Abstract: BACKGROUND: Falls are an urgent health concern globally and the most common cause of injury-related death in older adults.
OBJECTIVE: To analyze the characteristics of lower limb muscle exertion and the mechanical mechanisms affecting the risk of falls from sitting to standing. 
METHODS: Forty-five subjects recruited from current college students and retired employees over 65 years between July and September 2023 were divided into a young adult group, an elderly group of 65-69.9 years old and an elderly group of 70-75 years old. A three-dimensional infrared motion capture system and a dynamometer platform were used to capture the movements from sitting to standing. C3D files were input into AnyBody7.2 simulation software to simulate and process the lower limb muscles after completing the motion. A one-way analysis of variance was used to analyze the differences in lower limb muscle strength, contribution, active muscle, antagonistic muscle and functional ratios between the two groups.
RESULTS AND CONCLUSION: (1) Compared with the young adult group, the completion time of the elderly groups was significantly higher than that of the young adult group (P < 0.05). (2) There were significant differences in the strength of the adductor longus, adductor brevis, adductor maximus, obturator lateralis, quadratus femoris, piriformis, plantaris, biceps femoris brevis, femoris medialis, femoris lateralis, gastrocnemius lateralis, soleus lateralis, peroneus longus, and tibialis anterior muscles (P < 0.05). (3) There were significant differences in hip joint antagonistic muscle and functional ratio, knee joint active muscle, antagonistic muscle and functional ratio, and ankle joint active muscle and functional ratio (P < 0.05). (4) There were significant differences in the peak sagittal torque of the hip, knee, and ankle joints (P < 0.05). These two elderly groups showed significant differences in the strength of the pyriformis, tibialis anterior muscles, and ankle extensors (P < 0.05). To conclude, the main joints that exert force in the movement are the knee and ankle joints, with the hip joint playing a controlling role. The high contribution rate of deep muscle groups in the ankle joint can increase the stability of completing movements. Weakness in hip joint antagonistic muscle group strength, knee joint active muscle group strength, and ankle joint active muscle group strength leads to an imbalance in the human-machine performance ratio of older adults. The contribution rate of the deep adductor muscle group in the hip joint is low, and the strength of the antagonistic muscle group is insufficient, resulting in uneven coordination of muscle action. When completing movements, the body’s ability to control balance is weakened, the completion time is prolonged, and the risk of falls in older adults is increased.

Key words: older adults, from sitting to standing, lower-extremity biomechnics, simulation, active muscle, antagonistic muscle, functional ratio