Study Design. A systematic review and meta-analysis of studies on the putative mechanisms of action of lumbar supports in lifting activities. Objective. To summarize the evidence bearing on the putative mechanisms of action of lumbar supports. Summary of Background Data. A restriction of trunk motion and a reduction in required back muscle forces in lifting are two proposed mechanisms of action of lumbar supports. Available studies on these putative mechanisms of action of lumbar supports have reported contradictory results. Methods. A literature search for controlled studies on mechanisms of action of lumbar supports was conducted. The methodologic quality of the studies was assessed. The evidence for the two proposed mechanisms of action of lumbar supports was determined in meta-analyses. Results. Thirty-three studies were selected for the review. There was evidence that lumbar supports reduce trunk motion for flexion-extension and lateral bending, with overall effect sizes of 0.70 (95% confidence interval [Cl] 0.39-1.01) and 1.13 (95% Cl 0.17-2.08), respectively. The overall effect size for rotation was not statistically significant (0.69; 95% Cl -0.40-4.31). There was no evidence that lumbar supports reduce the electromyogram activity of erector spinae muscles (effect size of 0.09; 95% Cl -0.41-0.59) or increase the intra-abdominal pressure (effect size of 0.26; 95% Cl -0.07-0.59). Conclusion. There is evidence that lumbar supports reduce trunk motion for flexion-extension and latera bending. More research is needed on the separate outcome measures for trunk motion before definite conclusions can be drawn about the work conditions in which lumbar supports may be most effective. Studies of trunk motion at the workplace or during specified lifting tasks would be especially useful in this regard.