Neuromechanics of nonuse and compensation: implications for rehabilitation after stroke

Stroke often results in compensatory movement strategies that combine use of trunk movements with nonuse of the affected upper extremity, which is considered detrimental to recovery. In this study, we investigate whether compensation and nonuse can be more than a suboptimal counterpart to recovery. We explore the influence of neuromechanics on nonuse and compensation in 22 stroke survivors and 22 controls. We asked seated participants to reach a target under two nonuse conditions (spontaneous or with minimized trunk compensation) and under three arm weight conditions (control, lightened and weighted). We replicated the findings that stroke induces more trunk compensation, more anterior deltoid activation and greater effort. We found that, in stroke patients, reduced arm weight decreased nonuse, while in controls, increased arm weight induced trunk compensation similar to what is observed after stroke. Furthermore, reaching with nonuse after stroke required less effort and preserved a force reserve at the shoulder. Our results show that both poststroke and healthy individuals engage in similar nonuse and compensation in the face of their neuromechanical limitations, as predicted by an optimal control policy aiming to succeed at the task while keeping a force reserve at each joint. We derive two theoretical predictions for future clinical research. First, patients whose nonuse yields few functional gains should benefit most from interventions to reduce compensation. Second, improving shoulder strength after stroke should reduce relative muscle activation, thereby decreasing trunk compensation and upper extremity nonuse. Clinical Trial: NCT04747587