Ambulation after spinal cord injury is possible with the aid of functional electrical stimulation (FES). Individuals with incomplete spinal cord injury (iSCI) retain partial volitional control of muscles below the level of injury, necessitating careful integration of FES with intact voluntary motor function for efficient walking. In this study, the surface electromyogram (sEMG) of the volitionally controlled Erector Spinae was used to detect the intent to step and trigger FES-assisted walking in a volunteer with iSCI via 8-channel implanted stimulation system. The inference system was able to trigger the FES-assisted swing-phase of gait with a false positive rate of 1% during over ground ambulation on a level surface. The performance of the sEMG inference system highlights its potential as a natural command interface to better coordinate stimulated and volitional muscle activities than conventional manual switches and facilitate FES-assisted community ambulation.