Spinal segmental mapping
SCES, lower extremity muscle
RECRUITMENT, CONTRACTION, RECRUITMENT
Following both temporary and permanent implantation, participants were scheduled to perform the process of spinal segmental mapping. Spinal mapping is the process of identifying the right stimulation parameters (frequency, amplitude, and pulse duration) responsible for activation of the lower extremity muscle groups, polarity of the electrodes (cathodes vs. anodes), as well as the number of the channels responsible to evoke the desired contraction and joint movement pattern (hips, knees, and ankles). The mapping protocol may require stimulation for every specific muscle group and joint per each limb.Spinal mapping was carried out to identify optimal configurations (cathodal-anodal electrode arrangements and stimulation parameters) to enable multiple functions and movements withoutinducing unwanted activityFrom supine position, 12 EMG sensors were attached (left and right vastus medialis, rectus femoris, tibialis anterior, medial hamstring, medial gastrocnemius, and gluteus medius) after shaving and carefully cleaning the skin. The participant was then asked to make three major movements such as wiggling the big toe, dorsiflexion of the ankle joint or moving the entire leg into flexion followed by extension. We aimed to find a multiple electrode combination with the correct stimulation parameters to identify SCES configurations for standing, SCES-evoked recruitment curves from 5 standardized SCES configurations were established based on EMG peak-to-peak amplitude of each muscle group. Recruitment curves were collected by stimulating at 2 Hz from 1–10 mA; three recruitment curves were collected for each configuration at pulse widths of 250, 500, or 1000 µs. From these curves, optimum stimulation configurations that could yield tonic extensor activity for standing were determined. These optimum stimulation configurations were re-tested in a standing frame or standing with a standard walker to establish the standing configurations. Participants were then trained on how to use the SCES controller to activate paralyzed lower extremity muscles.The emphasis of the first 6 months of the study was to achieve overground standing, while the emphasis of the second 6 months was to achieve step-like activity to lead to overground ambulation. Thus, the initial temporary and permanent mapping periods prior to the first 6 months emphasized voluntary limb movement and extensor activity to facilitate overground standing.In the interim mapping phase, these original configurations were refined. Configurations which yielded tonic extensor activity in supine were refined to enable standing with the participant upright, first fully supported in a standing frame. Further refining was done by having participants attempt a sit-to-stand in an exoskeleton (EksoNR, Ekso Bionics, CA, USA) in “squat” mode, a manufacturer setting which provides assistive torque at the knees and hips sufficient to complete a sit-to-stand only if the user volitionally contributes to the movement. Lastly, configurations were refined with participants standing in parallel bars. In addition to refinement of configurations for standing, during the interim mapping phase, further mapping was conducted for rhythmic locomotor activity to facilitate stepping and overground ambulation. Different SCES configurations were tested at 2 Hz and 210 µs with incremental amplitudes to see which configurations preferentially recruited the vastus medialis muscles over the medial gastrocnemius muscles at lower amplitudes