Low-back pain and injury claims account for a large number of occupational illnesses each year. In the railroad industry, many maintenance and operation activities require a high degree of manual labor, often resulting in increased stress on the lowback. One of the most common functions of railroad yardmen is the setting and releasing of railcar hand brakes. A static three-dimensional low-back biomechanical model was developed to estimate the levels of compressive force on the L3/L4 spinal joint that existed in subjects during an experiment that simulated the hand brake setting task. We recorded three-dimensional body posture and resultant forces at the hands for analysis by the model. The model resolved the external forces acting on the body to a resultant moment about L3/L4 and then employed an optimization algorithm to estimate the internal lumbar muscle forces generated to resist the external forces. The muscle forces and external forces were added to arrive at a prediction of compressive force at L3/L4. The experiment investigated the effects of general body posture, left hand grip, subject anthropometry, and hand brake torque level upon predicted compressive force at L3/L4. An analysis of variance revealed that compressive force was significantly affected by each of the experimental variables. Additional analyses at subjects' maximum voluntary torque levels indicated that compressive force would exceed NIOSH guidelines for low-back compressive force, especially in males. Predicted L3/L4 compressive force at maximum torque ranged from an average of 2350N in small females to an average of 7485N for large males. We then used regression analysis to predict brake torque levels that would cause compressive force to exceed the NIOSH (1981) recommended maximum of 3400N. Based on the prediction methods used, hand brake torques of 40 to 80 Nm would be likely to cause compressive force to exceed this maximum. / Master of Science