Bernhard Berliner, a German neurologist, described his reflex hammer in 1910:
I constructed a hammer which [has a sufficiently heavy head and a large striking surface], and which in every case proved more useful and comfortable for the testing of tendon reflexes than the common percussion hammers. Such a device has not been available commercially and it appears, therefore, justified to introduce this hammer into our instrumentarium. The device has the shape of a hatchet, and an edge covered with rubber, which always strikes the tendon along its total width…It is made of metal and nickel-plated. The handle is tapered at the end and may be used for testing skin reflexes…
Primary reference: Lanska, DJ; Neurology Nov 1989 p1542-9; The History of Reflex Hammers