Figure 1.

Simulated changes in endpoint stiffness that would result from increasing the cocontraction of various muscle pairs. The mean NF stiffness ellipse for five subjects was used for baseline joint stiffness values (filled gray ellipse). Details of the simulations are given in Materials and Methods. The simulations were performed using the limb posture shown by the stick figure of the arm (C). A, Endpoint stiffness produced by global cocontraction of all muscles. B, Endpoint stiffness produced by cocontracting single-joint shoulder muscles. C, Endpoint stiffness produced by cocontracting single-joint elbow muscles. D, Endpoint stiffness produced by cocontracting biarticular muscles. E, Endpoint stiffness produced by cocontracting single-joint elbow and biarticular muscles.