If you work out regularly at the gym, then chances are high that you may be wearing your muscles out. How can you know when to stop pushing yourself over the edge while exercising? A study shows that when the muscles contract, they pump certain chemicals into your body.  When these chemicals cross a certain limit, you experience muscular fatigue. In the study, all these compounds were injected into the thumbs of the subjects individually as well as together. In the first case, the participants did not feel any sensation. In the second case, they reported swelling, tiredness and heaviness in their thumbs. So, be careful when your body gives you signals of exhaustion and rest your body.