Backpacks introduce various forces onto the human body. Understanding how these forces interact with the back and hips is important for comfort and well-being, guiding choices in backpack design, fit, and packing.
Understanding Load and Force
A backpack exerts force on the body due to the physical principles of mass, weight, and gravity. The mass of the items inside is converted into weight by Earth’s gravitational pull, creating a downward force the body must counteract. This force, distributed over the body’s surface, is known as pressure.
Consider placing a heavy book directly on your hand versus spreading its weight across a tray. The book’s weight remains the same, but pressure on your hand decreases significantly when distributed over a larger area. Similarly, a backpack’s design determines how its total weight translates into pressure on different body parts.
Forces on the Spine and Shoulders
A backpack exerts a downward compressive force on the spine. This occurs as the pack’s weight pushes down through the shoulders and trunk, directly affecting the vertebrae and intervertebral discs. Studies indicate that for a neutral spinal alignment, axial compression on the spine can be approximately 7.2 times the backpack’s weight. If posture shifts, such as leaning forward by 20 degrees, this force can increase to about 11.6 times the backpack’s weight.
Shoulder straps create pressure on the trapezius muscles and clavicles. This pressure can lead to discomfort, tingling, numbness, or weakness in the arms and hands if straps are too narrow or the load is too heavy. Strap design, padding, and width influence pressure distribution. Improper fit or movement can also introduce shear forces on the upper back, where forces act parallel to the surface, potentially causing strain as the pack shifts.
Forces on the Hips and Lower Back
The hip belt transfers a backpack’s load from the shoulders to the stronger pelvic bones and lower lumbar spine. When properly used, a hip belt can shift 60% to 80% of the pack’s weight to the hips. This transfer reduces direct compressive forces on the shoulders and upper spine, allowing the more robust muscles and bones of the lower body to bear the load, distributing the downward force by pressing into the iliac crests (top of the hip bones).
The hip belt’s load distribution alleviates pressure points on the shoulders. However, an improperly fitted hip belt can create its own pressure points on the hips. A snug fit, with the belt resting flush against the body and the iliac crest in its middle, is important to evenly distribute weight and prevent localized pressure. Research suggests that while a hip belt reduces pressure under shoulder straps, it can also reduce compressive impulses in the lower lumbar spine, especially during downhill walking.
Optimizing Force Distribution
Optimizing force distribution in a backpack involves its design features. Modern backpacks often incorporate internal frames that help transfer weight efficiently to the hip belt. Padded shoulder straps and hip belts are designed to spread the load over a larger surface area. Adjustable components, such as torso length adjustments, load lifters, and sternum straps, allow for a customized fit for proper weight transfer and comfort.
Proper backpack fitting manages forces on the body. The hip belt should be positioned snugly over the top of the hip bones, where the majority of the pack’s weight should rest, ideally around 80%. Shoulder straps should be adjusted to be snug but not overly tight. Load lifters help pull the top of the pack closer to the body, improving stability. The sternum strap connects the shoulder straps across the chest, stabilizing the pack and preventing side-to-side motion.
Smart packing techniques also aid force distribution and overall comfort. Placing the heaviest items close to the back and centered between the shoulder blades helps maintain a stable center of gravity and prevents the pack from pulling backward. Medium-weight items can be placed around the heavy ones, and lighter items can fill the top or bottom sections. Balancing the load from side to side is equally important to prevent uneven strain and maintain stability.