangular “spear,” which could be used to examine for anaesthesia or hyperaesthesia.

Ernst L.O. Tromner’s hammer design had a similar head to that of Krauss’ but the smooth thin handle was made of metal and came to a sharp point at the end. “The smooth handle of the hammer can be easily cleaned and in a pinch may be utilized as a tongue blade. Its sharpened edge, in addition, elicits cutaneous and vascular reflexes.”

Throughout the next several decades the reflex hammer continued to evolve and new designs were continuously being developed. Even today they are a wide variety of reflex hammers so that each physician can have one that fits their needs. So far, the reflex hammer has been one of the few diagnostic tools that has lasted more than 100 years in medicine. In the face of ever evolving and impressive imaging modalities used for neurological exams, the reflex hammer holds fast to its place in medical history. – by Leah Lawrence

For more information:
  • Krauss WC. A neurologist’s percussion hammer. J Nerv Ment Dis. 1894;21:686-688.
  • Lanska DJ. The history of reflex hammers. Neurology. 1989;39:1542-1549.
  • Pinto F. A short history of the reflex hammer. Practical Neurology. 2003;3:366-371.
  • Schiller F. The reflex hammer. Med Hist. 1967;11:75-85.
  • Tromer E. Ein neuer Reflexhammer; zugleigh ein Beitrag zur Pruefung von Sehenreflexen. Berl Klin Wochenschrift. 1910;21:997-998.