Abstract
Following a lateralized spinal cord injury (SCI) in humans, substantial walking recovery occurs; however, deficits persist in adaptive features of locomotion critical for community ambulation, including obstacle negotiation. Normal obstacle negotiation is accomplished by an increase in flexion during swing. If an object is unanticipated or supraspinal input is absent, obstacle negotiation may involve the spinally organized stumbling corrective response. How these voluntary and reflex components are affected following partial SCI is not well studied. This study is the first to characterize recovery of obstacle negotiation following low-thoracic spinal hemisection in the cat. Cats were trained pre- and post-injury to cross a runway with an obstacle. Assessments focused on the hindlimb ipsilateral to the lesion. Pre-injury, cats efficiently cleared an obstacle by increasing knee flexion during swing. Post-injury, obstacle clearance permanently changed. At 2 weeks, when basic overground walking ability been recovered, the hindlimb was dragged over the obstacle (∼90%). Surprisingly, the stumbling corrective response was not elicited until after 2 weeks. Despite a notable increase, between 4 and 8 weeks, in the ability to modify limb trajectory when approaching an obstacle, limb lift during obstacle approach was insufficient during ∼50% of encounters and continued to evoke the stumbling corrective response even at 16 weeks. A post-injury lead limb bias identified during negotiations with complete clearance, suggests a potential training strategy to increase the number of successful clearances. Therefore, following complete severing of half of the spinal cord, the ability to modify ipsilateral hindlimb trajectory shows significant recovery and by 16 weeks permits effective clearing of an obstacle, without contact, ∼50% of the time. Although this suggests plasticity of supporting circuitry, it is insufficient to support consistent clearance. This inconsistency, even at the most chronic time point assessed (16 weeks), is probably a contributing factor to falls reported for people with SCI.