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INTRODUCTION: The aim of this study was the analysis of human musculoskeletal system energy costs of normal walking and walking under reduced weight loading. METHODS: There were 15 subjects who participated in the study. We analyzed the biomechanical parameters of walking under different musculoskeletal system loads. The subjects walked on a treadmill at a pace of 90 steps/min under various loading conditions: 1) 100% bodyweight loading, corresponding to the terrestrial surface; 2) 38% bodyweight loading, corresponding to the surface of Mars; and 3) 17% bodyweight loading, corresponding to the surface of the Moon. Joint angles and angular velocities were recorded from the hip, knee, and ankle. RESULTS: We analyzed changes in joint phase trajectories and the ratio of kinetic extension energy to kinetic flexion energy in the joints. We observed changes in kinetic energy parameters associated with both flexion and extension motions in the joints of the feet while walking under various loads. In terrestrial conditions (walking under 100% bodyweight), flexion kinetic energy in the hip joint prevailed over extension kinetic energy by 90%, with a small variation equal to 22%. If weight loading decreased up to 17% (lunar conditions), the difference between flexion and extension kinetic energies diminished, and eventually reached only 9%. The ratio of flexion energy and extension energy in the ankle joint equalized under lower loading conditions. Thus, 38% bodyweight loading was sufficient for approximation of flexion and extension energy values. DISCUSSION: Our results revealed that phase trajectories shifted toward smaller joint angles and a decreased ratio between extension kinetic energy and flexion kinetic energy in the knee joint of all subjects. However, significant differences in the ratio of flexion and extension kinetic energy in the knee joint under bodyweight support were not found. The methods used for musculoskeletal system assessments that were proposed in our work can be used in clinical practice to evaluate the effectiveness of rehabilitation measures in a patient's musculoskeletal system disorders. [ABSTRACT FROM AUTHOR]