EQUILIBRIUM-POINT CONTROL HYPOTHESIS EXAMINED BY MEASURED ARM STIFFNESS DURING MULTIJOINT MOVEMENT

For the last 20 years, it has been hypothesized that well-coordinated, multijoint movements are executed without complex computation by the brain, with the use of springlike muscle properties and peripheral neu ral feedback loops. However, it has been technically and conceptually difficult to examine this ''equilibrium-point control'' hypothesis dir ectly in physiological or behavioral experiments. A high-performance m anipulandum was developed and used here to measure human arm stiffness , the magnitude of which during multijoint movement is important for t his hypothesis. Here, the equilibrium-point trajectory was estimated f rom the measured stiffness, the actual trajectory, and the generated t orque. Its velocity profile differed from that of the actual trajector y. These results argue against the hypothesis that the brain sends as a motor command only an equilibrium-point trajectory similar to the ac tual trajectory.