Well,
during performance of exercises where the upper arm is held behind the body, the long head is placed in a greater position of stretch compared to the short head and therefore is able to generate maximal force.
Test yourself slowly and only when you understand that you can bear the pressure, continue with
the performance of the exercise.
This may suggest that a large hip extension moment is critical to
the performance of this exercise.
As for the back squat, this again suggests that a large hip extension moment may be critical for
the performance of this exercise.