Analyzing Exfordance Use by Unilateral Upper-Limb Amputees* This work was supported by the US Army Medical Research & Materiel Command, grant W81XWH-14-1-0277.

Abstract

Humans often use features of their environment for assistance in picking up and manipulating objects or in stabilizing their own bodies. This ‘exfordance’ use occurs when external contact or gravitational or inertial forces are utilized to aid in task completion or stabilization. This paper presents a categorization of exfordance use and applies the new framework to quantifying how experienced unilateral upper-limb amputees use of exfordances during everyday activities, both in their affected and unaffected limbs. Head-mounted cameras were used to record video footage of participants in their homes while they completed self-selected activities of daily living. A total of 35 minutes of dense manipulation footage has been analyzed for each of 5 trans-radial amputees with different prosthetic devices, resulting in over 4,700 instances of observed exfordance use. The results indicate that participants used exfordance-based vs. non exfordance-based manipulation strategies approximately the same amount with both their intact and prosthetic hands, after adjusting for overall hand use. Furthermore, the specific exfordance use strategies vary substantially between limbs, with participants using environmental surfaces such as tables to guide the motion of their unaffected hand more frequently than with their prosthetic hand, possibly due to increased control and passive conformation ability. Also, participants used gravity-based exfordances (e.g. hanging a towel over the hand) much more frequently with their prosthetic, likely due to its reduced grasping capabilities.