How much does that backpack weigh? Studies have shown that the weight of a backpack and the amount of time carrying a backpack can impact neck and shoulder posture. If a backpack is 15% of an individual’s body weight, then it is too heavy for a child to maintain proper posture. Many children and older teens develop poor posture while carrying heavy backpack loads, and the affects can be hard to reverse. Avoiding heavy backpacks can mean avoiding chronic back pain.