A revised NIOSH equation for the design and evaluation of manual lifting tasks was presented. NIOSH first developed an equation in 1981 to assist safety and health practitioners evaluate lifting demands in the sagittal plane, but this could be applied only to a limited number of tasks. A new 1991 equation reflected new findings, and offered new guidelines. The principal objective was to prevent or reduce the occurrence of lifting related low back pain (LBP). Three criteria (biomechanical, physiological, and psychophysical) were used in defining the components of the lifting equation. The rationale for selecting these criteria was described, and the use of these criteria in determining the equation values was demonstrated. Biomechanical criteria selected were the joint between the lumbar 5 and sacral 1 (L5/S1) vertebrae as the site of the greatest lumbar stress during lifting, compressive force as the critical stress vector, and 3.4 kilonewtons as the compressive force that defined an increased risk of low back injury. Physiological criteria included the choice of 9.5 kilocalories per minute as the baseline measure of maximum aerobic lifting capacity (MALC) for determination of energy expenditure limits (EEL) for repetitive lifting tasks, a baseline MALC of 70% to establish an energy EEL for lifts that mainly required arm work, and 50%, 40%, and 33% baseline MALC for establishing EEL for lifting tasks lasting 1, 2, and 2 to 8 hours, respectively. Psychophysical criteria included the rationale for choosing a criterion acceptable to 75% of female workers, and one for using maximum acceptable weight of lift and strength to determine recommended weight limits. Other aspects discussed included the identification of hazardous lifting jobs using the lifting index, and the limitations of the 1991 equation.