Unlock your t-spine part 3
This exercise has quickly become one of my favorite in terms of teaching my clients to learn to engage the muscles of the upper back and get the shoulder blades back in their proper place all WHILE keeping the muscles of your core engaged.
Typically people can do one or the other, either they extend the upper back while increasing the curve of their lower back or flatten the lower back while hunching and rounding the upper back. Kind of like the standing equivalent of a cat/cow exercise.
This exercise is great for teaching you how to separate the two movements.