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Not All "Cardio" is Created Equal

Cardiovascular exercise, commonly referred to as cardio, is an essential component of a well-rounded fitness program. It involves activities that increase the heart rate and breathing, such as running, cycling, or swimming. While many people focus solely on the duration or frequency of their cardio workouts, varying the intensity is equally important for optimal health and fitness benefits.

One reason for varying the intensity of cardio exercise is to engage different energy systems. The human body has three primary energy systems: the Creatine-Phosphagen system (PCr for short), the Glycolytic system, and the Oxidative system. Each of these systems is responsible for producing energy for different types of activities.

PCr System- Power & High Intensity

The PCr system provides energy for short bursts of high-intensity exercise, such as sprinting or weightlifting. This is because the chemical pathway to producing ATP (the energy "currency" of our cells) is shorter and fast, making it ideal for the energy demands of exercises requiring immediate energy. The drawback of this process is that the ATP yield is low, which means this energy system won't be able to provide your body for more than a few seconds. Exercises beyond 10-15 seconds will transition to using the glycolytic system.

Glycolytic System - Anaerobic Training

The glycolytic system is used for longer periods of moderate to high-intensity exercise, such as running up a steep hill, engaging in a long duration set of strength training, and generally activity in the 1-3 minute range. This system is primarily anaerobic but relies on stored & ingested glucose for energy. The chemical pathway to create ATP is relatively short but not as immediate as the PCr system, but the yield is greater. For comparison, the PCr system poroduces 1 molecule of ATP immediately without any glucose needed. The glycolytic system produces 2 molecules of ATP for every glucose molecule, and a typical reaction involves about 11 glucose molecules.

Oxidative System - Aerobic Training

Finally, the oxidative system is used for lower-intensity, longer-duration exercise, such as jogging or swimming laps. This system is aerobic, meaning it relies on oxygen to produce energy, and it is the most abundantly used energy system because of how much energy it can produce. A single reaction in the oxidative system will produce 36 molecules of ATP for comparison to the other energy systems. This is what your body uses for energy on a regular basis even when not exercising.

By varying the intensity of cardio exercise, you can engage each of these energy systems and improve your overall fitness level. For example, performing high-intensity interval training (HIIT) can engage both the phosphagen and glycolytic systems, while low-intensity steady-state (LISS) cardio engages primarily the oxidative system.

Neglecting one or two of these energy systems can have negative consequences on your fitness and health. For example, neglecting the oxidative system by only performing high-intensity exercise can lead to a decrease in cardiovascular fitness and an increased risk of injury. Neglecting the phosphagen system by only performing low-intensity exercise can lead to a decrease in power and explosiveness.

In addition to engaging different energy systems, varying the intensity of cardio exercise can also prevent boredom and burnout. Doing the same type of exercise at the same intensity every day can lead to a plateau in progress and make it harder to stick to a fitness routine. Mixing up the intensity and type of cardio exercise can keep workouts challenging and interesting.

It's essential to remember that varying the intensity of cardio exercise should be done in a safe and controlled manner. It's important to gradually increase the intensity and duration of workouts to prevent injury and allow the body to adapt. Consulting with a fitness professional or healthcare provider can also ensure that you are engaging all energy systems safely and effectively.

Varying the intensity of cardio exercise is crucial for optimal health and fitness benefits. Engaging different energy systems, preventing boredom and burnout, and preventing injury are just a few of the reasons why varying intensity is so important. Incorporating a variety of cardio exercises at different intensities can help you achieve your fitness goals and improve overall health and well-being. If you have any questions about how to optimize your cardio training, reach out to

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