Sore thighs can be caused by physiological or pathological reasons, such as excessive exercise, synovitis, meniscus injuries and so on. 1. Physiological reasons: may be a long time lack of exercise, if suddenly over-exercise, there will be a large amount of lactic acid accumulation in the body, resulting in thigh muscle soreness, leading to thigh flexion and extension of the action of the difficulty, generally can be relieved after resting for a period of time. 2. Pathological causes (1) Synovitis: Increased secretion of joint fluid in the knee joint leads to swelling of the joint, which also causes soreness in the thigh. In addition, when the leg is bent, the synovial membrane inside the joint will be squeezed, resulting in pain and discomfort in the knee joint and inability to bend the leg. (2) Meniscus injury: Meniscus injury may be caused by wrong exertion during exercise or trauma, etc. Because the knee pain is radioactive, it is felt that the muscles of the thigh are also painful. At the same time, it will appear in the obvious joint pain, swelling and fluid accumulation, etc., and also thigh soreness and inability to bend. There are many other causes of thigh pain can not bend, such as lumbar disc herniation, etc., if the rest does not relieve, it is recommended to go to the hospital in a timely manner, improve the relevant examination, clear the cause of the disease to follow the doctor’s instructions for targeted treatment or treatment, so as not to delay the condition.