Knee Jerk: The Origins of the Reflex Hammer

As you probably already know, the popular phrase ‘knee-jerk reaction’ has its roots in medicine. Before the convenience of technology and diagnostic imaging, medical professionals often needed to ascertain what was happening internally by examining external clues. The stethoscope was one of the first tools developed to assist in this process, and the reflex hammer followed shortly after. Reflexes first came into the spotlight in the early 1800s, when research by Marshall Hall established that blinking, sneezing and vomiting were reflexes controlled by the human nervous system. Later, in 1875, two physicians, Heinrich Erb and Carl Friedrich Otto Westphal, published papers on the clinical utility of the muscle stretch reflex - specifically the patellar or “knee-jerk” reflex. These papers sparked widespread research, and smooth muscle reflexes became a vital part of neurological exams. As they incorporated this new field of study into their practices, many physicians used a tool that they already carried with them: a percussion hammer - a tool used to examine the chest. However, the design proved to be less than perfect in its new application, and other physicians found resourceful ways to incorporate entirely different tools. In the 1880's, William Gowers coined the phrase "myotatic reflex" for the knee-jerk reflex, and he recommended striking the patellar tendon with the rubber edge of the end of a stethoscope. Alluding to far more unorthodox methods, Bernhard Berliner is quoted as having said "it is not very elegant to percuss the knee or achilles tendon with a paper weight, the edge of a large electrode, the foot of a laboratory stand, a table lamp, or similar devices." Luckily for stethoscopes, table lamps, and patients' knees, the first official reflex hammer was designed in 1888 by neurologist John Madison Taylor. Taylor was working as the personal assistant to Silas Weir Mitchell at the Philadelphia Orthopedic Hospital and Infirmary for Nervous Disease, and his design featured the triangular rubber head that is frequently seen today. John Madison Taylor's reflex hammer 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.” The Taylor hammer was subsequently praised and popularized by many of the founding fathers of American neurology, including Mitchell, who used it in his neurological assessments of Civil War veterans, and Charles Mills, who considered it “the best hammer for tapping the much-abused patellar ligament.” It was later incorporated into the original logo of the American Academy of Neurology.