The weight of your phone

Expert insight:

Your head only weighs about 10 to 12 pounds, but as you tilt it forward, more weight is placed on the spine, Melendez says. When you look down at your phone at a 30-degree angle, for example, your spine bears about 40 pounds of pressure. 

This can cause headaches, jaw pain, dysfunctional breathing, and decreased mobility, he notes. Those last two effects are especially bad during workouts since they can lead to overcompensation, creating muscle imbalances and upping your injury risk.

The bottom line:

Phone use is the biggest culprit. Melendez suggests holding your device at eye level a few inches in front of your face and never looking at it for more than 15 minutes at a time. That's the point when your attention to posture starts to fade.