Neural Coupling between Upper and Lower Limbs during Recumbent Stepping.

Humans naturally coordinate upper and lower limb movements during rhythmic tasks. This innate coupling between upper and lower limbs has a neural basis that may be advantageous for gait rehabilitation. Adding upper limb effort to lower limb stepping could improve lower limb muscle recruitment during therapy. I developed a computer controlled recumbent stepping device with mechanically coupled handles and pedals to test principles of neural coupling of the arms and legs. Subjects drove the stepping motion using different combinations of upper and lower limb effort (active or passive). The first study demonstrated that upper limb effort increased passive lower limb muscle activation proportionally in neurologically intact individuals. These results indicated an excitatory neural coupling between the upper and lower limbs during rhythmic stepping movements. I then studied neurologically intact subjects performing maximal effort, velocity-controlled recumbent stepping. The results revealed that neural coupling between the upper and lower limbs is bidirectional and ipsilaterally biased. For the next study, I examined neural coupling in individuals with incomplete spinal cord injury to determine if adding upper limb effort enhanced muscle recruitment of volitionally maximally active lower limbs. The data indicated that maximal upper limb effort did not increase active lower limb muscle recruitment in individuals with incomplete spinal cord injury. Similar to neurologically intact individuals, spinal cord injured individuals also demonstrated increased passive lower limb muscle activation with greater upper limb effort. Lastly, I used computer simulations to examine potential neural mechanisms behind the upper and lower limb neural coupling. These models showed that excitatory sensory feedback, excitatory ipsilateral pathways, and supraspinal drive were all possible specific neural mechanisms explaining my empirical results. Future studies using more sophisticated neural techniques such as transcranial magnetic stimulation can test the specific neural mechanisms shown in the simulations. Overall, my findings provide a better understanding of interlimb neural coupling and have specific implications for the design of exercise therapies for gait rehabilitation after neurological injury.Ph.D.Biomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/63694/1/hjhuang_1.pd