The lifting task is the major activity contributing to the risk of lower back injury. The purpose of this study was to examine the mechanical stresses on the lower back during manual lifting and to analyze how people adapt to the stresses. This purpose was accomplished by approaching the mechanical stresses on the lower back from three different perspectives. The effects of manual lifting tasks were measured as: the fatigued condition, the knowledge of load and the previous lifts, and effort in lifting over a constraining barrier. A biomechanical model (linked segment and rigid body) was applied to estimate the peak compressive forces at the L5/S1 (lower back) joint during the manual lifting tasks. Three-dimensional locations of joint markers were digitized using a Peak Video and Analog Motion Measurement System. A strain-gage force platform was used to determine the location, direction and magnitude of external ground reaction forces acting on the feet. The peak compressive forces at the lower back were insignificantly different when individuals were fatigued. Some individuals increased and others decreased when they were fatigued. Individuals tended to lift faster and bring the load closer to the body when fatigued. Individuals applied different techniques of lifting when they were fatigued. Individuals lifted and adapted differently during the subsequent lifts when lifting known or unknown loads of different masses. Lifting under an unknown condition generated greater stresses on the lower back, especially when the load was light. When lifting a light load, individuals adapted to the lifts by bringing the load closer to the body, which reduced the stresses on the lower back. However, when the load was heavy, individuals adapted the lifts by changing the lifting technique, which did not reduce the stresses on the lower back. Lifting over a constraint barrier developed greater forces on the lower back. Individuals required more flexing of the lower back and less bending of the knees when lifting over the barrier. Individuals adapt to their internal or external lifting environment. These adaptations can increase or decrease the peak compressive forces.