Context dependence of intrinsic and reflex contributions to ankle stiffness

Overall joint stiffness arises from two components: intrinsic stiffness due to the mechanical properties of the joint, passive tissue, and active muscle fibers; and reflex stiffness arising from changes in muscle activation due to sensory response to stretch. This paper reviews work from my laboratory in which both empirical and system identification methods were used to assess the magnitude of intrinsic and reflex contributions to overall joint stiffness. Overall joint stiffness and the relative contributions of intrinsic and reflex mechanisms are shown to be strongly context dependent. Under stationary conditions, the perturbation properties, mean joint position and level of tonic activation have a strong effect on stiffness and its components. Changes under time-varying conditions are also substantial but differ from what stationary studies would predict. Finally, recent results are presented that indicate the CNS is capable of adjusting reflex stiffness in a task-dependent manner, even when all other variables are controlled. These results indicate that reflex contributions to joint stiffness are by no means fixed. Rather, they vary strongly with the task being performed and the context under which they are examined.