1: I haven't exercised for too long. When we suddenly go to exercise and fitness, not only will our knees hurt, but our muscles will also hurt, and even our whole body will not adapt. This is a normal phenomenon. We should warm up before exercise. When we exercise, we can't sit, squat or bend our knees with our hands, bow our heads and breathe immediately. You should walk slowly and do some simple stretching exercises. For example: lifting legs, reaching out, twisting waist, and so on!

2. Exercise too hard. Sometimes unprofessional exercise does not adjust yourself to the body. When we exercise, it is sometimes fast and sometimes slow, sometimes fierce and sometimes fierce, which leads to the body can't keep up, so the body will feel pain after exercise. As long as we can learn to master the rhythm of sports, this situation will be greatly improved.

3. Nutrition can't keep up. We need nutritional supplements during exercise. The most common is protein powder or egg milk, which can supplement our exercise consumption, enhance our physical function, supplement nutrition and improve health.

4: Calcium deficiency. This situation is rare, but it is also common now, mainly because of insufficient calcium and iron intake during meals, which leads to bone osteoporosis. This situation is more serious, so we should pay more attention to the intake of calcium and iron.

I hope my answer can help!