The knee jerk occupies an important place as a routine diagnostic procedure. It is a reflex that is elicited in the majority of normal people with ease, but in a small and disturbing minority with more or less difficulty. In most cases it is the patient's inability voluntarily to relax the quadriceps muscle group that interferes with the proper response. It is self-evident that the knee jerk cannot take place unless the quadriceps group is at least partially relaxed.
Many tricks have been devised with the object of "reenforcing" the knee jerk by diverting the patient's mind from his quadriceps, thus permitting it to relax. The method most commonly employed is the so-called "Jendrassik's method"1 in which the patient hooks both hands together, pulling them against one another, and looks up toward the ceiling, thereby shifting the focus of his attention and relaxing the muscles of the leg. In Laufenauer's
James M. Faulkner. A SIMPLE AND EFFECTIVE METHOD OF REENFORCING THE KNEE JERK. Arch NeurPsych. 1932;28(4):895–896. doi:10.1001/archneurpsyc.1932.02240040140009