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What Happens to Muscles During Exercise?

Adopting a regular exercise routine is one of the best things that you can do for your long-term health. With each passing year, researchers learn more about the benefits of frequent movement.

Exercise benefits all tissues in your body, including your heart, blood vessels, muscles, bones, ligaments, tendons, immune system, intestines, liver, pancreas and brain. That’s right, even tissues that have nothing to do with performing exercise receive the benefit of frequent activity.

In this article, I’ll share some important aspects of muscle physiology that can help you create a personalized plan for optimal athletic recovery.


Exercise is a Beneficial Stress

Exercise is considered a “stress” to many organ systems, but differs from the negative stress of everyday life in that it stimulates the breakdown, repair and growth of muscles, ligaments, tendons and bones in the process of creating a stronger and more resilient body.

During exercise, muscles must perform two main tasks:
(1) “Burn” available fuel for energy
(2) Contract in response to a rush of electrical signals from the brain

Here’s a behind-the-scenes look at how your muscles fuel themselves during your workouts.


Muscle Fuel During Exercise

Muscle is capable of burning multiple fuels during exercise, including glucose (from carbohydrates), fatty acids (from fat) and amino acids (from protein). The type of fuel that is burned for energy depends on the intensity and duration of exercise being performed.

In the same way that a car stores fuel in a fuel tank, muscles have evolved the ability to store glucose, fatty acids and amino acids on-board. All three fuels are burned for energy in the mitochondria, organelles within muscle cells that function much like a car engine.


Glucose is Stored as Glycogen

Glucose is stored within each muscle cell as glycogen. Glycogen is a quick-burning fuel used during high intensity exercise.


Fatty Acids are Stored as Triglyceride

Fatty acids are stored within muscle cells as triglycerides. Triglycerides provide a secondary fuel source for low intensity exercise.


Amino Acids are Stored as Muscle Protein

Finally, amino acids are stored within the muscle tissue as muscle protein itself. Unlike glucose and fatty acids, there is no storage tank for amino acids in the muscle tissue. The muscle itself is the amino acid storage tank.

Here’s another way to visualize the storage tanks in muscle tissue:


The Choice of Fuel Depends on Exercise Intensity

As you can see in the graph below, as the intensity of exercise increases, the dependence on carbohydrate goes up and the dependence on fatty acids goes down. This means your muscles will use your glucose storage tank to fuel your workout so you can eat more carbs!

At low intensities, fatty acids are the main fuel source and only small amounts of glycogen are broken down. As the intensity of exercise increases, larger amounts of glycogen are broken down and burned for energy, making glucose the predominant fuel source.

Amino acids from protein are the lowest priority fuel, given amino acids are the infrastructure of the muscle tissue itself. In order to preserve muscle mass, the muscle will burn glucose and fatty acids before resorting to amino acids. Brilliant design.


Muscle Microtrauma

Even though amino acids from muscle protein are the last choice for fuel during exercise, microscopic tears result from repeated muscle contractions, called microtrauma. These microscopic tears are one of the signals that the muscle requires in order to repair during rest.

Think of microtrauma as the repeated wear-and-tear that your car experiences from driving long distances. In the same way that you replace damaged engine parts with newer and more efficient technology, microtrauma requires repair work immediately following exercise.

Recovery is a vital, often overlooked aspect of your workout regimen. It’s very important to let your muscles rest – and replace your “fuel” with the right foods.



Muscle is the largest type of tissue in your body, and is extremely malleable because it responds to the type, duration and intensity of exercise that you perform. Frequently exercised muscle tissue is in a constant state of remodeling, leading to increases in endurance, strength, flexibility and power.

The next time you perform a workout, keep in mind that your muscle is performing a number of tasks at the same time, including:
(1) Choosing the right fuel
(2) Protecting against muscle protein breakdown
(3) Contracting up to thousands of times in a single exercise session



Source: FitStar

By: Cyrus Khambatta — FitStar Contributor; April 2014

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