  Increased effort during use of the paretic arm and hand can provoke
involuntary abnormal synergy patterns and amplify stiffness effects of muscle
tone for individuals after stroke, which can add difficulty for user-controlled
devices to assist hand movement during functional tasks. We study how
volitional effort, exerted in an attempt to open or close the hand, affects
resistance to robot-assisted movement at the finger level. We perform
experiments with three chronic stroke survivors to measure changes in stiffness
when the user is actively exerting effort to activate ipsilateral
EMG-controlled robot-assisted hand movements, compared with when the fingers
are passively stretched, as well as overall effects from sustained active
engagement and use. Our results suggest that active engagement of the upper
extremity increases muscle tone in the finger to a much greater degree than
through passive-stretch or sustained exertion over time. Potential design
implications of this work suggest that developers should anticipate higher
levels of finger stiffness when relying on user-driven ipsilateral control
methods for assistive or rehabilitative devices for stroke.
