Fig. 1

Experimental set-up and task protocol. a KINARM exoskeleton robotic system. b Task order of events (note hand is included for illustration but hands were occluded from participant view for the entire trial). Postural hold: stabilize white hand feedback marker in red target (with (Unload Task) or without (Load Task) background load). Perturbation: load is turned on (Load Task) or off (Unload Task) and hand feedback marker is removed. Return to Target: return the hand to the target without visual feedback. c Load profiles for each direction of perturbation (Sho: shoulder; Elb: elbow; Flex: flexion; Ext: extension). d Schematic of the direction of motion following each perturbation type. Representation of how elbow and shoulder angle are defined at the start of the trial (Ɵs is shoulder angle, Ɵe is elbow angle measured at the start position). e Hand path after perturbation onset (shoulder flexion trial). The yellow circle indicates the Maximum Displacement of the hand and the green square indicates the Deceleration Time. Return Time (when subject returned within 1 cm of their endpoint location) is indicated by the white square. Endpoint (position at end of trial) is indicated by white circle. Endpoint Error was calculated as the distance between endpoint and the target center