The Biology of Sport Endurance

The Biology of Sport Endurance


What do you think when you hear sports endurance? You might have thought about the time when you swore you would cycle to work but ended up catching the bus half way because you did not have enough endurance to make the trip. Endurance training includes exercises that help you increase your overall training. 

Before diving into sports endurance it is important to understand the biology of endurance. You should also ask yourself whether you are an endurance athlete or a speed athlete. Many endurance athletes go into the sports prematurely without gaining enough information about the sports, and they end up getting injured. 

We will be discussing more in the upcoming blogs on getting started, nutrition, injuries and challenging yourself.



Adrenaline is also known as epinephrine which helps to impede the secretion of insulin. Insulin slows the breakdown of your body's two most important energy sources. Endurance athletes tend to have higher adrenaline. If you are not an endurance athlete, fret not, the more endurance exercise you do, the more adrenaline you'll produce. 


I am sure you have heard about this term in your biology class. ATP is short for adenosine triphosphate, which is the primary energy currency. We gain ATP from food. The three main sources of energy in food are carbohydrates, protein, and fat. Food is being broken down in the body to give off energy (ATP). Endurance athletes need more energy because they break down energy aerobically (with oxygen). The more energy you have in your energy reserve, the less likely your body is going to switch to breaking down energy anaerobically (without oxygen) causing muscle fatigue. 


Capillaries are minute blood vessels at the end of your circulatory system's branching network. According to Scott Tinley, a world-renowned triathlon champ, "more capillaries mean that your muscles can remove heat and waste by-products more quickly. In short, it means that they will result in more efficient energy production".


Endorphins are painkillers that are released by our bodies to combat physical and mental stress. 

Glucose and Glycogen

For the endurance athlete, sugar means energy, and energy, of course, means everything. Glucose, simple sugar, or monosaccharide, is the end product of carbohydrate digestion. When exercising, it is the primary fuel for your muscles because it is readily available. Glycogen can be viewed as a secondary fuel source. It is made up of chains of glucose molecules, which are stored in the liver and muscle cells.

Lactic Acid

Lactic acid is a by-product of anaerobic metabolism. It accumulates when there is not enough oxygen to break down carbohydrates to create energy. Once the levels of lactic acid rise in your muscles, you will find your energy peters out and your fatigue snowballs.

Maximum Heart Rate

Knowing your heart rate is the best way to ensure that you are getting the workout you want. The most common way of measuring your heart rate is by subtracting your age from 220. Are you 40 years old? Your MHR is about 180 beats a minute. 

Maximum Oxygen Uptake (VO2 Max)

Maximum oxygen uptake is the amount of oxygen you are putting to use. It is a measure of your body's ability to take in oxygen, move it through the bloodstream and make it available for the breakdown of carbohydrates, fats, and proteins. These provide the energy that keeps you going. After all, if you can't deliver oxygen to your muscles and put it to good use, you can't perform.

Muscles and Muscular Endurance 

Muscles are organs that allow the various parts of your body to move. There are three kinds of muscles:

  1. Smooth or involuntary muscles, which are found, for instance, in the walls of your digestive tract.
  2. Striated or skeletal muscles, which are attached by tendons to the bones and are under voluntary control.
  3.  Cardiac muscles, are similar to skeletal muscles but are not under voluntary control and are found only in the heart.

As endurance athletes, we need to concern ourselves primarily with the muscles we can control, the skeletal muscles. 


It is important to understand that any type of training is highly specific. To be successful in your chosen sport, you need to stress the muscles and fibers unique to that sport. Once you understand the slow-twitch/fast-twitch distinction, it is easy to understand why you need to vary your training. 

  • Type IA, or high-oxidative, slow-twitch red fibers react slowly to nerve impulses, but they can produce great quantities of ATP over long periods. They fatigue slowly and are ideally suited to endurance energy. These are the fibers you would call on during a slow jog.
  • Type IIA or high-oxidative, fast-twitch red fibers react quickly to nerve impulses. They produce a lot of ATP quickly, but they fatigue far more rapidly. These are the fibers you would call on when you up the pace from a jog to a run.
  • Type IIB or low-oxidative, fast-twitch white fibers react very quickly to nerve impulses, but they fatigue far more rapidly than the other two types. These fibers produce ATP almost solely in the absence of oxygen. These are the muscle fibers you'd call on during a sprint.


Many athletes choose a hard/easy training program, following a day of intense training with one that is more recreational than purposeful. You can also alternate sports. If you are a runner, you can opt to do a hard interval workout on Tuesday, give your legs a break on Wednesday and spin easily on the bike. Whatever the pattern, the idea is the same. You do not ever want to stress the same muscle groups twice in a row.

Muscle Fatigue

Muscle fatigue can be a good thing. It is another mechanism that stymies our best efforts to destroy ourselves. Fatigue prevents us from doing irreversible damage to our muscles and other organs including the brain which hopefully will know better next time. 


Most endurance athletes do not research before getting into sports. You have taken the first step towards injury free training. Check out my other blog on the training tools and techniques to prepare you for endurance sports.


Scott Tinley and Ken McAlpine. Winning Guide to Sports Endurance, Rodale Press, 1994



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