Sore and weak legs and knees may be caused by strenuous exercise, calcium deficiency, synovitis, ligament injury and so on. 1. Strenuous exercise: If patients occasionally exercise strenuously, it will lead to an increase in lactic acid secretion, which will lead to the phenomenon of weak and achy legs and knees. 2. Calcium deficiency: Calcium deficiency will lead to a decrease in the ability of the nervous system to control the muscles, and the patient will experience symptoms such as weak and achy legs and knees, easy cramps and joint pain. 3. Synovitis: synovitis usually occurs in the knee area, the patient’s synovial membrane will produce inflammation after being stimulated, resulting in the imbalance of secretion fluid, thus forming a fluid accumulation, and the phenomenon of leg and knee pain and weakness. 4. Knee ligament injury: If you have a history of knee trauma and have weakness during activities, you need to go to the hospital for timely consultation, and if necessary, have a magnetic examination of the knee joint. If the patient has the phenomenon of weak and achy knees, it is recommended to go to the hospital in time, diagnosed by a professional doctor, and make reasonable treatment.