So people who carry heavy backpacks sometimes lean forward. Because of the heavy weight and this unnatural position, they can develop shoulder, neck, and back pain. If you wear your backpack over just one shoulder, or carry your books in a messenger bag, you may end up leaning to one side to offset the extra weight.

Neck pain from carrying a heavy bag is usually caused by muscle strain and poor spinal alignment. When you carry a heavy load—especially on one shoulder like a messenger bag or a "one-strapped" backpack—your body naturally leans to the opposite side to compensate for the weight. This puts an "uneven load" on the trapezius muscles and the cervical vertebrae, leading to inflammation and tension. Furthermore, heavy straps can compress the brachial plexus, a network of nerves running from the neck to the arms, causing tingling or radiating pain. To prevent this, always use both shoulder straps of a backpack, ensure the bag sits high on your back (not sagging toward the waist), and utilize a chest or waist strap to distribute the weight across your stronger core and hip muscles. Ideally, your bag should not weigh more than 10-15% of your body weight to avoid long-term "text neck" or postural issues.