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  • Christopher Breen

Metabolic Adaptation Part 1

Some athletes know all about the inner workings and mechanics of their bicycles. They can discuss ceramic bearings and how many watts it saves an athlete at length, but they take the human body for granted and forget that it is the most scientific machine of all. Afterall, it is the human body that is pushing that bicycle forward. Despite approximately 20 years of furthering technology the finishing times of the top competitors at the Ironman World Championship held in Hawaii are not drastically different. In 1996 the top pro male Luc Van Lierde swam 2.4 miles in 51:36, biked 140.6 miles in 4:30:44, and ran 26.2 miles in 2:41:48 for a total time of 8:04:08. Nineteen years later in 2015 the top pro male Jan Frodeno completed the same distances in 50:50, 4:27:27, and 2:52:21 for a total time of 8:14:40. You see the human body is the ultimate limiter in how fast we perform in a triathlon, no matter what technological advances are made in swimsuit fabric, bike technology, and running sneakers.

It is important therefore to understand just what is happening to our bodies as we train. Only by understanding how our bodies adapt to aerobic training can we devise training plans to improve upon one's endurance. In this short blog post and subsequent blog posts I will be writing about some key points of endurance training and the metabolic and cardiorespiratory adaptations that occur with it. Although I have kept it short, if science is not what you are interested in skroll to the bottom to find out the key points.

The following are metabolic adaptations to muscle as it pertains to increases in endurance training:

*Endurance training, specifically at low to moderate intensities such as those training in Zones 1 and 2 of heart rate or power (recovery to aerobic zones) requires the recruitment of slowtwitch muscle fibers first, also known as Type I muscle fibers and then the recruitment of fast twitch (a) fibers or Type IIa. Endurance training does not increase the percentage of slow twitch fibers, it does however recruit the use of fast twitch (b) fibers (Type IIb) thereby asking them to perform more like fast twitch (a) fibers which do have a moderately high oxidative capacity although not quite as high as slow twitch fibers.

*A significant known adaptation to endurance training is the increase in muscle capillaries. An increase in capillary production allows more blood to get to a working muscle, this in turn allows for a greater exchange of both nutrients and waste between the blood and a working muscle.

*Myoglobin is an iron and oxygen binding protein found in muscle fibers. When oxygen is delivered to the muscle it binds to myoglobin and the myoglobin releases it to the mitochondria when oxygen is limited during muscle action. Endurance training has been shown to increase muscle myoglobin by as much as 80%.

*Mitochondria are organelles that produce a cell's energy. The ability for a cell to use oxygen and produce energy relies directly on the number and size of its mitochondria and how efficiently its mitochondria are working. We know that increases in the volume of endurance training leads to both an increase in the size and number of mitochondria in a cell.

*The production of energy relies on the efficiency of mitochondria to use certain types of oxidative enzymes. The activities of these enzymes are increased with increases in endurance training, thus contributing to a muscle's aerobic capacity.

It is here where one can begin to understand what happens to an athlete's body as it is adapting to an aerobic training stimulus. In layman's terms the take home points are, as we train and increase our training volume our muscles become able to endure longer workloads and utilize oxygen for energy more efficiently. Blood flow to our working muscles is increased allowing them to get much needed nutrients and get rid of unwanted waste. Finally, muscle's powerhouse cells (mitochondria) are increased allowing our muscles to work longer, harder and more efficiently. It is true that genetics limits a good portion of these changes. However, we can maximize on our own abilities by developing proper nutrition plans to maximize energy and developing proper individualized training plans that take into account the appropriate volume, frequency, and intensity for a given athlete at certain times of their training cycles.

In my next installment I will be writing on adaptations to our bodies energy sources.


Wilmore, Jack H. (1994). Physiology of Sport and Exercise. Illinois: Human Kinetics.

Hermansen,L., & Wachtlova, M. (1971). Capillary density of skeletal muscle in well-trained and untrained men. Journal of Applied Physiology, 30. 860-863.


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