Abstract: BACKGROUND:  Overweight and obesity are becoming more and more common in young children. Studies have shown that excessive body mass affects the fundamental movement competence of young children. However, there are fewer studies using biomechanical methods for analysis, and there is a lack of researches on using simulation to analyze the muscle force during exercise.  
OBJECTIVE: To analyze the kinematics, dynamics and lower limb muscle strength of children of different weights during vertical jump using sports biomechanics research methods, and compare the differences in these data.
METHODS:  Totally 52 children aged 4-5 years old were selected from a kindergarten in Beijing using a random sampling method. According to the body mass index, they were divided into a normal body mass group (n=20), an overweight group (n=16) and an obesity group (n=16). The BTS three-dimensional infrared motion capture system, the Kistler three-dimensional force plate and the VIXTA video analysis system were used to synchronously collect the kinematics and dynamics data of the children during the vertical jump. AnyBody7.1.2 simulation modeling software was used to calculate the lower limb muscle strength indicators.  
RESULTS AND CONCLUSION: (1) The vertical jump height and knee changes in the normal body mass group were higher than those in the overweight group and the obesity group (P < 0.05 , P < 0.01). The minimum knee flexion and the trunk angle were greater in the normal body mass group than those of the obesity group (P < 0.05). (2) The soleus lateralis, gastrocnemius lateralis, and gluteus medius anterior in the normal body mass group were smaller than those in the overweight group (P < 0.05). (3) The soleus lateralis, gastrocnemius lateralis, peroneus longus, inferior funicle of vastus medialis, and anterior funicle of gluteus medius in normal body mass group were smaller than those in the obesity group (P < 0.05). (4) The semitendinosus and the anterior funicle of gluteus minimus in the normal body mass group were larger than those in the obesity group (P < 0.05). The inferior and superior of gemellus and obturator internus in the normal body mass group were larger than those in the overweight group (P < 0.05). (5) Peroneal brevis muscle was larger in the obesity group than that in the normal body mass group (P < 0.05) and overweight group (P < 0.05). (6) Posterior funicle of gluteus minimus was smaller in the obesity group than that in the normal group (P < 0.05) and overweight group (P < 0.05). (7) The results showed that the vertical jump height of overweight and obese children was lower than that of normal-weight children of the same age, and showed the kinematic characteristics of the initial vertical jump stage. During the landing stage, overweight and obese children have greater ground reaction forces in the front and rear and left and right directions. The main muscles that children use when they complete the vertical jump are the soleus lateralis, the gastrocnemius lateralis, the peroneus brevis, the peroneus longus, and the quadriceps, gluteus medius, gluteus maximus muscle, and adductor magnus muscles. The normalized muscle strength of the quadriceps femoris and the active hip joint of the overweight and obese children was lower than that of the normal weight children of the same age.

Key words: children, vertical jump, lower limbs, kinematics, dynamics, simulation, body mass index, biomechanical characteristics