RESONANCE TUNING IN RHYTHMIC ARM MOVEMENTS

The hypothesis was tested that the preferred frequency of rhythmic mov ement corresponds to the resonant frequency of tile muscle-limb system , as proposed by the hybrid spring-pendulum model (Kugler & Turvey, 19 87). In contrast to previous studies, the resonant frequency and stiff ness of the system were estimated independently, which permitted quant itative predictions of the preferred frequency to be made. Human subje cts (N = 5) were asked to oscillate their forearms in the vertical pla ne at their preferred frequency under conditions of added mass and ext ernal spring loading. Subjects also oscillated their arms at frequenci es below and above the preferred frequency, which enabled the investig ators to estimate the resonant frequency and stiffness of the elbow jo int by using the phase transfer method (Viviani, Soechting, & Terzuolo , 1976). The preferred frequency corresponded to !:he resonant frequen cy of the muscle-limb system under each condition, as predicted. The o scillation amplitude varied inversely with the preferred frequency, wh ich was also predicted. Finally, the internal joint stiffness was modu lated so that it matched the impedance of the external springs but was unaffected by added mass. The results are consistent with an autonomo us oscillator model that incorporates proprioception about the dynamic s of the periphery.