If you're not training all three, you're limiting your fitness.

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 without wanting to die!

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 (also called the metabolic pathways), which fuel every single thing you do. (Related: The Basics on Your Aerobic and Anaerobic Energy Systems)

Understanding the metabolic pathways can help can you train with more intention, not just for fitness performance but also for life.

Photo: SolStock/Getty Images

The Basics of Metabolic Pathways

Before getting into the nitty-gritty of the metabolic pathways, you have to understand that your body uses food for energy by converting it into ATP (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 One Medical Provider. 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. The human body has three different systems (metabolic pathways) that it can use to produce ATP: the phosphagen pathway, glycolytic pathway, and oxidative pathway, explains Dave Lipson CrossFit Level 4 Trainer and Founder of Thundr Bro, an educational fitness platform. "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."

The Phosphagen Pathway = Sprints

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.

There isn't very much creatine phosphate stored in the muscle, so 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. 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
  • 1 rep max deadlift

Yup. "Even a 1 rep max every 3 minutes for 15 minutes falls into this category," says Lipson. (Related: What You Need to Know About Training with Your 1 Rep Max)

"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. of NYC Surgical Associates.

Glycolytic Pathway = Longer Intervals

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., founder of Flawless Fitness in Pasadena, CA. This makes the body incredibly efficient at using glycogen for energy via a process called glycolysis. (That's why, if you're on the keto diet you may have a hard time training at intensity because your glycogen stores are so low.)

"This pathway provides a fast source of energy for exercise lasting up to about 90 seconds," explains Schoenfeld. That could include things like:

  • 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." (Related: Do You Have to Do HIIT To Be Fitt?)

If you've ever done a remotely challenging workout, you're probably familiar with the hurts-so-good, burning 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).

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. For example, burning fat and boosting your metabolism are just two of the benefits of HIIT.

Oxidative Pathway = Endurance Work

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 system, the aerobic system can provide lots of energy for a long time, says Schoenfeld. (Related: Should I Be Working Out In The Fat Burning Zone?)

"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".

  • 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 us humming in life-whether we'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, she explains. "That's why it's considered the slowest form of energy creation." Once started, it's the system that keeps you going for endurance activities like mountain biking, marathon running, and long swims.

The oxidative pathway is highly adaptive, says Sanjiv Patel, M.D., cardiologist at MemorialCare Heart & Vascular Institute at Orange Coast Medical Center in Fountain Valley, CA. 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," he says. (See: You Don't Need to Do Cardio to Lose Weight-But There's a Catch)

Why the Metabolic Pathways Matter

Many people specialize in 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.

And the three systems aren't actually mutually exclusive: Doing Tabata 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. (That's why the answer to the age-old question: "Which Is Better: Running Faster or Longer?" is both.)

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. Switch up your workouts throughout the week to incorporate exercise that trains each system. (Related: Here's What a Perfectly Balanced Week of Workouts Looks Like)

That might look like a week with:

ICYWW: Can you combine two pathways into a single workout? For example, test a 1 or 3 rep max (phosphagen pathway) and then do this TRX 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." (Related: Does The *Order* You Perform Your Exercises Matter?)

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, his suggestion is to stick with what you enjoy.

But if you've hit a plateau or want to become as fit as possible? A training program that utilizes all three metabolic pathways can help you level up.