Neuronal function in chronic spinal cord injury: divergence between locomotor and flexion- and H-reflex activity

Knowledge about the long-term course of spinal neuronal function after spinal cord injury (SCI) is important if regeneration therapies become available in the future. The objective of this study was to examine the behavior of locomotor EMG activity and of spinal reflexes in patients with chronic motor-complete SCI.

EMG activity from rectus femoris (RF), biceps femoris (BF), medial gastrocnemius (GM) and tibialis anterior (TA) of both sides was investigated during locomotor movements assisted by a robotic device in 10 chronic (>1 year after accident) complete SCI and 5 healthy subjects. H-reflexes (recorded from GM) were induced during the onset and flexion reflexes (recorded from BF and TA) at the end of the stance phase.

Only in the chronic SCI subjects an exhaustion of EMG activity--i.e. a decrease in amplitude--occurred within a few minutes in all leg muscles. The EMG exhaustion was not associated with a change in the H- or flexion reflex amplitude during a walking session.

Exhaustion of neuronal function in chronic complete SCI might be restricted to unused motor tasks, i.e. locomotion. The fact that H- and flexion reflexes show a normal behavior might be due to the fact that they still become activated.

Training/pharmalogical approaches may be required to maintain neuronal function of unused tasks as a basis for future successful regeneration therapies.