Medicine Physicians that specialize in sport's medicine say that having bad posture can put stress on certain muscles and joints, forcing them to be overworked and causing the to fatigue. https://health.usnews.com/10-ways-poor-posture-can-harm-your-health
There are many negative effects of bad posture, a couple of these include: 1. The one experienced more often which is soreness and pain 2. Poor circulation, not only does it cause you pain, but it also messes with the fluids and gases flowing through your body. http://www.cryousasolutions.com/5-negative-effects-bad-posture
There are many ways to treat bad posture now. 1. Assessment and diagnosis of postural habits 2. Postural education and training 3. Manual therapy and soft tissue massage 4. Corrective excersises and movements to improve flexibility, strength and posture.