“Good” posture is imperative, but achieving it can be much more complex than you would think. “Postural activity precedes and accompanies every purposeful movement.” So simply “straightening up” will likely not have the impact one would envision. Generally, in addressing posture, stabilization (or lack there-of) is the first piece of the puzzle. This stabilization does not include crunches, situps, “ab”work, or planks. It entails a reflexive trunk strength and stabilization that starts with the breathing pattern. Diaphragmatic function acts as the foundation of true trunk stabilization, and without it…well, “No movement exists without stabilization of the trunk as a whole.”
Kolar, Pavel et al. (2013). Clinical Rehabilitation. Prague: Kobesova, Alena.