Pre-clinical evaluation of a natural prosthetic elbow control strategy using residual limb motion and a model of healthy inter-joint coordinations

Abstract

A gap has been growing between the mechanical features of newly commercialized prosthetic devices and the control strategies available to the users. The prosthetic joints are controlled sequentially via myoelectric control, and each actuation requires the user's attention. Because of a complex control scheme, transhumeral amputees are generally equipped with a 1-degree-of-freedom myoelectric hand, a myoelectric wrist rotator, and a manually locked elbow. The prosthetic forearm position, adjusted before the movement, is not involved in the overall upper limb movements, resulting in the development of compensatory strategies. A promising solution to improve prosthetic control utilizes the residual limb motions to control the elbow. Previous studies have shown that elbow motion could be predicted from measures of the residual limb movements and an inter-joint coordination model. This study is the first to report the utilization of an automatically driven prosthetic elbow by a transhumeral amputee. The participant pointed at targets with a prosthesis prototype including a modified motorized elbow. The prosthetic elbow motion was derived from a generic model of inter-joint coordinations, and IMU-based residual limb measurements. The participant performed also the task with the prosthetic elbow implemented with his own myoelectric control strategy. Body movements were assessed with the data recorded with a motion capture system. The patient achieved the pointing task with a better precision when the elbow was myoelectrically-driven. However, these movements required important trunk compensations. Trunk movements were smaller with residual limb motion-based elbow control, which enabled a more natural overall body behavior with synchronous shoulder and elbow motions. Due to socket impairment, but also to post-amputation body scheme modifications and discrepancies between healthy and artificial limbs, the participant's residual limb amplitudes were different of the ones of healthy shoulder movements for the same tasks. This work questions the paradigm whereby a prosthetic elbow can be intuitively and naturally used by an amputee while its motion is derived from healthy individual data. Although there is a need for novel modeling approaches to build an inter-joint coordination model adapted to each user, residual limb motion-driven prosthetic elbow enables simultaneous control of elbow and end-effector, and restores a more natural body behavior.