The New Originals


The Biology of Running


You stretch, you head out the door and hit the trail. You sweat, you get sore, you rest, and you repeat.

Running seems simple enough. It's been universal to just about all humans throughout history. After learning to walk, running comes naturally. But beneath the surface, it's not quite so simple. The human body is a complex system, and has been evolving over thousands of years to become a lean, mean running machine. While we all know what running looks like from the outside, we were curious to find out more about what happens on the inside, so we asked nutritionist and TNF athlete Stephanie Howe to walk us through the various processes that take place in the body while running.

There are many signals that are released during running to signal and prepare the body for activity. The first thing that happens is stimulation of the sympathetic nervous system, which releases the neurotransmitter epinephrine. The influx of epinephrine prepares the body for flight or fight; heart rate and blood pressure increase to bring blood to the working muscles, glucose and fatty acids are mobilized to provide energy, breathing rate goes up, and we start to heat up.

During running, the muscles require oxygen to release energy. To get to the working muscles, oxygen travels through the body by way of the arteries. This process starts with the lungs: we breathe in and oxygen enters the lungs where it binds to red cells in the pulmonary blood vessels. Oxygen is then transported through the body to the muscles and tissues, where it is taken up for energy production.

The body has three main energy systems. When we first begin a muscle contraction, the body will use the stored energy, ATP, within the muscles. This stored ATP is only able to fuel the first couple seconds of exercise. The next system, anaerobic glycolysis, takes over and can supply the body with additional energy to fuel the activity for up to two minutes. This system is referred to as anaerobic glycolysis, because it does not require oxygen to break down glucose to yield ATP. Since anaerobic glycolysis is limited as well, another type of glycolysis called aerobic glycolysis can also break down glucose to fuel activity. The aerobic glycolysis works in combination with another aerobic system, the citric acid cycle, which uses fat and byproducts shuttled from aerobic glycolysis to produce ATP. Still with me? I know, metabolism is super complex! Just know that all three systems work in harmony to adequately fuel the activity. Still, after one and a half to two hours of activity, stored energy supplies in the body start to diminish and we have to supplement to continue moving at the same intensity. Otherwise we hit the proverbial wall.

Digestion and absorption are complex physiological processes in the body. Normally, food is consumed, broken down in the stomach, absorbed across the small intestine, and enters the bloodstream, where nutrients are carried to the liver and sent out to via the blood vessels to the body.

There are three types of macronutrients: carbohydrates, fat, and protein. All three nutrients are essential to health and optimal functioning of the body. While running, carbohydrate and fat are the primary sources of fuel. Your body uses both carbohydrate and fat, with the amount depending on the intensity of the run. Although many nutrition fads exist, it’s important to understand how metabolism works to provide sports nutrition recommendations. Unfortunately there is a lot of misinformation floating around that is largely based on anecdotal evidence. To best fuel your body during races and long training runs, focus on simple carbohydrates found in sources like gels and sports drinks

To maximize your ability to utilize fat, make sure to run your distance and long runs at an appropriate intensity. The biggest mistake new runners make is running their easy and long runs too fast! Slow down and enjoy the views! Save the fast running for your hard workout days. Protein is not used as a fuel during exercise, but is important for literally all functions in the body, such as signaling, sending messages, building and repairing, and much more. It’s important to consume protein on a regular basis, but focus on quality protein. Contrary to popular belief, it’s not actually difficult to get enough protein. In fact, most Americans overconsume protein. However, endurance athletes require slightly more protein due to the constant breaking down and building up of tissues. Just make sure to vary your sources so you get everything you need.

The initiation of running involves many physiologic process that work synchronously to allow us to run. It’s incredible actually, if you stop to think about it. There are so many things that have to go right to create movement.