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There is considerable normal individual variation in the degree of the flexion angle of the knee at which the patellar reflex is most readily elicited. With the patient in the dorsal position, the angle at which the optimum response is obtained can usually be found by slowly and rhythmically tapping the patellar ligament while the previously extended leg is passively and slowly flexed.
Often it is necessary to reenforce the knee jerk in order to determine whether the reflex is absent or hypo-active. The more commonly employed methods of reenforcement frequently are too difficult for the young child to comprehend. Then, too, the physician often finds it cumbersome to determine whether or not the reenforcement is being consistently performed and at the same time to observe the quadriceps response.
The knee jerk can often be reenforced by active plantar flexion of the foot and toes. The physician can usually obtain
Frank E. Stevenson. REENFORCEMENT OF THE KNEE JERK IN CHILDREN. Arch NeurPsych. 1934;31(3):614. doi:10.1001/archneurpsyc.1934.02250030154011