What's happening when you get a second wind?

An energy-carrying molecule known as adenosine triphosphate (ATP) fuels every living thing — you, me, plants, animals, all of it — and, when you get right down to it, it's what fuels your second wind.

Adenosine nucleotides are part of the energy production systems of your body, specifically the energy metabolism of your cells. ATP is created from the process of metabolizing the carbohydrates, fats and proteins you consume. It's formed by a high-energy bond between lower-energy phosphates, adenosine diphosphate (ADP) and inactive phosphate (Pi).

The body makes an ongoing supply of ATP, and it starts with the breakdown of sugars from food. First, a reactive process called glycolysis traps and converts glucose, a monosaccharide, and converts it into fructose 1,6-bisphosphate. Next, that fructose 1,6-bisphosphate is split into two molecules of three-carbon pyruvic acid (CH3COCOOH); that's important, because ATP is produced when those three-carbon molecules are oxidized into pyruvate, the final product of the glycolysis energy-conversion process. In short, your body is constantly breaking down the food you eat and converting it to stored energy, which can fuel that second wind.

Because ATP is critical and stored only in limited amounts, the process of hydrolysis and resynthesis is circular and ongoing. ADP and Pi combine to synthesize and replenish the body's ATP, and through hydrolysis, ATP is broken down into ADP and Pi as needed for energy. That equation that looks like this: ATP + H2O → ADP + Pi + energy [source: Encyclopedia Britannica].

The human body fuels itself through three types of energy production methods, depending on how intense and how long you engage in that physical activity: phosphagen, anaerobic and aerobic energy production.

When energy is needed in a hurry, it's the phosphagen system that gives the body immediate energy, lasting only for seconds; ATP is able to fuel some pretty intense muscle contractions, but not for very long. Because the supply of ATP stored in the muscles is limited, the body can only sustain short bursts of energy, like sprinting for no more than five to six seconds [source: muscles.

After that first five seconds, the rate of glycolysis — that's the process that converts glucose to pyruvate, which is needed for cellular respiration — dramatically increases by 1,000 times than while the body's at rest. The anaerobic energy system, which uses carbohydrates but no oxygen to provide for the body's energy demands, takes over [source: Stipanuk et al.]. ATP is rapidly generated during anaerobic glycolysis, to be used during intense physical activities lasting between 30 seconds and three minutes [source: Gagliardi]. If the body's demand for oxygen becomes and remains greater than what you're supplying, there is an increased risk of lactic acidosis, when pH levels decrease in the body and byproducts of the breakdown of glucose to pyruvate accumulate in the body's tissues and bloodstream.

Most of the body's energy needs, though, are produced through a process called aerobic metabolism, also known as mitochondrial respiration. During aerobic endurance exercise, oxygen is required to generate energy from carbohydrates and fats — and to keep up the production of ATP, although its synthesis is low when the aerobic metabolism has kicked in. When the measure of your oxygen consumption (V02) reaches the maximum volume of oxygen your body can use (V02max), you've arrived at your second wind. You're what some refer to as "in the zone" — you're focused, you're not in pain, and your breathing deepens to provide maximum levels of oxygen to your working muscles and maximum ATP regeneration.

As your body gets accustomed to exercising and regulating its energy needs, the odds increase that you'll see your second wind kick in more frequently because your muscles, including your heart, will be more efficient.

"I know that I'm going to have a number of highs and lows over the course of an ultra, to the extent that I don't really think of it as a 'second wind' anymore," says Rob Colenso, ultra-marathoner and RRCA-certified running coach. "It's more like, I was able to properly eat and hydrate over the last hour, and so now I feel better and have gotten a burst of energy."