Changes in limb stiffness under conditions of mental stress

In 2 experiments, the effects of mental stress on limb stiffness were inves tigated. The relative contribution to arm stiffness of individual muscle ac tivity, co-contraction, muscle reflexes, and postural adjustments were exam ined. In each experiment, participants (N = 24, Experiment 1; N = 16, Exper iment 2) held their supinated hand under a tray that they were required to return to horizontal after it had been suddenly released. Electromyographic activity in the biceps and triceps muscles was recorded, as were elbow and wrist angles and tray displacement. In Experiment 1, mental arithmetic str ess was shown to lead to decreased tray displacement (i.e., increased resis tance) compared with displacements under the control, unstressed condition, as well as to increased elbow flexion before tray release. In Experiment 2 , the increased resistance to perturbation caused by mental stress was foun d to be independent of initial elbow angle, but to vary as a function of th e amount of upward force exerted before tray release. The authors conclude that stress-induced increases in limb stiffness result from changes in the initial position of the elbow, specified by its angle, together with the in itial force exerted by participants to counteract the mechanical perturbati ons.