Modulation of elbow joint stiffness in a vertical plane during cyclic movement at lower or higher frequencies than natural frequency.

The purpose of the present study was to determine how joint stiffness during cyclic movement in a vertical plane is modulated at lower or higher frequencies than the natural frequency of the system. Five male subjects were instructed to swing their forearms rhythmically in a vertical plane under various frequency conditions (0.7-2.25 Hz). 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-forearm system (1.09+/-0.08 Hz). The resonant frequency showed the similar tendencies to joint stiffness and was significantly different from movement frequency in the lower frequency range (0.7-0.9 Hz). In addition, the ratio of joint stiffness to the background torque (ST(ratio)) was greater in the frequency conditions below the natural frequency than in the frequency conditions above the natural frequency and was relatively constant in the latter. These results suggested that: (1) the modulation of joint stiffness for movement in a vertical plane, by which the resonant frequency of the system is kept close to the movement frequency, may be limited to the movement frequency range above the natural frequency; and (2), in the case of movement in a vertical plane, the mechanism by which joint stiffness is modulated may change according to the relation between natural frequency and movement frequency.