Abstract
INTRODUCTION: The majority of tasks we perform every day require coordinated use of both hands. Following a stroke, the paretic hand contribution to bimanual tasks is often impaired, leading to asymmetric hand use. Grip strength is a commonly used clinical indicator of progress towards stroke motor recovery. The extent to which the paretic hand's contribution to bimanual tasks improves with increasing grip strength is not known. The purpose of this study is to determine how grip strength capacity of the paretic hand influences its contribution to bimanual tasks. METHODS: Twenty-one chronic stroke participants and ten older control participants volunteered to take part in this study. The individuals with stroke were recruited in two distinct groups based on the grip strength capacity of paretic hand, i.e., paretic hand strength/non-paretic hand strength, expressed as a percentage. The low strength-capacity group was identified as individuals with grip strength capacity less than 60% and the high strength-capacity group was individuals with grip strength capacity greater than or equal to 60%. All groups performed isometric, grip force contractions in two bimanual tasks - a maximum force production (MVC) task and a submaximal force control task. We quantified the magnitude of force contributed by the paretic and non-paretic hands during both tasks. Additionally, in the force control task we quantified the amount and structure of force variability using coefficient of variation (CV) and approximate entropy (ApEn) for both hands. RESULTS: The amount of force contributed by the paretic hand increased in bimanual tasks with an increase in its grip strength capacity, (maximal force production: r = 0.85, p < 0.01; submaximal force control: r = 0.62, p < 0.01). In the bimanual MVC task and bimanual force control task, both hands contributed equal magnitudes of force in the high strength-capacity group but unequal forces in low strength-capacity group. Surprisingly, the amount and structure of force variability in bimanual force control tasks did not change with the increase in grip strength capacity, (CV of force: r = - 0.07, p = 0.77; ApEn: r = - 0.23, p = 0.31). Both low and high strength-capacity stroke groups showed significantly higher CV of force and heightened ApEn compared with the control group. CONCLUSION: With the increase in grip strength capacity, the paretic hand contributes greater magnitude of force but continues to show persistent deficits in force modulation in bimanual tasks. Therefore, stroke rehabilitation should emphasize retraining of the paretic hand for force modulation to maximize its use in bimanual tasks.