Intrinsic, reflex and voluntary contributions to task-dependent joint stiffness

Dynamic joint stiffness defines the dynamic relationship between the position of the joint and the torque acting about it. Joint stiffness consists of two components: intrinsic and reflex stiffness. Previous work from our lab has shown that subjects can alter their reflex stiffness voluntarily and independently of intrinsic stiffness. Numerous studies have investigated whether reflex stiffness is altered in a task-dependent fashion; however the results of these studies are inconclusive. We designed an experimental paradigm where subjects were faced with 3 tasks: one task where joint stiffness aided subjects, a second where joint stiffness hindered the subjects and a third where joint stiffness had no effect. We found that subjects did not alter their joint stiffness to perform the different tasks. Rather, they performed the tasks by voluntarily producing the appropriate torque based on visual feedback. Thus, with the paradigm used in this study, reflex stiffness was not modulated in a task-dependent manner.