Comparing squat variations, Schwanbeck et al. (2009) explored lower erector spinae muscle activity in the back squat with either a barbell or in a smith machine with the same relative loads (8RM). (strengthandconditioningresearch.com)
Additionally, they reported superior lower erector spinae muscle activity during the bent over row and inverted row compared to the standing cable row. (strengthandconditioningresearch.com)
Assessing the effects of stability during split squats, Andersen et al. (2014) explored erector spinae muscle activity in the split squat with the foot placed on the floor or on a foam cushion, using 6RM. (strengthandconditioningresearch.com)