Why Do My Muscles Grow Tired When I Exercise?
If you use your muscles for a long time when you exercise, a waste material called “lactic acid” collects in the muscle tissue. As a result, your muscles lose some of their ability to contract, or tighten. The tired feeling you get is a signal for you to rest.
As you relax, your body removes the lactic acid and replaces the energy you have used up. Then you are ready to get back into action. Muscles that are exercised a lot get bigger and stronger.
Larger muscles are able to store more energy; they have more room for lactic acid to collect, so that they don’t tire as quickly. In reality, this is a natural defense mechanism for the body; it prevents permanent damage during extreme exertion by slowing the key systems needed to maintain muscle contraction.
Once the body slows down, oxygen becomes available and lactate reverts back to pyruvate, allowing continued aerobic metabolism and energy for the body’s recovery from the strenuous event.