A rather messy situation exists in the motor control literature with the notion of muscle and joint viscosity. The source of the confusion lies in attempts to study human movement using a simple mechanical concept that was proposed for uncomplicated objects such as a gel or gas. Although A.V. Hill (1938) wrote, “The 'viscosity' hypothesis must be dismissed” (p. 193), the concept of muscle/joint viscosity is still very popular in the literature. Distressingly, the reported values of muscle/joint viscosity differ greatly. Eleven dissimilar “viscosities” are briefly discussed. They represent disparate characteristics of the human motion system. It seems that the concept of viscosity is inadequate for describing motor control phenomena.
Vladimir M. Zatsiorsky is with the Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802.