In the study by Cynn et al. (2006), the cue was assisted using a biofeedback unit and the increase in gluteus medius muscle activity was associated with reduced lateral pelvic tilt angle. (strengthandconditioningresearch.com)
Comparing the split squat with the back squat, DeForest et al. (2014) assessed the muscle activity of gluteus maximus in resistance - trained males performing a split squat with elevated rear foot, a split squat, and a standard back squat. (strengthandconditioningresearch.com)