Athletes and serious exercisers can get into an overtraining syndrome in which they become injured or suffer frequent infections, feel exhausted, don't perform up to their usual levels, and even lose interest in training.  One of the most difficult problems for athletes is knowing when they are training too much.
You make a muscle stronger only by stressing that muscle, feeling sore on the next day, and taking easy workouts or days off until the soreness goes away. Then you are supposed to take a hard workout again. If you do not feel soreness on the day after a hard workout, you have not injured your muscles, and they will not become stronger. However, if you try to work hard when your muscles feel sore, muscles do not recover and will feel sore all the time.
Every athlete knows that sometimes your muscles still feel little sore several days after a hard workout. You may think that you have recovered from your previous hard workout and you think you are ready to stress your muscles again, so you go ahead and try to run very fast and you start to feel sore all the time. Your joints, muscles and tendons ache. You feel tired. You can still run with the soreness in your muscles and tendons, but the soreness prevents you from running fast. Each succeeding day, the soreness increases and you think that you are sick, so you go to your doctor. He does a complete work-up and everything is normal, so you are stuck with a diagnosis of training too much.
Now you need to go back to background training. The same principles apply for all sports. If your sport is running, jog on the days that you can. Take days off when you feel sore. After several weeks, your muscle start to feel fresh again and you are able to start running. You are ready to start training again, but first you must promise yourself that you will never try to go hard when you feel soreness in your muscles and tendons. Set up a schedule in which you take a hard-fast workout, feel sore on the next day, and then go at an easy pace in your workouts until the soreness has completely disappeared.
Checked 9/23/19