What Happens to Muscles After Exercise?

If you enjoy exercising frequently, increasing your body’s ability to recover following intense exercise can take your athleticism to the next level.  If instead you are exercising to control and reverse the symptoms of chronic health conditions like diabetes, heart disease, hypertension or obesity, then it is especially important that you focus on athletic recovery in order to keep you moving frequently.

In last week’s article I covered the basic physiology of muscles during exercise, and learned that:

  • Muscles burn mainly fat during low intensity exercise
  • Muscles burn mainly carbohydrate during high intensity exercise
  • Muscles spare protein “infrastructure” from being burned as fuel
  • Microscopic tears (microtrauma) that occur during exercise need to be repaired

In this week’s article, I’ll focus on the physiology of the post-exercise state, in order to gain appreciation for the nutrients that muscles require to ensure optimal recovery.

Metabolic Equivalents (METS) Measure Your Exercise Work Rate

Exercise can place a significant stress on muscles by increasing the rate at which they burn fuel by up to 30-fold.  Beginner athletes can increase muscle work rate by 12-fold, intermediate athletes typically increase muscle work rate by 16-fold, and advanced athletes are capable of 25 to 30-fold increases.

The term Metabolic Equivalent (MET) refers to the change in energy expenditure that results from exercise, above and beyond your resting energy expenditure.  The reference value of 1 MET has a value of 1 kcal/kg/hour, and refers to the resting metabolism of an “average” individual.

If you go on a brisk walk, you might increase your metabolic rate 3-fold (from 1 kcal/kg/hour to 3 kcal/kg/hour), so we say you are working at 3 METS.  If you run a mile at an 8:00 pace, you might increase your metabolic rate 14-fold, and if you sprint a mile at a 4:15 pace you might increase your metabolic rate 25-fold.

Think of a MET as the fold-change in work rate due to a particular activity.  The more intense the activity, the higher the work rate, the higher the number of METS.  Low intensity exercise increases METs from baseline, albeit to a lesser degree than high intensity exercise.

Energy Expenditure MET Diagram

The Balance Between Onboard and Imported Fuel

Onboard Fuel Tanks

In order to sustain these large increases in energy production, 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%, contributing to the feeling of exhaustion.

Imported Fuel

As these two onboard fuel tanks run low, muscles start importing glucose and fatty acids from the blood.  This process happens on-the-fly, and the amount of imported fuel depends on the amount of remaining onboard fuel.

When fuel stores are high, onboard muscle fuels are depleted first.

When onboard fuel stores fall low, fuel is imported from the blood.

Fuel Stores During Exercise

This is exactly how endurance athletes are capable of performing exercise for many hours – because they have trained themselves to recognize when their muscles are running low on fuel.  Eating during a workout provides a rapid influx of fuel into the blood that is then directed towards active muscle for immediate energy.

Fatigue and Exhaustion During and After Exercise

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 exhaustion to tissues all around your body, not just from the muscles that performed work.

Even though the muscles are the exercise workhorses, peripheral organs work in overdrive to support the delivery of fuel and the clearance of waste products, in an effort to keep you moving efficiently and for long periods of time.

Here is a short list of some of the effects of exercise on tissues in your body:

  • Depleted glycogen and triglyceride stores in the muscle
  • Depleted glycogen stores in the liver
  • The accumulation of waste products in muscle
  • The development of muscle microtears
  • Insufficient sodium and potassium salts in the blood (electrolyte imbalance)
  • Nervous system fatigue (brain)
  • A rapid exchange of oxygen and carbon dioxide across lung tissue
  • 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 image below summarizes a few of the key effects of exercise:

Post Exercise Exhaustion


The Window of Opportunity Following Exercise

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 high work rate.

 Construction Workers


This is exactly how enzymes in the muscle tissue operate.  In the 2-3 hours following exercise, the enzymes located around the glycogen molecule are activated and ready for a fresh supply of glucose from carbohydrates.  When the glucose is present, these glycogen “construction workers” are capable of refilling the glycogen tank 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 from protein.

Why “Carbo-Loading” is Foolish

Athletes are told to carbo-load before a big race in order to maximize stored glycogen in the 24-48 hours prior to competition.  They do so by eating large plates of pasta, bread and potatoes, in the hopes that simply consuming more carbohydrate will increase the size of their onboard fuel tank.  True or not true?

True.  Carbo-loading will increase glycogen content, but only by a few percent at the maximum.

It is foolish to think that your muscles and liver will dramatically increase glycogen content with a single meal.  Studies have shown that eating a single meal high in carbohydrates immediately prior to competition is less effective at improving performance and recovery than eating an overall diet high in carbohydrates.

This is because your muscles and liver respond to frequent carbohydrate intake more effectively than a single meal containing carbohydrates.  I’ll talk about this in way more detail in next week’s article.

Take Home Messages

  • Following exercise, tissues throughout your body are low on energy, not just your muscles.
  • The higher your work rate (METs), the more fuel is depleted from onboard fuel tanks.
  • In the two to three hours following exercise, your muscles are BEGGING for carbohydrate energy.
  • Carbo-loading in a single meal is far less effective than carbo-loading every day.
  • In next week’s article, I’ll go into detail about which foods are excellent choices for refueling and repairing muscle post-exercise, to provide the right ratio of carbohydrates, fat and protein.
  • Until then, enjoy your workouts and embrace the feeling of post-exercise fatigue. If nothing else, it’s an opportunity to eat like a champion in order to perform like one.
About The Author

Cyrus Khambatta

Diagnosed with type 1 diabetes at the age of 22, I have spent over a decade learning the fundamentals of nutrition at the doctorate level. My goal is to share my knowledge of practical nutrition and fitness with people with prediabetes, type 1 and type 2 diabetes. Diabetes is an OPPORTUNITY to attain excellent health. Reversing the effects of insulin resistance can be a fun and enjoyable process if the right system is in place. That's why I've spent over 10 years developing a rock solid system that can minimize blood glucose variability and insulin resistance.

Leave a comment or question below