Many studies have shown the importance of proper lifting methods. For example, lifting heavy weights from below the waist without bending the knees puts much more pressure on the low back than bending the knees while lifting. Likewise, lifting and holding objects away from the body (with the arms straight) increases the forces through the spine.
Other studies have shown that lifting when the back already hurts increases the stress on the back by up to 75 percent. In the injured worker, at least 10 extra muscles forcefully contract to help protect the back. The overall effect is a powerful compressive force through the spine. This can cause more damage to the spine.