An open-source musculoskeletal model for predictions of lumbar muscle forces is presented in this paper. The model is based on a previously published anatomically detailed lumbar spine model, with upper extremity, lower extremity and head-neck models incorporated. It is implemented in OpenSim, an open-source musculoskeletal modeling platform. The features of this model include the ability to predict joint reactions, lumbar muscle forces and lumbar muscle activation patterns. To validate the ability of the model to predict lumbar loading, the loads acting on the vertebral bodies under several conditions were determined and compared with loads measured in vivo using instrumented vertebral body replacements and pressure transducers in the intervertebral discs. Model-predicted loads were strongly correlated with measured loads (r = 0.934); thus, the model successfully predicts variations in loading between different conditions. The model introduced here should ultimately aid in the ergonomics of machine design, since current machines are still mainly designed without knowing reasonable lumbar loads.