Aerobic vs. Anaerobic: How Do Workouts Change the Body?

Aerobic vs. Anaerobic: How Do Workouts Change the Body?

Reading Time: 4 minutes 11 seconds


Date: 2018-09-26T00:00:00-04:00

When working with clients you need to find all kinds of ways to motivate them to try the workouts you have planned. Some clients inevitably prefer low-intensity, endurance work, while others like to run fast or lift heavy. It's important to find a balance that matches each individual's goals but you'll often find you come up against some resistance. Therefore, it's so important for you to be able to quickly explain the benefits and outcomes of aerobic exercise and anaerobic exercise. Use your knowledge of what these different workouts do for the body to help your clients understand how and why you develop the variety in their training plans.

Why Aerobic Exercise?

Workouts with a primary focus on aerobic exercise are beneficial because they burn fat, improve cardiovascular health and fitness, and improve the body's ability to recover and repair after intense exercise. To better help your clients understand why you insist they do some low intensity, longer duration workouts, teach them about all the ways in which this exercise triggers important adaptations in the body.

Changing the Composition of Muscle Fibers

Aerobic exercise mostly relies on slow twitch muscle fibers, which contract slower and at a lower intensity. These fibers are important because they allow you to do endurance work for long periods of time before fatigue sets in. Aerobic workouts increase the size and number of these Type I muscle fibers, which improves endurance performance.

This kind of workout also converts Type IIx fibers to Type IIa. The IIx fibers are fast twitch and found in especially high quantities among sedentary people. They can be recruited quickly for bursts of power and strength, but they're also usually destroyed once used. By converting IIx to IIa fibers, clients can improve fitness and increase the amount of reusable fast twitch muscle fibers.

More, Bigger Blood Vessels

The underlying distinction between aerobic and anaerobic exercise is the use of oxygen for energy. During aerobic workouts, oxygen is flowing through the body and to the muscles which result in the production of more blood vessels (to carry that oxygen). It also increases the size of blood vessels.

These changes to blood vessels help more nutrients and oxygen get to the muscles and then take waste away. All of this supports recovery and muscle growth.

More Mitochondria, More Myoglobin

Aerobic workouts trigger important metabolic changes in muscle tissue, including an increase in mitochondria and the protein myoglobin. Mitochondria, the powerhouses of the cells, are necessary for creating ATP for energy. Myoglobin, on the other hand brings in the oxygen needed for that process. These changes improve the aerobic capacity of muscles.

Using More Fat for Energy

All of these changes from aerobic activity lead to a better ability to burn fat. Greater blood flow to the muscle tissue, more oxygen transport, larger numbers of mitochondria, and a jump in the levels of enzymes used to metabolize fat all let muscles to better access fat and burn it for energy. This leads to fat loss throughout the body.


  • By changing the composition of muscle fibers, aerobic workouts increase endurance.

  • Aerobic exercise promotes recovery and repair of muscles after workouts, which in turn assists with muscle growth.

  • Increased myoglobin and mitochondria in muscle tissue triggered by aerobic workouts promotes greater aerobic ability.

  • Aerobic workouts trigger numerous changes that all help the body burn more fat.

Why Anaerobic Workouts?

For clients who prefer to pound it out with a long slow run, introducing intervals, sprints, or intense weight training can be a challenge. Help them see the importance of incorporating anaerobic workouts into their sessions by explaining the important changes this kind of exercise produces and how it can benefit overall fitness and health.

More Fast Twitch Muscle Fibers for Strength

While aerobic workouts produce more slow twitch muscle fibers for better endurance, anaerobic exercise increases the size and quantity of powerful fast twitch fibers. This shift improves the power and strength of muscles and also increases hypertrophy, or size.

Better Lactic Acid Tolerance for Endurance

Anaerobic activity is short-lived compared to aerobic workouts because the lack of oxygen triggers a production of lactic acid. The buildup of lactate is what causes fatigue and forces you to take a break from the exercise. But, the more you engage in these kinds of workouts the greater your tolerance will be to high levels of lactic acid. This improves strength and muscle endurance.

Increased Glycolysis, ATP, CP, and Creatine

Some of the metabolic changes triggered by an anaerobic workout include increased breakdown of glucose, the process known as glycolysis. This kind of exercise will also boost levels of ATP, the primary source of energy in muscles, as well as CP, creatine phosphate, which can be quickly changed to ATP and used for energy. Levels of creatine also go up, which helps supply energy for muscle contraction.

Increased Growth Hormone and Testosterone

High-intensity workouts that last for about 45 to 75 minutes will trigger important hormonal changes, including a boost in the production of testosterone and growth hormone. These along with other hormones are necessary for increasing muscle hypertrophy.


  • Intense, anaerobic workouts increase fast twitch muscle size and quantity, improving muscle power, strength, and size.

  • Anaerobic exercise helps build tolerance to the lactic acid that causes fatigue, improving muscle endurance.

  • Metabolic changes due to anaerobic activity help increase the amount of energy available to muscles, which allows them to act more quickly and powerfully when recruited.

  • Hormones that promote muscle growth are boosted by anaerobic workouts.

The next time your client puts up some resistance to 30 slow minutes on the stair climber or an interval workout, hit him with these facts. Aerobic and anaerobic workouts each have their place, producing important, productive changes in the body that are necessary for all fitness goals.

You can learn more about the nuances of cardio programming and how to use both aerobic and anaerobic exercise to get your clients to their goals in our comprehensive Personal Trainer Certification program.

Aerobic vs. Anaerobic: How Do Workouts Change the Body?

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