Recommendations for tracking the residual forearm in people with trans-radial limb difference using marker-based motion capture.

Upper-limb prostheses users report unsatisfactory 'socket fit', leading to poor outcomes. Previous attempts to measure socket-residuum coupling as a proxy for upper-limb socket fit were limited by their modelling approach (<6-degrees of freedom (DoF) models) and focused on trans-humeral prosthesis users. Prince described a 6-DoF model to track trans-radial (TR) socket-limb coupling, but this required bespoke measurement equipment and was only demonstrated in one intact-limbed participant. Current International Society of Biomechanics (ISB) recommendations for marker-based forearm tracking include ulna styloid markers, therefore being unsuitable for people with TR limb difference. To identify alternative marker frames, ten intact limb participants performed six different movements, nine times each. Displacement of an ulna marker 'cloud' of markers was evaluated, for 18 alternative tracking frames, none of which utilised wrist markers. The optimal frames were a) for a long residuum/full forearm - markers on humeral epicondyles and mid-distal ulna bone (62.5-75 % of a typical ulna), and b) for a shorter residuum - markers on humeral epicondyles and distal ulna bone (25-50 % of a typical ulna). These frames compared favourably to the ISB frame in the point cloud displacement metric. This study provides novel solutions to tracking the ulna bone of people with TR limb difference.