Focus ~ Food ~ Fitness


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.

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.
The Choice of Fuel Depends on Exercise Intensity

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 remodelling, 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

What Happens to Muscles After Exercise

Work Rate
Exercise increases the muscle work rate up to 30-fold. In order to sustain these large increases in work rate, glycogen and triglyceride fuel stores are depleted on-demand. Following a challenging workout, muscle glycogen and triglyceride stores are depleted by as much as 70%.

The Balance Between Onboard and Imported Fuel

As the glycogen and triglyceride fuel tanks run low, the muscle starts importing glucose and fatty acids from the blood. This process happens on-the-fly, and the amount of imported fuel depends mainly on the amount of fuel currently in the muscle tissue. When fuel stores are high, onboard fuel is depleted first. When onboard fuel stores fall low, more fuel is imported from the blood. This is how endurance athletes are capable of performing exercise for many hours – because they have trained themselves to recognise when their muscles are running low on fuel. Eating during a workout provides a rapid influx of fuel into the blood that becomes available for active muscles during exercise.

Fatigue and Exhaustion

You may have noticed that exercise becomes more difficult towards the end of a workout, as you become physically and mentally fatigued.

These feelings of exhaustion result from:

(1) Depleted muscle fuel stores
(2) Muscle microtrauma
(3) The accumulation of waste products
(4) Electrolyte imbalance in the blood
(5) Nervous system fatigue
(6) A rapid exchange of oxygen and carbon dioxide across lung tissue
(7) Rapidly increased heart rate

It’s no wonder that you get tired the longer you work out given that your liver, heart, lungs and brain are working in overdrive to support a high muscle work rate. 

The Window of Opportunity

In the 2-3 hours following exercise, fatigued muscles are incredibly hungry. That’s why it’s important to feed your muscles in the few hours following exercise in order to ensure that they receive the nutrients they require. During this 2-3 hour window of opportunity, the enzymes required for glycogen synthesis, triglyceride synthesis and protein synthesis are revved up and ready to do their job. Think of muscle enzymes in the post-exercise state as a collection of eager construction workers ready to build. If they show up to the work site but lack raw materials, their talent cannot be put to use. Instead, if you provide them with the proper raw materials, watch as they perform incredible work at a fast work rate.

In the 2-3 hours following exercise, the glycogen and triglyceride “construction workers” are capable of refilling fuel tanks quickly and efficiently. Similarly, the enzymes needed for muscle protein synthesis are activated and ready to do their job, eagerly anticipating the arrival of amino acids. Your mission is to provide high quality and bioavailable carbohydrates, fat, protein, vitamins, minerals, water, fiber and antioxidants in the right ratio, and let your muscles do the rest of the work. 

Remember, enjoy your workouts and embrace the feeling of post-exercise fatigue – it’s a sign of upcoming growth and repair.

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