Reflex responses to passive motion in normal and spastic man

Responses elicited by passive stretch have been the focus of many investigations by clinicians and neurophysiologists, but responses during passive shortening have received little attention. The receptors responsible for stretch responses have been identified as the primary muscle spindle afferent. The most likely source of shortening responses is the secondary spindle afferents located in antagonistic muscle. This hypothesis is based on the known reflex effects of these afferents, the larger shortening responses in the flexors than the extensors and the increased shortening responses when the test position exaggerated the stretch of the antagonist. The lack of correlation between shortening responses and active tension in the antagonist argues against Golgi tendon organs which is the other possible receptor source.