You sit too much so your hip flexor tightens up. Your hip flexor tightens up and starts tugging on your back, impacting your posture. Your posture is bad and your chest starts caving in and your upper back rounds—your breathing is impacted.
Whoa! Put the brakes on all of that and do this hip flexor stretch:
Hip Flexor Stretch
Kneel on the ground, placing one knee on a pillow (or soft earth) with the other leg in front, foot flat on the ground. Place your hands on your hips and push your hips forward and then pull them back, rhythmically.
You should feel this stretch in about the same place as you can see my hand. The hip flexors connect to the back. Many people experience back pain due to tight hip flexors. Tight hip flexors pull on the back and can alter the natural curve of the back.
As you push your hips forward, you can simultaneously look up and arch your back rearward. This helps to open the chest, which is another important feature of good posture and another area that excessive sitting impacts negatively.
Do 5-10 oscillations, then switch to the other leg.