Objectives: The main objective of this study has been qualitative investigation of the effects of external loading on the lumbar extension moment during squat lifting. Findings of this study may allow to determine the factor with the most considerable effect on the lumbar extension moment and may help determine the lumbar spine risk factors at temporo-spatial coordination during squat lifting. Material and Methods: Twelve healthy men volunteered to perform slow and fast squat lifting of a box of varied mass (4 kg, 8 kg and 12 kg). The eight-channel electromyography was applied to detect the activities of abdominal (rectus abdominis and external oblique) and lower back muscles (iliocostalis lumborum and multifidus). The lumbar extension moment was calculated using 3D linked segment model. Ground reaction forces and kinematic data were recorded using a Vicon system with 2 parallel Kistler force-plates. Results: Significant increases (both p-values < 0.05) were detected for the peak lumbar extension moment with increases in the lift speed and box weight. Moreover, a significant interaction (p = 0) was detected between the lift speed and box weight. Furthermore, insignificant differences (all p-values > 0.05) were detected between the lumbar angles related to the lower trunk muscles peak activities and lumbar angle related to the peak lumbar extension moment in most of the lifts. Conclusions: According to the findings, the inertial force of the lifted box is the most important factor that affects the lumbar extension moment during squat lifting. Moreover, critical lumbar angles are seemingly those ones in which the lifted box reaches the peak acceleration. Int J Occup Med Environ Health 2017;30(4):665–679