Abstract
Kinematic
redundancy of the human body provides abundant movement patterns to
accomplish the same motor goals (motor equivalence). Compensatory
movement patterns such as excessive trunk displacement in stroke
subjects during reaching can be viewed as a consequence of the motor
equivalent process to accomplish a task despite limited available ranges
in some joints. However, despite compensations, the ability to adapt
reaching performance when perturbations occur may still be limited when
condition-specific changes of joint angles are required. We addressed
this hypothesis in individuals with and without stroke for reaching a
target placed beyond arm reach in standing while flexing the hips
(free-hip condition). In randomly selected trials, hip flexion was
unexpectedly blocked, forcing subjects to take a step (blocked-hip
condition). In additional trials, subjects took an intentional step
while reaching the target (intentional-step condition). In blocked-hip
trials, healthy subjects maintained smooth and precise endpoint
trajectories by adapting temporal and spatial interjoint coordination to
neutralize the effect of the perturbation. However, the ability to
produce motor equivalent solutions was reduced in subjects with stroke,
evidenced by substantial overshoot errors in endpoint position, reduced
movement smoothness and less adaptive elbow-shoulder interjoint
coordination. Movement adaptability was more limited in stroke subjects
who used more compensatory movements for unperturbed reaching. Results
suggest that subjects with mild-to-moderate stroke only partially
adapted arm joint movements to maintain reaching performance.
Therapeutic efforts to enhance the ability of individuals with stroke to
find a larger number of task-relevant motor solutions (adaptability)
may improve upper limb recovery.
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