Leg soreness in a 12-year-old child may be due to physiological factors, such as growing pains and lactic acid buildup, or it may be due to pathological factors, such as calcium deficiency. 1. Growing pains: In the process of growth and development, children may show recurrent, intermittent lower limb pain due to the pulling of the periosteum, especially in the anterior side of the knee and calf. This is a normal physiological phenomenon, and usually can be relieved without treatment. With the maturation of bone development, the symptoms of leg pain can be reduced or disappeared. 2. Accumulation of lactic acid: During strenuous exercise, the anaerobic metabolism of the leg muscles increases, resulting in excessive accumulation of lactic acid in the legs, which can lead to leg pain in children. After the lactic acid is metabolized, most of the symptoms of sore legs will be relieved by themselves. 3. Calcium deficiency: If the child’s growth and development is too fast, it may cause vitamin D and calcium deficiency in the body. If calcium and vitamin D are not supplemented in time, it may cause leg soreness and even muscle spasm. In addition to the above reasons, fatigue osteochondritis dissecans, tumors and so on may also cause children’s leg soreness. If a 12-year-old child with sore legs is difficult to relieve on their own, it is recommended to seek medical attention in time to identify the cause and standardize the treatment under the guidance of a doctor.