The Effect of Inertia and Initial Body Posture on Lower Extremity Kinetics and Trunk Muscle Activity in Lifting

Document Type : Original research papers


1 Department of Biomechanics, Faculty of Medical Sciences and Technologies, Islamic Azad University of Science and Research Branch, Tehran, Iran.

2 Department of Biomechanics, Faculty of Mechanical Engineering, Abhar Branch, Islamic Azad University, Abhar, Iran

3 Mechanical Engineering Department, K.N. Toosi University of Technology, Tehran, Iran

4 Pediatric Neurorehabilitation Research Center, University of Social Welfare and Rehabilitation Sciences, Tehran, Iran


Objectives: Because the function of the lower limb joints and the role of the trunk muscles in bearing and lifting loads are important, this study aimed to investigate the intervention of the initial body position when controlling inertia and its effect on the strategy of kinetic patterns in the lower limb joints as well as trunk muscle activity during symmetric lifting.
Methods: This experimental study included 10 healthy young man. They were asked to lift an 8.4 Kg box at 3 different knee angles (-10, 35, and 70 degree angle of the thigh to the horizon line) symmetrically and at a constant velocity. The extensor moment and power of lower limb joints were calculated using the 3D linked-segment model (LSM). To measure the muscular activity of abdominal muscles (rectus abdominus (RA) and external oblique (EO)) and lumbar muscles (iliocostalis lumborum (IL) and multifidus (MU)), an 8 channel electromyogram (EMG) were used.
Results: The results showed the similarity of extensor moment pattern of lower limb joints and lumbar joint (L5/S1) in accelerated lifting and constant velocity lifting with similar techniques. The results revealed that there was no significant effect of squat lifting while controlling inertia on lumbar extensor moment and back muscles activity. In all knee postures. There was also a significant effect of different knee postures on the extension power of lower limb joints.
Discussion: According to the findings, in the lifting without the effect of the inertial force of the lifted box, lumbar extension moment and back muscle activity was independent of different knee postures. However, the extensor power of the lower limb joints was affected by the knee postures. Moreover, the lumbar extensor moment and lumbar muscles activity in different knee postures had a similar behavior.


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