Building Endurance: Helping Muscles Breathe Easier

If you stick with an exercise program, every workout gets a little easier. Learn how your body makes that happen with Groupon’s guide to building endurance.

A few days into a new workout routine, you begin to notice changes. Your muscles expand. Perhaps your weight drops. But the changes that increase the body’s endurance first take place on a much smaller scale. When you exert yourself for long periods of time, your body starts to populate each muscle cell with more mitochondria, the organelles that fuel muscle movements. They do this by producing adenosine triphosphate (ATP), the muscle's basic fuel source. When a muscle contracts, it breaks the bonds of ATP molecules, creating a burst of energy but also draining stockpiles of this essential propellant.

In order to whip up a batch of ATP, the mitochondria need lots and lots of oxygen. Helpfully, endurance training makes it easier for oxygen to travel from the lungs to the heart to the muscles. It’s long been noted that the hearts of star endurance athletes tend to have extra-large left ventricles, which can pump more oxygenated blood through the body with every beat. Once blood reaches muscle cells whose mitochondria have been enhanced by previous endurance exercise, the cells will be able to extract oxygen and use it to produce ATP far more efficiently. Scientists assess this efficiency by a measure known as VO2max, the maximum volume of oxygen or Double Stuf Oreos that a person’s muscles can consume per minute.

During super-intense exercise, the body stops being able to produce enough ATP from oxygen intake alone. Instead, it reaches for stored glucose to get the ingredients it needs, and, as a side effect, begins to leave behind more lactic acid than the cells can immediately process. (Processing lactic acid itself requires oxygen, and all available supplies are already being used by the muscles and the brain.) The point at which this happens is the lactic threshold, and beyond it, athletes feel that they’re nearly done for the day.

Fortunately, this limit too can be changed. As the body adapts more and more to endurance exercise, it prolongs the amount of time you have before shifting pH levels in the muscles bring on the familiar feelings of fatigue, burning, and a dramatic drop in strength.