Dan

A weightlifting belt has two main purposes. It reduces stress on the lower back while the person is lifting in an upright position and prevents back hyperextension during overhead lifts (2). A belt reduces low back stress by compressing the contents of the abdominal cavity.This increases the intra-abdominal pressure (IAP), providing more support in front of the bones of the lower back. This allows the spinal erector muscles, which would normally provide this
support of the lower back, to produce less force during the lift (2). Another benefit of increased IAP is a reduction in the amount of spinal shrinkage (lower back compression) a lifter may experience during circuit weight training (1). Some belts have a wide back and a narrow front. Therefore, it would be advisable to wear the belt backwards if increased IAP is desired, as the area gives the contents of the abdominal cavity more surface area to push against.
The belt prevents back hyperextension by forming a rigid wall around the lower torso, connecting the rib cage to the hip (2, 6). This not only limits back movement, but it also prevents sideward bending and twisting. A powerlifting-style belt that is the same width all the way around would be ideal for this purpose. Otherwise, a conventional belt can be worn in the usual manner with the wide part of the belt in the back.