Multiple stretch reflexes occur in muscles adjacent to or remote from the tap. The response may be ipsilateral or bilateral. These reflexes are encountered not only in normal subjects with brisk stretch reflexes but particularly in patients with lesions of the upper motor neuron. The concussion obtained by the blow is conducted along bone to muscle. Muscle spindles are stimulated, and in this manner independent stretch reflexes are produced in these muscles. This mechanism is responsible for the phenomenon of multiple stretch reflexes. The thorax and pelvis play important roles in the contralateral responses by transmitting these mechanical events across the midline.
Teasdall, Robert D.
"The Phenomenon of Multiple Stretch Reflexes,"
Henry Ford Hospital Medical Journal
: Vol. 34
Available at: https://scholarlycommons.henryford.com/hfhmedjournal/vol34/iss1/6