This report focuses on welding work for the assembly of large steel structures as an example of physical jobs. Task simulations using a digital human model, including metabolic energy expenditure analysis, have been carried out using the biomechanical approach for typical welding postures. Moreover, necessary rest time to recover from fatigue has been studied, and the optimal work cycle in a day was examined. As a result, it can be concluded that the flat position for welding, the most widely applied posture, requires the greatest energy expenditure, whereas the overhead position is requires the least. Furthermore, it is concluded that the rule of taking short breaks and often is preferable from the viewpoint of recovery from fatigue, especially for work where the consumption of energy is large. Finally, an optimal work cycle is proposed.