Speed-dependent locomotor adjustments following staggered thoracic lateral hemisections in adult cats.

Animals adjust their locomotor pattern to increased speed demands by decreasing stance/extensor phase duration while the swing/flexor phase remains relatively unchanged, which we refer to here as 'stance/extensor dominance'. The control of locomotor speed involves dynamic interactions between spinal circuits, supraspinal drive and somatosensory feedback. While complete spinal cord injuries abolish brain-spinal cord interactions, incomplete lesions, such as lateral hemisections, preserve some connectivity between brain and spinal circuits. In this study, we investigated adjustments in the locomotor pattern at different treadmill speeds before and after staggered lateral thoracic hemisections performed on opposite sides of the spinal cord (first at right T5-T6 and then at left T10-T11). We collected kinematic and electromyographic data during treadmill locomotion from 0.4 to 0.8 m/s before and eight weeks after each spinal lesion in eight adult cats. Our main results show left-right asymmetries in hindlimb phase durations after each lesion, with prolonged swing on the ipsilesional side and prolonged stance on the contralesional side across speeds. Hindlimb stance dominance was also weakened on the side of each lesion, first on the right and then on the left after the first and second hemisections, respectively. In contrast to phase durations, hindlimb stride lengths remained symmetric after both injuries across speeds. Using our recent computational models (1,2) and experimental data of the present study, we provide predictions of altered interactions between supraspinal drive and somatosensory feedback onto flexor and extensor half centers to explain left-right changes in hindlimb phase durations across speeds after staggered lateral thoracic hemisections.