This result was not investigated further, the focus of subsequent work being on the more robust effects of abductor drive on distal joints, including elbow, wrist, and fingers. Specifically, the paretic and control arms had similar secondary torque generation patterns in internal and external rotation that were different from the non-paretic arms. compared control, non-paretic, and paretic internal and external rotation torques generated during shoulder abduction and found inconsistencies with the expectations of the abnormal synergy hypothesis. Using an isometric task in single directions, Dewald et al. Shoulder abduction is reported as being accompanied by shoulder external rotation, elbow flexion, supination, and wrist and finger flexion, while shoulder adduction is often accompanied by shoulder internal rotation, elbow extension, and wrist and finger flexion. One factor that contributes to the loss of multi-joint coordination after stroke is an unintentional co-contraction of muscles throughout a limb, described as an abnormal synergy, and loss of independent joint control. Weakness and loss of multi-joint coordination involving the upper extremity may affect activities of daily living that require control of arm position, stiffness, damping, and inertia to enable the individual to accomplish tasks such as feeding themselves, dressing, preparing food, carrying objects, or opening doors. Currently, 6.6 million Americans are living post stroke, approximately 30–60% of whom have chronic upper extremity motor impairments, including weakness, loss of multi-joint coordination, hypertonia, and spasticity. The inclusion of external rotation in flexion synergy and of internal rotation in extension synergy may be over-simplifications.Īpproximately 610,000 new strokes occur each year in the US and 16.9 million occur worldwide. Additional non-load-dependent effects such as increased antagonist co-activation (hypertonia) may cause the observed decreased performance in individuals with stroke. ConclusionĬommon biomechanical constraints (muscle actions) explain limitations in external and internal rotation strength during adduction and abduction dual-tasks, respectively. Simulations incorporating increased co-activation mirrored the drop in performance observed across all dual-tasks in paretic arms. OpenSim musculoskeletal modeling mirrored the experimental results of control and non-paretic arms and also, when adjusted for weakness, paretic arm performance. Load-level and group interactions were not significant, indicating that abduction and adduction loading affected each group in a similar manner. There was a significant effect of load in three of four load/task combinations for all groups. Paretic arms were less able to generate internal and external rotation during abduction and adduction, respectively. Group was significant in all task combinations. The protocol was then modeled using OpenSim to understand and explain the role of biomechanical (muscle action) constraints on task performance. Four linear mixed-effects models tested the effect of group (control, non-paretic, and paretic), load (0, 25, 50% adduction or abduction), and their interaction on task performance one model was created for each combination of dual-task directions (external or internal rotation during abduction or adduction). Once established against the vertical load, each participant generated maximum internal and external rotation torque in a dual-task paradigm. A robotic device controlled abduction and adduction loading to 0, 25, and 50% of maximum strength in each direction. MethodsĢ4 participants, 12 with impairments after stroke and 12 controls, completed this study. Objectiveĭetermine the ability to generate internal and external rotation torque under different shoulder abduction and adduction loads in persons with chronic stroke (paretic and non-paretic arm) and uninjured controls. Humeral internal and external rotation have been included in definitions of abnormal synergy but have yet to be studied in-depth. After stroke, motor control is often negatively affected, leaving survivors with less muscle strength and coordination, increased tone, and abnormal synergies (coupled joint movements) in their affected upper extremity.
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