We have extensively used arm cycling to study the neural control of rhythmic movements such as arm swing during walking. Recently rhythmic movement of the arms has also been shown to enhance and shape muscle activity in the legs. However, restricted information is available concerning the conditions necessary to maximally alter lumbar spinal cord excitability. Knowledge on the neuromechanics of a task can assist in the determination of the type, level, and timing of neural signals, yet arm swing during walking and arm cycling have not received a detailed neuromechanical comparison. The purpose of this research was to provide a combined neural and mechanical measurement approach that could be used to assist in the determination of the necessary and sufficient conditions for arm movement to assist in lower limb rehabilitation after stroke and spinal cord injury. Subjects performed three rhythmic arm movement tasks: (1) cycling (cycle); (2) swinging while standing (swing); and (3) swinging while treadmill walking (walk). We hypothesized that any difference in neural control between tasks (i.e., pattern of muscle activity) would reflect changes in the mechanical constraints unique to each task. Three-dimensional kinematics were collected simultaneously with force measurement at the hand and electromyography from the arms and trunk. All data were appropriately segmented to allow a comparison between and across conditions and were normalized and averaged to 100% movement cycle based on shoulder excursion. Separate mathematical principal components analysis of kinematic and neural variables was performed to determine common task features and muscle synergies. The results highlight important neural and mechanical features that distinguish differences between tasks. For example, there are considerable differences in the anatomical positions of the arms during each task, which relate to the moments experienced about the elbow and shoulder. Also, there are differences between tasks in elbow flexion/extension kinematics alongside differential muscle activation profiles. As well, mechanical assistance and constraints during all tasks could affect muscle recruitment and the functional role of muscles. Overall, despite neural and mechanical differences, the results are consistent with conserved common central motor control mechanisms operational for cycle, walk, and swing but appropriately sculpted to demands unique to each task. However, changing the mechanical parameters could affect the role of afferent feedback altering neural control and the coupling to the lower limbs.
Traditionally, tasks used to assist in walking rehabilitation focus on the lower limbs and do not actively engage the arms. However, recent evidence suggests that rhythmic arm movement can have beneficial effects on the neural control of the lower limbs, such as enhancing leg muscle activity during locomotor tasks. Ambiguity remains regarding the specific details of how upper limb movement influences the control of the legs. This is in part due to the limited mechanical characterization of rhythmic arm movements. We present a neural and mechanical comparison of three rhythmic arm movements (arm cycling, stationary swinging, and swinging while walking) in an attempt to expose considerations for the use of arm movement in walking rehabilitation. Our results suggest that while there are different kinematics across tasks the general neural control is conserved. The anatomical oscillatory “anchor point” position of the arm during the tasks is distinctive which relates to different moments about the elbow and shoulder. Results from mathematical analyses are consistent with conserved common central motor control mechanisms operational for arm cycling, arm swing while walking, and arm swing alone, but appropriately sculpted to demands unique to each task. However, changes in the mechanics of the tasks can alter the neural control and coupling to the legs and therefore it is necessary to document and explore neuromechanical interactions with different arm tasks. This comparison can assist in the determination of the necessary and sufficient conditions for arm movement to assist in lower limb rehabilitation after stroke or spinal cord injury.
Support was provided by grants from the Heart and Stroke Foundation of Canada (BC & Yukon) (M.K. and E.P.Z.), the Natural Sciences and Engineering Research Council of Canada (E.P.Z.), and the Michael Smith Foundation for Health Research (E.P.Z.). D.P.F. and R.H.S. were supported by the Christopher Reeve Paralysis Foundation and the Paralyzed Veterans of America Spinal Cord Research Foundation.
A. Can common core arm activities activate the legs?
D. Data acquisition
E. Data analysis
F. Mathematical analysis
G. Additional statistics
D. Mathematical analysis
A. Neuromechanical considerations for the use of arm movement to facilitate leg muscle activity during locomotor activities
1. Arm swing while walking (walk)
2. Arm cycling (cycle)
3. Arm swing alone (swing)
VI. FUNCTIONAL IMPLICATIONS AND CONCLUSION
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