Modulation of elbow joint stiffness and co-contraction of agonist-antagonist muscles during human rhythmic movement.

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The purpose of this study is to investigate the modulation mechanism of joint stiffness during human rhythmic movement. To estimate the mechanical properties of the elbow joint, external perturbations were applied by an electromagnetic torque motor system to the forearm of each subject during the movement. Joint stiffness showed a significant quadratic trend with a minimum close to the natural frequency of the apparatus-limb system. This results suggested that 1) the mechanism by which joint stiffness is modulated may change according to the relation between natural frequency and movement frequency, and 2) the co-contraction mechanism is less important for maintaining joint stiffness during slow movement than during fast movement.