Stimulation parameter optimization for FES supported standing up and walking in SCI patients

Functional Electrical Stimulation (FES) to restore leg movement for standing up and walking (stepping) in SCI patients with intact lower motor neuron is used by several groups. Usually quadriceps muscles are stimulated for hip and knee extension, gluteus muscles for hip stabilization, and the common peroneal nerve to elicit the flexion reflex. The requirement to get a natural movement would need a huge number of stimulation channels--a request that could be easily fulfillled from the engineer's point of view but not from the point of practicability since each stimulated muscle requires two skin-attached electrodes resulting in a prolonged time for donning and doffing. In the described project a newly developed eight channel stimulator that can vary the stimulation parameters in many ways and over a wide range is used. The goal is to achieve a natural movement with a minimum of surface electrodes by optimizing the stimulation parameters. Seven experienced FES users and five unexperienced persons (all between Th4-Th11) participate in this study. Standing up can be significantly improved by optimizing the time delay between the onset of quadriceps and gluteus muscles (0.2-0.4 s) and the duration of the ramp. A 0.2 s delay gives good results in heavy patients while slower ramps (0.4 s) are required in slim patients. During stepping, gluteus muscle timing is not very crucial. Gluteus stimulation is turned off 0.1-0.2 s before quadriceps muscle and with the same delay turned on again. Of major influence on the gait quality is the timing during heel strike when peroneal stimulation is switched off and quadriceps stimulation is turned on. Six patients require 0.0-0.1 s where neither peroneal nor quadriceps stimulation is applied, the others require an overlap of 0.1-0.2 s. Activation of adductor muscles during standing up and during the swing phase helps to avoid hip abduction and improves knee trajectories.