The action of plantar pressure on flexion reflex pathways in the isolated human spinal cord

Objective To investigate the conditioning effects of plantar pressure on flexion reflex excitability in patients with motor complete spinal cord injury (SCI). Methods In five motor complete SCI subjects, the non-nociceptive flexion reflex was evoked via electrical stimulation of the right sural nerve and was recorded from the ipsilateral tibialis anterior muscle. Pressure ranging from 25 to 80 kPa was applied to the metatarsal heads through an adjustable platform incorporated into a foot rest and a comparison of the reflex size made between control conditions and during pressure application. Results In all subjects, a significant depression of the long latency flexion reflex was observed when pressure was applied to the foot sole. The short latency flexion reflex appearing at latencies less than 100 ms was absent in all patients. Conclusions The results demonstrate that flexion reflex excitability in the isolated human spinal cord can be modulated by adequate activation of plantar mechanoreceptors. Significance Activation of plantar mechanoreceptors is a feature of normal standing and walking. Rehabilitation for standing and walking in SCI commonly uses body weight support based protocols. The strong inhibitory actions of plantar pressure on reflex pathways in the isolated human spinal cord suggest that sensory feedback from the foot sole may be an important factor in successful rehabilitation of standing and stepping in SCI patients.