Objective Based on OpenSim platform, an improved musculoskeletal model was developed to analyze the force of lumbar muscle groups under forward flexion. Methods The existing lumbar musculoskeletal model was improved via modifying constraints of lumbar vertebrae to restore them into 6 degrees of freedom (DOF). The 30 year-old and 70 year-old muscle models were established respectively by adjusting muscle parameters according to Thelen’s model, adding stiffness matrixes and abdominal pressure (AP) characterized by concentrated force to investigate the effects of arm swing, movement velocity, varying AP and muscle aging on the force distribution of 9 lumbar muscle groups during forward flexion. Results A multi-body musculoskeletal model with 9 lumbar muscle groups was developed. Based on the computations of 0°-70° flexion, the results showed that arm swing reduced the force of psoas and external oblique abdominis, while under the movement of forward flexing to 70° and returning up-right, the force of psoas, erector spinae, rectus and external oblique abdominis obviously increased, when the time of flexing-returning process was reduced from 5 s to 2.5 s, and in the 5 s case, increasing AP reduced the force of psoas but increased the force of transversus, internal and external oblique abdominis. In the 2.5 s case, there was no obvious difference between the 30 year-old and 70 year-old muscle models under different conditions. Conclusions The developed model provides an effective method to analyze the force of lumbar spine and muscles, and it certainly shows a potential application in the fields of kinematic mechanics and rehabilitation engineering with further development of basic theory.