Because people's physical function is relatively low when they don't exercise for a long time, after some intense fitness exercises, those muscles that don't exercise for a long time will start to work and secrete some hormones. These hormones are the main culprit that makes you feel sore. Generally, muscle soreness will be eliminated in two or three days, and there will be no more soreness after repeating the previous exercise, because the body is used to such exercise, and if you increase the amount of exercise, your muscles may still be sore.
However, there are also special circumstances. If the urine is brown and the muscles are sore after a lot of exercise, it may be "rhabdomyolysis" and you should go to the hospital as soon as possible. Generally, proper fitness is enough, not too ambitious. Fitness is a long-term examination, and you don't want to do it overnight, so it won't hurt your body. Because we can't exercise for a long time, our bodies will become very weak, which will make our bodies unable to bear some stimulation brought by strenuous exercise, but the human body can bear great pressure, which will make us feel very uncomfortable and sore after fitness.
It is also possible that when we go to the gym for the first time, we will do some exercise, and at the same time we don't notice the limit that our bodies can bear, which leads to overload. Overload exercise has a great influence on the body, so you must do what you can when you exercise.