Energy System Interplay

Energy System Interplay

Three different metabolic energy systems power your workouts — and your day. Here’s how each one works, and how to make the most of them all.
Most of us understand our bodies about as well as we understand our cars. We know we’re supposed to take them out for a spin once in a while, and keep them well fueled.


Like a hybrid engine, your body has several ways of turning the stuff you eat into the stuff you do. All of these metabolic energy systems are switched on during physical activity, but each plays a different role depending on available energy and the specific demands of the task. Each burns a particular type of fuel at a particular rate — thereby affecting fat loss and muscle gain in a particular way.

The adenosine triphosphate–creatine phosphate (ATP-CP) system, or phosphagen system, supports very brief, high-intensity activities like the 100m and long jump.
The glycolytic system provides energy for activities of slightly longer duration and lower intensity like the 400m and strength training.
The oxidative system supports long-duration, lower-intensity activities like walking or distance running.
In recent years, exercise physiologists have learned how to target each system with specialized training to better prepare individuals for a specific event or sport.

Here you’ll discover how your body’s energy systems interact, and learn how to challenge each one so you can reach your fitness goals faster and with less wasted effort.


Whether you’re running the 100m, jumping up to answer the phone, or catching a child falling off the monkey bars, the adenosine triphosphate–creatine phosphate (ATP-CP) system is first to respond. Among your three energy systems, it’s the one most prepared for emergencies. It kicks in whenever the oxidative system, your body’s normal method for providing energy, isn’t up to the demands you’re placing on it.

All three of your energy systems ultimately run on ATP: It’s the fuel source for all your physical functions, from eating to breathing to running hill sprints. Your glycolytic and oxidative systems make most of this ATP to order, cobbling it together from the food you eat and the air you breathe as need arises.

But a small quantity of ATP is stashed away in your muscles for when you need to expend a short burst of energy in a hurry. Let’s say you’re doing a single barbell squat with close to max weight. As you power the weight up, the muscles of your hips, thighs and lower back immediately burn through their ATP stores. Once the ATP has done its job, it’s either further broken down or recycled (with the help of another substance, creatine phosphate, or CP), so it can provide more energy to your working muscles.

How fast does the ATP-CP system gear up? Blink and you’ll miss it. “Once you begin hard activity,” says Christopher Scott, PhD, associate professor in the Department of Exercise, Health and Sport Sciences at the University of Southern Maine and an expert in metabolism, “it takes just thousandths of a second for the phosphagen system to kick in.”

There’s a cost for this speed and efficiency, however: You can store only enough ATP and CP in your muscles for about 6 to 10 seconds of serious effort. Though training the ATP-CP pathway will improve your explosive speed and power (so you can jump higher, sprint faster and throw farther), it won’t increase your storehouses of ATP-CP — or give you the ability to operate at full throttle for longer than a few seconds. That’s why activities like javelin throwing, Olympic weightlifting, and the 100m are “one-and-done” endeavors, even at the elite level. Most trained athletes need three to five minutes of rest before their ATP is replenished and they can perform near the level of their previous effort.

“As we age, we lose a lot of our ability to exert strength quickly,” says Scott. “So doing some of this training is important simply for maintaining quality of life.”

ATP-CP training doesn’t typically burn a lot of fat or build a lot of muscle,  but ATP-CP training is the best way to build serious strength, speed and power.

How do you train the ATP-CP system? With Intermittent training. This means very brief periods (10 seconds or less) of high effort with lots of rest (two minutes or more) between activities.

Training Your ATP-CP System

Speed: Fast
Primary Fuel: Adenosine triphosphate and creatine phosphate, stored in your muscles
Sample Activities: sprinting to 100m, lifting a heavy weight, long jump
How to Train It: Heavy strength training, medicine-ball throws, jumps, short sprints (less than 40m), sports-specific drills
Sets/Reps/Rest: Three to eight sets
Brief, maximum-effort sets lasting eight to 15 seconds; one or two heavy reps in strength-training activities
Long rest between sets (up to five minutes); full recovery between efforts
Frequency: Up to three times a week

How fast does the ATP-CP system gear up?
Blink and you’ll miss it.

ATP-CP athletes are fast, strong and explosive, specializing in brief, single-effort activities like Olympic weightlifting, high-jumping, and sprinting. Athletes in field and team sports like soccer, lacrosse, tennis, martial arts, basketball and other activities also rely heavily on the ATP-CP system during the highest-effort moments of sprinting, serving, kicking or hitting.



As your ATP-CP system sputters out, your glycolytic system steps in and keeps you moving for about another minute or so before it, too, begins to run out of fuel.

Because glycolysis relies on energy converted from carbohydrate (glucose) into ATP, your glycolytic system is slightly less responsive than your ATP-CP system. But it can still provide as much as half the energy you need in the first few seconds of intense exercise.

If you’ve ever done an all-out set of max pushups, or a 400-meter sprint, you’re familiar with what it feels like to exercise the glycolytic system at close to its maximum. In a word, it hurts.

Contrary to popular belief, the burning sensation you get when you exercise intensely is caused not by lactic acid (another fuel source) but by a buildup of hydrogen ions, a byproduct of glycolysis, which can inhibit muscle contraction, giving you “wobbly knees” after a minute or so of full-out running or cycling.

The more you train your glycolytic system, however, the better you’re able to buffer these ions and the faster you can recover between sets of medium-to-high-intensity exercise.

The discomfort that comes from glycolytic training is well worth it. Increasingly, fitness pros (we have been advocating this type of training for 14 years) are recommending this type of training for people who want to gain muscle, lose fat and get the most out of their time at the gym.

“A 200-meter sprinter is a great example of an athlete whose training is mostly glycolytic,” says Giuseppe. “It’s a nice compromise between strength and endurance work.”

One reason glycolytic training burns fat so effectively is that it creates a significant “metabolic disturbance,” Giuseppe explains. And recovering from it requires work from all three energy systems. In this way, glycolytic training improves not only the functioning of each individual system, but also your ability to transition smoothly
among them.

Giuseppe argues that such “metabolic flexibility” is a significant, though little-known, component of long-term health and fitness. “Diabetics and obese people can’t transition well between energy systems — they’re metabolically inflexible,” he says. “Smart training doesn’t just develop the three systems in isolation — it also develops your ability to transition from one fuel source to another so all three metabolic pathways work together effectively.”

The best way to train your glycolytic system is through repeated high-effort activity, with less-than-complete recovery between efforts: 20- to 30-second sprints on foot, in a pool or on a bike, with a minute of rest between them, or strength training in sets lasting 30 seconds to one minute.

Many field and team sports also train the glycolytic pathway.

Training Your Glycolytic System

Speed: Medium-fast
Primary Fuel: Carbohydrate
Sample Activities: Traditional strength training; 200- to 400-meter sprinting; 50-meter freestyle swimming
How to Train It: Medium-intensity strength training; interval training; running stadium stairs or hills; shaking “battling ropes”; jump-rope sprints; swimming repeats
Sets/Reps/Rest: Two to four sets
High effort for sets lasting 20 to 40 seconds; eight to 12 reps in strength-training activities
Short rest between sets (two minutes or less); partial recovery between efforts
Frequency: Twice a week per muscle group or area of the body trained

If you’ve ever done a 400-meter sprint, you’re familiar with what it feels like to exercise the glycolytic system.
In a word, it hurts.

Glycolytic athletes specialize in activities lasting 30 seconds to two minutes or so. They’re fast and seemingly tireless — though perhaps not quite as strong as the ATP-CP athlete, nor as enduring as the oxidative athlete — and they tend to be muscular and lean. This type of training is ideal for burning fat (in recovery) and building muscle mass. Strength training using sets of eight to 12 reps and sprinting 400 meters or less typify glycolytic training.

The oxidative (or aerobic) system is your slow-burning furnace, always humming in the background, whether you’re fast asleep or running hard. It’s fueled largely on fat and glucose, and, of the three metabolic pathways that support exercise, it’s the only one that directly requires oxygen to function.

“We’re predominantly aerobic creatures,” says Giuseppe. “We can go weeks without food, days without water, but if we’re deprived of oxygen for more than a few minutes, we’re dead.”

So although it’s last to kick in after you start to exercise, the oxidative system is the most important energy system of all. If it doesn’t work, neither do you.

Athletes in any long-distance endurance sport — cycling, running, triathlon — all need exceptional aerobic capacity, as do athletes in all continuous-action field and team sports, like basketball, lacrosse and soccer.

Fortunately, the aerobic system is very responsive to exercise. “Through training, you can increase the capacity of your aerobic metabolism up to 240 percent,” says Giuseppe. “And the better it works, the more effectively you burn fat in your workouts.”

Although the oxidative system is continuously active and produces loads of energy, the process of converting fat into usable energy can take a while. Once it gets started, though, it’s your body’s most reliable engine over long periods of time. In a 10-second sprint, Hartman says, your aerobic system is able to kick in only about 13 percent of the necessary energy; on an intense four-minute run, however, that figure rises to 80 percent.

Exercise physiologists used to believe that the best way to develop the oxidative system was through long, slow cardio exercise — an hour or more several times a week. Your aerobic system certainly responds well to this type of training, but recent research suggests that the oxidative system also works hard — very hard, in fact — to help you recover after a high-intensity anaerobic effort like a set of squats or a hill sprint.

Do a second, third and fourth set before you’ve fully recovered from the previous one, and the oxidative system ramps up its efforts even higher.

“A strength-training workout resembles a series of escalating waves of effort for the oxidative system,” says Giuseppe. That’s why you’re winded after high-intensity bouts of strength training and sprinting, even though the activities themselves are technically anaerobic. The oxidative system shifts into overdrive to replenish the depleted ATP-CP stores and clear out the glycolytic byproducts produced by your other two energy systems.

Because low-intensity aerobic activity speeds recovery from the minor damage caused by other forms of exercise, perhaps the best use of oxidative training is as a restorative tool on your off-days.

Training Your Oxidative System

Speed: Slow to medium
Primary Fuel: Fat
Sample Activities: Jogging, slow swimming, cycling, walking, hiking, martial arts, continuous-action team sports (basketball, football)
How to Train It: Light circuit training; running five minutes or more; long-distance cycling; traditional cardio machines; long, slow swimming
Sets/Reps/Rest: Either three to six one- to five-minute medium-high efforts with one to five minutes rest between sets, or
One to three eight- to 20-minute medium efforts, resting four to 10 minutes between reps
Frequency: One to three times a week

Although it’s last to kick in, the oxidative system is the most important energy system of all.
If it doesn’t work, neither do you.

Oxidative athletes are typically leaner and lighter than the other two athletic types. They can go on forever at a slow-to-medium pace, burning mostly fat — the ultimate high-efficiency, slow-burning fuel. Oxidative training is essential for endurance sports, but athletes in field sports shouldn’t neglect this kind of training. For the fitness enthusiast, though, the take-home lesson is that, unless you’re a competitive endurance athlete, lots of long, slow cardio is probably not the best way to exercise your aerobic system. Higher-intensity activities may be a more effective and efficient way to build your cardiovascular system — and to burn fat.