Using Energy How Do Muscles Do It ?

When you exercise aerobically, your muscles have the energy to contract because ATP is present in your system.You can keep going comfortable as long as your body can keep up.When you exert your muscles for a high intensity activity, such as lifting a weight for a very short time, your muscle is tired because the cells run out of ATP at the part of the muscle that controls muscle contraction. The fibers of the muscle have no energy and can't contract. When you exercise heavily for a longer period of time the muscles get tired because lactic acid, a byproduct of anaerobic methods for producing energy, accumulates in them.Usually, any excess lactic acid that your body produces while exercising clears from the muscles within about an hour after you stop.Some of the lactic acid is metabolized in the muscle cell, and some goes to the liver. Your clients muscles may be tired and feel sore while exercising because this  lactic acid is irritating nerve endings, but they will feel better later.When you exercise for an hour or longer, the fatigue your muscles feel is from the lack of glycogen. Glycogen is glucose in its stored from, and you probably remember learning about glycogen and its importance to the body in lesson 6. If you've tried an endurance activity, such as running a marathon, and hit the wall you've experienced glycogen depletion. You may have plenty of oxygen in your system, but without glycogen, it doesn't matter.