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Sternum straps are any type of material that connect two shoulder straps of a backpack, harness, or seatbelt system across the sternum. The purpose of sternum straps is to keep the shoulder straps in place, especially during impact and to ensure they do not slide off the shoulder at any point. The sternum strap on a backpack also helps keep the shoulder straps in position for comfort while the backpack is loaded down. These pieces of material are usually not load-bearing straps, though they will be rated for a load should an impact occur.
One of the common applications of sternum straps is a connecting piece on racing seat belts. Automobile racers are often secured into the seat of the car using a lap belt as well as shoulder belts, since the driver is likely to experience pull from forces during acceleration, deceleration, and turning. In this application, sternum straps will be used to keep the shoulder straps from shifting during turning, acceleration, and deceleration. Preventing movement of the shoulder straps also prevents limited mobility of the arms, as the driver will need to have his arms free during the entire duration of the race.
Backpacks worn during physical activity will have a tendency to shift, and all straps included with the backpack will be positioned to prevent such movement. Backpacks used for backpacking tend to be much larger than other packs, and they will feature two shoulder straps as well as a waist strap. Sternum straps in this case will be mounted on the shoulder straps, and the sternum strap is usually connected using a buckle system. This allows the user to take off the backpack quickly and easily when necessary; it also allows the user to adjust the tightness of the strap. Wearing the strap too loosely can allow the shoulder straps to work out of position, and wearing it too tightly can cause discomfort on the neck and shoulders.
Most sternum straps are made from a durable nylon webbing that resists stretching. Harnesses usually use nylon webbing that is rated to support a load, while backpack sternum straps can be made from lighter, thinner nylon. The straps are connected to the shoulder straps using a plastic or metal buckle in many cases, and the straps can be adjusted up or down so it can be positioned over the person's sternum properly.