INABILITY TO ACTIVATE MUSCLES MAXIMALLY DURING COCONTRACTION AND THE EFFECT ON JOINT STIFFNESS

In order to determine the maximum joint stiffness that could be produc ed by cocontraction of wrist flexor and extensor muscles, experiments were conducted in which healthy human subjects stabilized a wrist mani pulandum that was made mechanically unstable by using positive positio n feedback to create a load with the characteristics of a negative spr ing. To determine a subject's limit of stability, the negative stiffne ss of the manipulandum was increased by increments until the subject c ould no longer reliably stabilize the manipulandum in a 1 degrees targ et window. Static wrist stiffness was measured by applying a 3 degrees ramp-and-hold displacement of the manipulandum, which stretched the w rist flexor muscles. As the load stiffness was made more and more nega tive, subjects responded by increasing the level of cocontraction of f lexor and extensor muscles to increase the stiffness of the wrist. The stiffness measured at a subject's limit of stability was taken as the maximum stiffness that the subject could achieve by cocontraction of wrist flexor and extensor muscles. In almost all cases, this value was as large or larger than that measured when the subject was asked to c ocontract maximally to stiffen the wrist in the absence of any load. S tatic wrist stiffness was also measured when subjects reciprocally act ivated flexor or extensor muscles to hold the manipulandum in the targ et window against a load generated by a stretched spring. We found a s trong linear correlation between wrist stiffness and flexor torque ove r the range of torques used in this study (20-80% maximal voluntary co ntraction). The maximum stiffness achieved by cocontraction of wrist f lexor and extensor muscles was less than 50% of the maximum value pred icted from the joint stiffness measured during matched reciprocal acti vation of flexor and extensor muscles. EMG recorded from either wrist flexor or extensor muscles during maximal cocontraction confirmed that this reduced stiffness was due to lower levels of activation during c ocontraction of flexor and extensor muscles than during reciprocal con traction.