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Hand impairment is prevalent in individuals after stroke. Regaining independent finger control is especially challenging. An objective and continuous assessment of finger impairment could inform clinicians and allow them to prescribe targeted therapies. The objective of this preliminary work was to quantify the neuromuscular factors that contribute to impairment in independent finger control in chronic stroke survivors. We obtained high-density electromyographic (HD-EMG) signals of extrinsic finger muscles and fingertip forces, while stroke or control participants were instructed to produce independent finger forces. We observed an impaired ability to isolate individual muscle compartment activation (i.e., co-activation of muscle compartment). This muscle co-activation pattern correlated with finger independence as well as clinical assessment scales on hand impairment. Our preliminary work showed that HD-EMG recordings can be used to continuously monitor activation abnormalities of small finger muscles in contribution to impaired finger independence. With further development, the outcomes can provide a basis for clinical decision making to reduce hand impairments of stroke survivors. 

Objective: Functional electrical stimulation (FES) is a common technique to elicit muscle contraction and help improve muscle strength. Traditional FES over the muscle belly typically only activates superficial muscle regions. In the case of hand FES, this prevents the activation of the deeper flexor muscles which control the distal finger joints. Here, we evaluated whether an alternative transcutaneous nerve-bundle stimulation approach can activate both superficial and deep extrinsic finger flexors using a high-density stimulation grid. Methods: Transverse ultrasound of the forearm muscles was used to obtain cross-sectional images of the underlying finger flexors during stimulated finger flexions and kinematically-matched voluntary motions. Finger kinematics were recorded, and an image registration method was used to capture the large deformation of the muscle regions during each flexion. This deformation was used as a surrogate measure of the contraction of muscle tissue, and the regions of expanding tissue can identify activated muscles. Results: The nerve-bundle stimulation elicited contractions in the superficial and deep finger flexors. Both separate and concurrent activation of these two muscles were observed. Joint kinematics of the fingers also matched the expected regions of muscle contractions. Conclusions: Our results showed that the nerve-bundle stimulation technique can activate the deep extrinsic finger flexors, which are typically not accessible via traditional surface FES. Significance: Our nerve-bundle stimulation method enables us to produce the full range of motion of different joints necessary for various functional grasps, which could benefit future neuroprosthetic applications.