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I know from experience that the proper form for best muscle activation and the most power is opening the chest. Done by tightening the traps to retract the scapula and contracting your lats to bring your shoulders down and sometimes also arching the back. I'm just curious as to why, more specifically, from a biomechanicol point of view, is this optimal? What's different between that and a flat back and/or concaved shoulders and chest that makes it so much better?

edit: Actually, I know the arch causes a slight decline, shifting focus to the lower chest which is stronger. I'm more curious about the upper back and lat involvement, and the positioning of the scapula

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It's mainly in how much of the muscle fibers get activated due to the stretch placed on the muscle.

When you have rounded shoulders and a concave chest, you are limiting the range of motion (ROM), and removing some of the stress at the lowest point of the lift. As you open up the chest, you stretch out the muscle more which causes the motor units to recruit more fibers.

Additionally, how the upper arms are placed in relation to the torso can emphasize or de-emphasize the triceps to some extent.

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  • There's also another thing that has slightly confused me. This may be a whole new question, but I've always wondered, why is the form the same for back exercises and not the opposite. For the best upper back and lat involvement, you pull back the scapula and shoulders, same as you would for bench. Why is the chest better when open and stretched, but the back is stronger when concaved and pre-tightened?
    – Ethan
    Jan 19 at 20:28

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