Body posture is a basic indicator for assessing health and quality of life. Maintaining the correct sagittal alignment is an important factor in the proper functioning of the spine. The aim of the study was to determine the impact of changes in standing body posture in the subsequent decades of life on the functioning of the human musculoskeletal system. The analysis of loads on the musculoskeletal system was based on numerical simulations carried out in the AnyBody Modeling System. The normative values of whole-body sagittal alignment in the asymptomatic adult population presented by Hu et al.[1] were used as input data to develop the mathematical model. Simulations of the standing position in the subsequent decades of life were based on the following parameters: knee flexion angle (°), pelvic tilt (°), and sagittal vertical axis value (mm). The simulations were performed taking into account the gender division: women (avg. height: 159.1cm, weight: 54.9kg) and men (avg. height: 168.5cm, weight: 69.5kg). The dimensions of the body segments and the maximum values of muscle force were determined using the ScallingLengthMassFat. The simulations made it possible to determine the impact of changing body posture on the resultant values of reaction forces in individual lumbar spine segments and muscle fatigue. The simulation results showed that changing human body posture over the years affects the functioning of the musculoskeletal system. At the age of 20-29, in the L5-Sacrum segment for both women and men, reaction force values of 0.82%BW (Body Weight) were recorded, while in the age group 80-89 these loads increased by 15-18% and amounted to 0.98%BW for women and 0.95%BW for men. The increase in loads may be due to the posterior pelvic tilt that deepens with age [2]. An increase in muscle fatigue with age was also noticed. FUNDING This research was funded by the National Science Centre Poland, project: “Mathematical model of prediction of health consequences in musculoskeletal system as a result of sedentary lifestyle”, grant number: 2019/35/O/ST8/02719. REFERENCES [1] Hu Z. et al. 2020 Young Investigator Award Winner: Age-and Sex-related Normative Value of Whole-body Sagittal Alignment Based on 584 Asymptomatic Chinese Adult Population from Age 20 to 89. Spine (Phila Pa 1976),45:79–87,2020. [2] Michnik R. et. al. The effect of the pelvis position in the sagittal plane on loads in the human musculoskeletal system. Acta Bioeng Biomech,22:33–42,2020.