Why do muscles fatigue with exercise?

I was recently asked the question, "Why do muscles fatigue?" I was kind of stumped, I knew it had something to do with the generation of ATP, but didn't know specifically why muscles fatigue. Well I had to do some digging in my old exercise physiology text book to find the answer.

Why muscle fatigues, depends on a few factors. First of all, what activity is the muscle trying to accomplish. Is it aerobic or anaerobic? Is it a short burst or a marathon? These are questions that must be asked first. For the sake of this discussion we will focus on anaerobic exercise-resistance training.

Fatigue is directly associated with the mismatch between the rate at which the muscles uses ATP and the rate at which ATP can be supplied. What is ATP? ATP stands for Adenosine Triphosphate. To put it simple ATP is our major energy source. When we workout ATP is used for our muscles to contract. During our workouts our ATP is depleted. This is where fatigue sets in. Fatigue is a protective measure for ours cells not to get severely damaged. When we rest is when more ATP is created giving us the energy to go at it again.

Many factors and chemical reactions take place in our bodies all the time. Our body is one complex system. The more you understand, the more you'll be amazed at what you body does for you. Good luck with your workouts.