Assistance Delivery and Muscle Coordination

1. Pre-experimental session:

First, reflective markers will be placed on the dorsum of the hand, i.e., metacarpophalangeal (MCP) and distal/proximal interphalangeal (DIP/PIP) joints of the index finger, index fingertip, MCP of the little finger, and carpometacarpal joint of the middle finger, which will be used to establish a local coordinate system. ten-camera motion capture system (Optitrak Prime X13; NaturalPoint Inc., OR, USA) will be used to record the finger movements.

Bipolar surface electrodes (Myosystem 14; Noraxon Inc., Scottsdale, AZ) will be placed to record the activity of the following muscles: extensor digitorum communis (EDC), flexor digitorum superficialis (FDS), first dorsal interosseous (FDI), first palmar interosseous (FPI), extensor carpi radialis (ECR), and flexor carpi radialis (FCR) muscles.

Once the reflective markers and the electromyography (EMG) electrodes are attached, subjects will place their hand in the device. Their forearm will be secured once a comfortable wrist posture for each subject is determined. The location of the pulleys and linear guides will be adjusted to accommodate the forearm posture, determined by the subject-specific wrist angle. Once the fingertip is secured to the end-effector actuator, whose location will then be fixed/locked, they will perform maximum contraction (3-second) of the following muscles, extensor digitorum communis (EDC), flexor digitorum superficialis (FDS), first dorsal interosseous (FDI), extensor carpi radialis (ECR), and flexor carpi radialis (FCR) by performing finger extension, finger flexion, finger abduction, wrist extension, and wrist flexion, during which maximum EMG signals for the target muscles will be obtained. Rest periods of minimum 30-second (or as needed) will be administered between contractions. The fingertip will then be disconnected from the end-effector. 2. Experimental session (1,2):

After rest, subjects will perform finger extension movements with and without assistance. In each session, finger extension task will be performed without assistance in the block 1 (10 trials), followed by the assisted finger extension in the block 2 (15 trials). In the block 3, unassisted finger extension will be performed to gauge residual impact of the motor adaptation by assistance (Fig. 1B).