Everything You Need to Know About Muscle Metabolism Before Your Next Workout
Our bodies use energy constantly: while working out, at the office, and even at rest. But where do we get the energy we need? And what exactly is going on inside of our muscles that allows us to lift weights and run marathons? We answer all your muscle metabolism questions in this article.
What is Muscle Metabolism?
Our bodies need energy in order to function properly. Everything from moving to regulating body temperature requires that all internal metabolic processes are running smoothly1.
As you probably already know, food is one of our main sources of energy. A balanced meal will contain the three essential macronutrients: carbohydrates, fat and protein. After we’ve finished eating, our bodies convert this food into energy by breaking it down through chemical processes. About 60% of this energy is put towards maintaining our body temperature. And the rest is reserved for metabolic processes and muscle maintenance(2).
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Our bodies also store energy away for us to use later—that way we don’t collapse after skipping just one meal. Known as your energy reserves, they are depleted and replenished during muscle training. Exactly how our bodies replenish these reserves depends on the intensity and duration of the workout we’re doing. (More on that in a bit.)
What is ATP?
Adenosine triphosphate (or ATP for short) is the molecule that fuels our muscles. Composed of adenosine and three phosphate groups, it’s responsible for storing and transferring energy to our cells.
During a muscle contraction, our bodies transform adenosine triphosphate into ADP (adenosine diphosphate). This process releases energy and generates heat—which is why you usually feel hot when you exercise—and prevents muscle fatigue from forming.
Afterwards, ADP is converted back into ATP. The released phosphate sends a signal to the metabolism and ATP is reformed in a process called resynthesis. There are several different ways this resynthesis can happen and it depends on the type of energy reserves our bodies rely on.
What Are the Types of Muscle Metabolism?
Our bodies have access to three different types of energy reserves: creatine phosphate, carbohydrates, and fat. Oxygen reacts differently with each of these reserves and produces four different forms of energy production: creatine kinase, anaerobic glycolysis, aerobic glycolysis, and lipolysis. This may seem overwhelming, but identifying these different processes is easy to do when you know what to look for.
Let’s start at the beginning. In the first few seconds of a physical effort—like throwing a ball or lifting a weight—your body uses ATP that’s stored in the mitochondria (also known as the powerhouse of the cell). The ATP reserves only contain enough energy to sustain this act for those first seconds, and the body must produce more ATP in order to continue.
Creatine Kinase: How the Body Uses Creatine Phosphate as an Energy Source
When the cells have used up all of that initial ATP, it’s creatine that they turn to next. Creatine is an organic compound formed in the liver, kidneys, and pancreas which passes through the bloodstream to reach the muscles. When it reaches the muscles, it undergoes a chemical reaction to become energy-rich creatine phosphate3.
Inside the muscle tissue, creatine phosphate combines with ADP to create new ATP molecules. This process supplies the body with a big burst of energy for a short amount of time. It’s also known as anaerobic alactic metabolism because it doesn’t require oxygen nor produce lactate (lactic acid).
While we’re on the subject, what is lactic acid?
Lactic acid is a chemical byproduct that the body produces in the absence of oxygen. It most often occurs at the beginning of a workout, when blood circulation is not yet able to supply the muscles with enough oxygen4.
However, lactic acid won’t have enough time to form during the creatine kinase process, because it takes longer than 6 to 8 seconds to develop. This type of muscle metabolism is particularly important for short, intense efforts that require maximum effort, like strenght training, sprinting or HIIT.
Nutrition comes into play even before the body has used up all of its creatine stores. Carbohydrates, fats, and proteins are rapidly broken down so they can be reformed into ATP.
Glycolysis: How the Body Uses Carbohydrates as an Energy Source
After creatine, carbohydrates are the next energy reserve that the body relies on. After digestion, carbohydrates are stored as glycogen in the muscles and liver, and they can be transformed into energy either aerobically or anaerobically.
What’s the Difference Between Aerobic and Anaerobic Muscle Metabolism?
This is when the body produces ATP from carbohydrates without oxygen. This process provides energy quickly because it doesn’t require waiting for oxygen to travel all the way from the lungs and blood to the muscle cells.
But there’s a catch: When the body forms ATP in this way, lactic acid is also produced. (It’s also why this process is called anaerobic lactic metabolism.) If the concentration of lactic acid increases, muscle fatigue may happen more quickly(5).
On the other hand, aerobic glycolysis is when the body uses carbohydrates and oxygen to create energy. The process takes longer because of the distance the oxygen has to travel, but glucose is broken down more completely and efficiently and lactic acid doesn’t develop. CO2 and water are the only waste products the body produces here.
Since aerobic glycolysis is a slow process, it’s better for longer, endurance workouts than it is for things like HIIT. Remember: your muscle glycogen stores are not endless. When they’re depleted, you will be, too6.
Lipolysis: How the Body Uses Fat for Energy
When the body is out of carbohydrates, the fat stores are the last energy source it turns to. Accessing these energy stores requires oxygen, which means that this is an aerobic process. Although lipolysis takes much longer than glycolysis, the two processes happen at the same time. As the body has fewer carbs to burn, it will rely more on fatty acids instead.
Protein as an Energy Source
The body can also use protein as an energy source, but only when it has nothing else to rely on. For example, an extreme diet with a huge calorie deficit may cause the body to turn to protein for energy. But this isn’t good, because the body will start to break down muscle mass in order to conserve energy. Eating a balanced diet is the best way to ensure your muscles and overall health are protected. Even if you’re trying to lose weight by cutting calories, it’s never advised to eat fewer than 1,200 calories a day.
Overview of the Different Energy Sources
|Energy Source||Delivery Time||Storage Location||Advantages||Disadvantages|
|Phosphocreatine||A few seconds||Muscle cells||Quick burst of energy||Only produces a small amount of ATP.|
|Anaerobic glycogen||Up to 60 seconds||Muscle cells and liver||Quick burst of energy without oxygen||Glycogen stores are limited, overproduction of lactate.|
|Aerobic glycogen||Up to 100 minutes||Muscle cells and liver||Doesn’t produce lactate and uses up all glycogen stores||Energy arrives more slowly because of the time it takes for oxygen to travel to the muscles.|
|Fat||A few hours||muscles, subcutaneous tissue||Almost endless energy stores||Slow delivery of energy|
Muscle Metabolism in Sports
The roles of muscle metabolism vary from sport to sport. The intensity and duration of a workout and the amount of energy used also play a part.
For example, explosive strength is necessary for high-impact sports like karate, boxing, and even sprinting. In these cases, the energy from creatine phosphate will predominate.
Our tip: Creatine is essential for preventing muscle fatigue during short, yet intense exercise. Supplementing at least 3 grams of creatine per day can greatly enhance your ability to make use of this source of energy.
Endurance sports like cycling, running, or swimming rely more heavily on aerobic glycolysis and lipolysis—when the body relies on fat or carbs in combination with oxygen. Whether or not your body uses these energy sources will depend on the level of difficulty and duration of your aerobic exercise. And eating a balanced diet is the best way to guarantee you’re always on top of your performance.
Along with these energy stores, muscle fibers also play a role in the energy production process. Red muscle fibers are responsible for long endurance efforts that rely on oxygen and use carbs or fat for fuel. On the other hand, white muscle fibers are responsible for short, fast, powerful movements and rely on a supply of phosphates. Your muscles work very differently from one sport to the next.
The proportion of red and white muscle fibers varies from person to person and is impossible to control. But it is possible to strengthen them with the right types of workouts. Regular endurance training helps the red muscle fibers use oxygen more easily. And hypertrophy training can increase the size of the white fibers and give you more strength and speed as a result7.
Energy Metabolism: Our Conclusion
- Your body is constantly using energy for metabolic processes, muscular activity, and body temperature maintenance.
- When muscles contract, the molecule (ATP) is converted into energy and body heat.
- ATP resynthesis is the process through which new ATP molecules are formed. The body can use creatine phosphate, glucose, or fatty acids to do this.
- The duration and intensity of an exercise determines whether your body relies on anaerobic and aerobic metabolism.
- Energy production can be aerobic (with oxygen) or anaerobic (without oxygen).
- Lactic acid is a metabolic byproduct of anaerobic metabolism. Increased lactic acid production can decrease your ability to perform and increase muscle fatigue.
- Each type of muscle metabolism differs from sport to sport and is always related to either red or white muscle fibers.
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- 7Dr. Kurt A. Moosburger (1994): Die Muskuläre Energiebereitstellung im Sport. In: Sportmagazin Jan. 1995.↩