You Should Be Doing These Three Types of Cardio

Gabrielle Kassel
·7 min read

When you think about the benefits of exercise, you likely think about the gains you can see, feel, and measure: My biceps are bigger! Lifting that thing was easier! I just ran a mile without stopping! But have you ever thought about how the heck your body gets the energy to squat heavy, run long trails, or take a HIIT class — and what exactly happens to make it easier the next go-around?

The answer comes down to the body's three main energy systems (aka metabolic pathways), which fuel every single thing you do. FYI, different types of cardio tap into each pathway — and understanding these differences can help can you train with more intention, not just for fitness performance but also for life.

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Photo: SolStock/Getty Images

The Basics of Metabolic Pathways

Before getting into the nitty-gritty of the metabolic pathways, it's important to understand that your body uses food for energy by converting it into ATP (aka 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. Basically, ATP does to your body what fuel does to a car: keeps it running.

Because your body can't store a ton of ATP, you're continuously making more of it — and that's where your metabolic pathways come in. Basically, they're 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 constantly working together, but they'll take turns being the dominant engine, depending on what exercise you're doing, how long you're doing it, and the intensity," he explains.

Below, what to know about the three metabolic pathways, including which types of exercise work each the hardest.

Phosphagen Pathway

The phosphagen pathway (also called the phosphocreatine pathway) uses the molecule creatine phosphate to make ATP very quickly. Like, blink and you'll miss it. Since there isn't very much creatine phosphate stored in the muscle, that means that there's a limited amount of energy available. "You can express a lot of power using this pathway, but not for very long," says Lipson. In fact, it only lasts about 10 seconds. (Here's what to know about taking creatine supplements.)

So, when are you using this engine? Whenever you're expressing 100 percent of your power or intensity. Think:

  • 100-meter sprint

  • 25-yard swim

  • One-rep max deadlift

Yup. "Even a one-rep max every three minutes for 15 minutes falls into this category," says Lipson. (Here's how to calculate your one-rep max, plus how to program them into your strength training sessions.)

"Training this system will improve your explosive speed, strength, and power so you'll be able to jump higher, sprint faster, and throw harder," says David Greuner, M.D., the surgical director of NYC Surgical Associates.

Glycolytic Pathway

You might think about the glycolytic pathway as the "middle" engine. When you're using this pathway, your body primarily breaks down glycogen — which comes from carbohydrate sources — into ATP, explains Melody Schoenfeld, C.S.C.S., the founder of Flawless Fitness in Pasadena, California. This makes the body incredibly efficient at using glycogen for energy via a process called glycolysis.

"This pathway provides a fast 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 periods

  • Sports requiring quick bursts of speed, such as basketball

  • High-intensity interval training programs

One important point: "It's not the overall duration of your workout that determines what pathway you're in," explains Lipson. "If you're doing 30 to 60 seconds of work and then resting 30 seconds before repeating, you're still in the glycolytic pathway," he notes.

Related: Do You Have to Do HIIT To Be Fit?

If you've ever done a remotely challenging workout, you're probably familiar with the hurts-so-good sensation of lactic acid building up in your muscles. That's because lactic acid is a waste byproduct of the glycolytic pathway. "Lactic acid builds up in the muscles, causing pain and tiredness, which makes it difficult to maintain intensity," explains Dr. Bhuyan. (This is known as your lactic threshold, BTW.)

Good news: The more you train in the glycolytic pathway, the more efficient you become at creating ATP, so you create less waste, says Dr. Bhuyan. Ultimately, that means you're able to exercise at that intensity for longer. "You get a big bang for your buck here," adds Lipson.

Oxidative Pathway

The oxidative pathway's primary fuel source is fat. It's called the oxidative pathway because it requires oxygen in order to produce ATP, explains Dr. Greuner. So, the phosphagen and glycolytic systems are anaerobic and don't require oxygen; the oxidative pathway is aerobic, meaning that it does. Unlike the phosphagen and glycolytic systems, the aerobic system can provide lots of energy for a long time, says Schoenfeld.

"Many people exercise only in this pathway," says Dr. Bhuyan. If you're a marathoner or live and breathe by slow-and-go (or LISS) cardio, that's probably true for you. The oxidative pathway is what's used during exercise that is traditionally categorized as cardio. For example:

  • Daily life activities

  • 30-minute jog

  • 40 minutes on the elliptical

  • Biking 20 miles

Yes, this comes into play when you're exercising, but it's also what keeps you humming in life — whether you're watching The Bachelor, meal prepping, or showering.

Although the oxidative pathway is always active, the oxidative process of converting fat to energy takes a lot longer than the anaerobic processes, explains Dr. Bhuyan. "That's why it's considered the slowest form of energy creation," she adds. Once started, it's the system that keeps you going for endurance activities such as mountain biking, marathon running, and long swims.

The oxidative pathway is highly adaptive, says Sanjiv Patel, M.D., a board-certified cardiologist at MemorialCare in California. That means the more you use it, the better it works. Anyone who's ever done a couch-to-5K knows this phenomenon to be true. "Oxidative pathway (or aerobic) training can have excellent benefits to the heart and fat loss," notes Dr. Patel. (See: You Don't Have to Do Cardio to Lose Weight — But There's a Catch)

Why the Metabolic Pathways Matter

Many people who exercise specialize in (or prefer) one of these metabolic pathways while neglecting activities that train the other two. But it's really important to train all three so your body becomes more efficient at using energy in all scenarios, says Dr. Bhuyan.

It's also worth noting that the three systems aren't actually mutually exclusive. For example, doing treadmill sprints will make you a better long-distance runner, just as training for a marathon can improve how quickly you're able to recover from a HIIT class. "Working all three will make you a more well-rounded athlete," adds Lipson.

Related: Which Is Better: Running Faster or Longer?

How to Incorporate Metabolic Training Into Your Workouts

So, how do you develop capacity in all three metabolic pathways? "Training with variety is key to working out smarter, not harder," says Dr. Bhuyan.

So, switch up your workouts throughout the week to incorporate exercise that trains each system. That might look like a week with:

But can you combine two pathways into a single workout — for example, test a one- or three-rep max (phosphagen pathway) and then do a HIIT workout (glycolytic pathway)? Lipson says yes. "But if you have to fit both of those into the same session, you may lose the potency of the workout because it takes a long time to warm yourself up to a one-rep max. There's always a risk that both get sacrificed," he explains.

Related: Here's What a Perfectly Balanced Weekly Workout Schedule Looks Like

If this is all really overwhelming, take a breath: "For the general population, I just want to see more people exercising — period," says Dr. Patel. So if you're new to working out, stick with what you enjoy, he suggests. But if you've hit a plateau or want to develop your strength? A training program that utilizes all three metabolic pathways can help you level up.