Genova , Italy CONTROL OF HUMAN ARM IMPEDANCE IN STATICS

The idea that the nervous system can flexibly modify the mechanical behavior of the periphery [1], known as impedance control, has been the focus of recent research in studies of motor control. Impedance control to counteract the effects of destabilizing loads during movement has been reported [2, 3]. In contrast, there is limited evidence that subjects can modify the pattern of limb impedance while the limb is stationary. Indeed the only demonstrations to date of impedance control in statics have involved the use of biofeedback in which subjects modified impedance based on visual displays of either electromyographic activity or hand stiffness patterns [4, 5]. Impedance control presumably involves a process of learning or adaptation, as has been reported for other predictive adjustments to neural signals that are needed to compensate for loads experience at the periphery. In the present study, we demonstrate that impedance control in statics is acquired over time and permits the voluntary control of the pattern of stiffness at the hand. In initial reports of human arm stiffness in a horizontal plane, Mussa-Ivaldi et al. [6] documented the spring-like properties of muscles acting about the shoulder and elbow. The pattern of stiffness at the hand was depicted as an ellipse which had a major axis that corresponded to direction of maximum stiffness. It was suggested that the size of the ellipse and hence the overall stiffness magnitude could be changed by cocontracting shoulder and elbow muscles. A number of studies have shown that the orientation or shape of ellipse is dominated by musculoskeletal geometry [6, 7, 8]. Consistent with the latter idea is the demonstration that the major axis of stiffness ellipse is generally aligned with a line connecting the shoulder to the hand. It remains unclear whether, as suggested by Hogan [1], the pattern of stiffness as measured in statics is modifiable or can only be uniformly scaled in magnitude. Lacquaniti et al. [9] reported evidence of voluntary modification of stiffness during the preparation phase of ball catching. However, in the Lacquaniti et al. data the largest changes in stiffness occur subsequent to catching rather than preceding it. It has also been suggested that if one could independently cocontract shoulder, elbow or biarticular muscles, voluntary change in the orientation of the pattern of arm endpoint stiffness could be achieved [4]. Gomi and Osu trained subjects to independently cocontract different muscle groups using EMG biofeedback from six singleand double-joint muscles of the shoulder and elbow. It was found that although shoulder and elbow muscles could be cocontracted independently, the coactivation of biarticular muscles was tied to cocontraction at the elbow. Counterclockwise rotation of the stiffness ellipse was achieved in response to shoulder muscle cocontraction and clockwise rotation was observed for cocontration of elbow and biarticular muscles. Perreault et al. [5] obtained similar results in a force production task when the arm endpoint stiffness ellipse was used to provide biofeedback to subjects. Subjects can thus change the pattern of arm endpoint impedance based on knowledge of results. However it remains uncertain whether the voluntary modulation of endpoint impedance can be achieved over the course of learning using only the sensory information that would ordinarily accompany limb movement and placement. We have applied mechanical perturbations to arm to assess the extent to which subjects can modify the pattern of impedance at the hand. The training spanned three consecutive days with the goal of examining the way in which the pattern of impedance develops as a function of time. We demonstrate that depending on the direction of the destabilizing load, subjects gradually modify the orientation of maximum stiffness at the hand over the course of training. Moreover, there is an asymmetric pattern in rotation of stiffness ellipse towards the Y or the X axis. Stiffness ellipse rotation towards the Y axis for subjects who trained with the destabilizing forces in Y is substantially greater than the rotation of ellipse towards the X axis for subjects who trained with destabilizing forces in X.