Muscle activation during unilateral stepping occurs in the nonstepping limb of humans with clinically complete spinal cord injury

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Abstract
Study design: Comparison of different kinematic and loading conditions on muscle activation in clinically complete spinal cord-injured subjects stepping unilaterally with manual assistance. Objective: To determine if rhythmic lower limb loading or movement could produce rhythmic muscle activation in the nonstepping limb of subjects with clinically complete spinal cord injury (SCI). Setting: Human Locomotion Research Center, Department of Neurology, University of California, Los Angeles, USA. Methods: We recorded electromyography, joint kinematics, and vertical ground reaction forces as four subjects with clinically complete SCI stepped with manual assistance and partial bodyweight support. For all trials, one limb continuously stepped while the other limb underwent different conditions, including rhythmic lower limb loading in an extended position without limb movement, rhythmic lower limb movement similar to stepping without limb loading, and no lower limb loading or movement with the leg in an extended or flexed position. Results: Three subjects displayed rhythmic muscle activity in the nonstepping limb for trials with rhythmic limb loading, but no limb movement. One subject displayed rhythmic muscle activity in the nonstepping limb for trials without ipsilateral limb loading or movement. The rhythmic muscle activity in the nonstepping limb was similar to the rhythmic muscle activity during bilateral stepping. Conclusions: The human spinal cord can use sensory information about ipsilateral limb loading to increase muscle activation even when there is no limb movement. The results also indicate that movement and loading in one limb can produce rhythmic muscle activity in the other limb even when it is stationary and unloaded. These findings emphasize the importance of optimizing load-related and contralateral sensory input during gait rehabilitation after SCI.