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Objectives: Restoration of walking to individuals with neurological impairments is challenging because of the complex interplay between neural control and musculoskeletal biomechanics. But, rehabilitation strategies to retrain muscle recruitment or strengthen muscles, for example, are based on qualitative simple, phenomenological cause-effect models of unimpaired walking. Our immediate goal is to develop scientific, experimentally based, computer-implemented models of lower limb motor tasks to identify basic principles of neural control and musculoskeletal biomechanics. Future goals are to use the models and perform experiments to elucidate the motor control deficits in veterans and other individuals with hemiparesis after stroke, to design rehabilitation strategies based on these modeling and experimental studies, and to evaluate the efficacy of these strategies.
Methods: Pedaling,rather than walking, is used to identify neural-control strategies for locomotion. Both pedaling and walking propulsion are dominated by sagittal-plane mechanics involving flexion-extension alternation of the legs at similar cyclic rates. Pedaling is ideal for experimental study because, in contrast to walking, balance and level of body weight support can be easily controlled. Moreover, bilateral lower limb coordination mechanisms difficult to study in walking can be found in pedaling using a split-axle pedaling ergometer with computer-controlled servomotors connected to each crank. Computer simulations are generated from pedaling models and experimental data to understand intra- and inter-limb muscle coordination, such as the role of each muscle in accelerating the body segments. Four pedaling experiments on 60 neurologically healthy subjects, along with computer simulations, have been completed and neuromotor control strategies identified.
Results: Basically, two-legged backward or forward pedaling is achieved by coordinating muscles to cyclically execute six biomechanical functions, organized antagonistically (flexor-extensor, anterior/posterior, plantarilexor/dorsiflexor). In one-legged pedaling tasks, the coordination of these functions is different, even when the loading is identical to the loading experienced during two-legged pedaling. The two-legged coordination pattern is restored when the contralateral leg generates rhythmic force and is moved antiphase to the ipsilateral leg. Our results suggest that excitatory and/or inhibitory interlimb influences operate between a biomechanical function and it's antagonistic function in the contralateral leg.
Conclusions: The neural control strategy to execute pedaling, a complex lower limb motor task, depends on the sensorimotor state of the contralateral limb. Bilateral coordination of muscles is believed to result in robust control, e.g., to assure alternation of the limbs. Also, the control of each muscle is quite simple and depends on its ability to contribute to the execution of the biomechanical functions. These conclusions form the basis for an approach toward understanding changes in sensorimotor state that occur post-stroke. Computer simulations of unimpaired walking are now being used to assess how applicable these control strategies are to walking.
Acknowledgments: Supported by NIH grant NS17662 and the Department of Veterans Affairs (VA), Rehabilitation R&D Service.