They report that the lateral gastrocnemius displayed superior muscle activity during the concentric upward phase of the back squat at 75 and 90 % of 1RM when compared to the same relative load in the overhead squat. (strengthandconditioningresearch.com)
The gastrocnemius displays its greatest muscle activity during the concentric upward phase of the exercise regardless of variation. (strengthandconditioningresearch.com)
Therefore, it appears that the gastrocnemius displays superior muscle activity during the concentric upward phase during the narrow stance back squat. (strengthandconditioningresearch.com)