Regulation of Stiffness by Skeletomotor Reflexes

Abstract

Since the early 1950s when Merton (44) first introduced his follow-up servo hypothesis of movement control, the dominant opinion has held that stretch and unloading reflexes function to control the length of a muscle in opposition to changes in mechanical load, thus providing load compensa­ tion. In this article I review recent evidence indicating that this notion is wrong, or at least incomplete, and proceed to develop an alternative idea (47) that neither muscle length nor force are regulated as individual vari­ ables, but that a property called stiffness (the ratio of force change to length change) is maintained relatively constant by skeletomotor reflexes. This leads to a discussion of the actions of descending motor commands on the segmental system, from which I conclude that these may be much less versatile than was formerly believed. The reader is referred to another review (55) and to Matthews' mono­ graph (42) for additional information.