When you contemplate the advantages of physical activity, you probably think about the progress you can see, sense, and evaluate: My triceps are larger! Lifting that object was simpler! I just ran a mile without pausing! However, have you ever considered how your body obtains the energy to perform heavy squats, run long distances, or participate in a HIIT class — and what exactly occurs to make it easier the next time?

The solution lies in the body’s three primary energy systems (also known as metabolic pathways), which provide fuel for every single action you take. By the way, various forms of cardiovascular exercise engage each pathway — and comprehending these distinctions can assist you in training with greater intention, not only for physical performance but also in daily life.

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The Fundamentals of Metabolic Pathways

Before delving into the specifics of the metabolic pathways, it’s important to acknowledge that your body uses food for energy by converting it into ATP (otherwise known as adenosine triphosphate). “ATP is a molecule stored in our muscles and is the direct source of energy for muscle contraction in life and exercise,” explains Natasha Bhuyan, M.D., a provider at One Medical. Essentially, ATP does for your body what fuel does for a car: keeps it operational.

Because your body cannot store a large amount of ATP, you are constantly generating more of it — and that’s where your metabolic pathways play a role. Essentially, they are the three systems in the human body that are used to produce ATP. The three metabolic pathways are the phosphagen pathway, glycolytic pathway, and oxidative pathway, notes Dave Lipson, a CrossFit Level 4 trainer and founder of Thundrbro. “All three are continuously collaborating, but they will take turns being the dominant engine, depending on the type of exercise, its duration, and intensity,” he explains.

Below, discover what you need to understand about the three metabolic pathways, including which forms of exercise exert the most effort on each one.

The Phosphagen Pathway

The phosphagen pathway (also referred to as the phosphocreatine pathway) employs the substance known as creatine phosphate to rapidly generate ATP. It happens so quickly that you could miss it if you blink. Since there is not much creatine phosphate stored in the muscles, this means there is only a limited amount of energy available. “You can exhibit a significant amount of power using this pathway, but not for an extended period,” says Lipson. In fact, it only lasts for approximately 10 seconds. (Here’s what to know about using creatine supplements.)

So, when do you utilize this engine? Whenever you’re showcasing 100 percent of your power or magnitude. Consider:

• 100-meter dash
• 25-yard freestyle
• Maximum weight deadlift for one repetition

Yes. “Even a maximum weight lift every three minutes for 15 minutes falls into this category,” says Lipson. (Here’s how to determine your maximum weight lift for one repetition, plus how to incorporate them into your strength training sessions.)

“Training this system will enhance your explosive speed, strength, and power so you’ll be capable of leaping higher, sprinting faster, and throwing with more force,” says David Greuner, M.D., the director of surgery at NYC Surgical Associates.

Glycolytic Pathway

You might consider the glycolytic pathway as the “intermediate” engine. When you’re using this pathway, your body primarily breaks down glycogen — which originates from carbohydrate sources — into ATP, explains Melody Schoenfeld, C.S.C.S., the founder of Flawless Fitness in Pasadena, California. This makes the body highly effective at utilizing glycogen for energy through a process known as glycolysis.

“This pathway provides a rapid source of energy for exercise lasting up to about 90 seconds,” explains Schoenfeld. That could include workouts such as:

• 400-meter sprint
• Lifting weights for short durations
• Sports requiring quick bursts of speed, like basketball
• High-intensity interval training programs

One important note: “It’s not the total duration of your workout that determines which pathway you’re in,” explains Lipson. “If you’re performing 30 to 60 seconds of work and then resting for 30 seconds before repeating, you’re still in the glycolytic pathway,” he mentions.

If you’ve ever engaged in a slightly challenging workout, you’re probably familiar with the pleasurable pain sensation of lactic acid accumulating in your muscles. That’s because lactic acid is a waste byproduct of the glycolytic pathway. “Lactic acid builds up in the muscles, causing discomfort and fatigue, which makes it challenging to maintain intensity,” explains Dr. Bhuyan. (This is known as your lactic threshold, by the way.)

Good news: The more you train in the glycolytic pathway, the more efficient you become at generating ATP, so you produce less waste, says Dr. Greuner.

Bhuyan. In the end, that implies you can engage in physical activity at that level of intensity for a greater duration. “You receive a significant value for your investment here,” further comments Lipson.

Oxidative Pathway

The aerobic process relies on fat as its primary source of fuel. It is named the oxidative pathway because it necessitates the presence of oxygen to generate ATP, as explained by Dr. Greuner. In contrast, the anaerobic phosphagen and glycolytic systems do not require oxygen, while the aerobic system does. Unlike the phosphagen and glycolytic systems, the aerobic pathway can provide sustained energy over an extended period, according to Schoenfeld.

“Numerous individuals engage exclusively in this pathway,” states Dr. Bhuyan. If you are a marathon runner or predominantly perform slow-and-go (or LISS) cardio, this is likely true for you. The oxidative pathway is activated during activities traditionally classified as cardio exercises. Examples include:

• Everyday activities
• A 30-minute jog
• 40 minutes on the elliptical
• Biking for a distance of 20 miles

This process is involved not only during exercise but also in various aspects of daily life, such as watching The Bachelor, meal prepping, or showering.

While the oxidative pathway is continually active, the conversion of fat into energy through this process takes considerably longer compared to anaerobic processes, as explained by Dr. Bhuyan. She adds that this is why it is considered the slowest form of energy production. However, once initiated, this system sustains endurance activities like mountain biking, marathon running, and long swims.

According to Sanjiv Patel, M.D., a board-certified cardiologist at MemorialCare in California, the oxidative pathway is highly adaptable. The more it is utilized, the more efficiently it operates. This is a phenomenon that people who have completed a couch-to-5K program can attest to. Dr. Patel notes that aerobic training has significant benefits for the heart and weight loss. (See: You Don’t Have to Do Cardio to Lose Weight — But There’s a Catch)

Why the Metabolic Pathways Matter

Many individuals who engage in exercise tend to specialize in or prefer one of these metabolic pathways, neglecting activities that train the other two. However, it is crucial to train all three in order for the body to become more proficient in utilizing energy in all situations, according to Dr. Bhuyan.

It is important to note that these three systems are not mutually exclusive. For instance, engaging in treadmill sprints can enhance long-distance running ability, just as marathon training can improve recovery speed after a HIIT class.

Engaging in all three endeavors will enhance your overall athletic abilities,” affirms Lipson.

So, how do you foster capacity in all three metabolic pathways? “Diversified training is crucial for exercising more intelligently, not harder,” asserts Dr. Bhuyan.

Therefore, alternate your workout routines throughout the week to include activities that target each system. This could involve a week with:

• Endurance-based exercises to enhance the aerobic pathway
• Resistance training to bolster the phosphagen pathway
• High-intensity interval training (HIIT) to stimulate the glycolytic pathway

However, is it possible to combine two pathways into a single workout? For instance, testing your one- or three-rep maximum (phosphagen pathway) followed by a HIIT workout (glycolytic pathway)? Lipson states that it is indeed plausible. “However, if you attempt to squeeze both into the same session, you might compromise the effectiveness of the workout. It takes a significant amount of time to prepare yourself for a one-rep max, and there’s always a risk of sacrificing both,” he explains.

If all of this seems overwhelming, take a moment to breathe. “When it comes to the general population, my primary goal is to see more individuals engaging in physical activity — regardless of the specifics,” advises Dr. Patel. Therefore, if you are new to exercising, stick with activities that bring you joy, he suggests. However, if you have reached a plateau or wish to enhance your strength, a comprehensive training program that incorporates all three metabolic pathways can help you progress.

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