Optimise Your Body’s Energy Systems

You’ve got two, and training them the right way makes everything – from powering up stairs to your regular workouts – easier and more efficient.

Portrait of Tammy Strobel

How much longer exercisers lasted in the aerobic energy system zone when they had caffeine beforehand, according to a new study in Medicine & Science in Sports & Exercise. The caffeine kick didn’t boost the anaerobic system, though. “Caffeine helps liberate fat, so that’s why you can go longer – you’re tapping into more potential energy,” Gary says.

What we call cardio is actually more nuanced than what that word implies. Our bodies have aerobic and anaerobic (without oxygen) energy systems, and we use both during exercise. Why split hairs? Because if both aren’t trained, you can be hard-core gym-committed and still get breathless walking up stairs. Here’s the drill for firing on all cylinders.


On a basic level, your body runs on adenosine triphosphate (ATP). Every move you make requires tapping this organic chemical for its ready-to-use energy. For quick bursts of activity like that dash upstairs, you need ATP pronto, so your body has to use whatever stores it has available, since there’s no time for creating more with the help of oxygen (via the aerobic process; more on that later). “With no warm-up, the body doesn’t have time to prepare ATP and therefore relies on functioning anaerobically regardless of how fit you are – hence you get winded,” says Gary Liguori, the dean of the College of Health Sciences at the University of Rhode Island. And that drained feeling in your legs? It’s caused by the rapid spike in lactic acid production. But you can increase your anaerobic capacity – meaning you’ll do more with your ATP on tap before fatigue sets in – by adding some all-out intervals: Warm up and then do sprints uphill or on a flat surface for 20, 30, or 40 seconds with sufficient recovery in between, Gary says.


The aerobic system kicks in when you ease into exercise, using available oxygen to turn the body’s stores of glycogen (aka carbs), fat, and even protein into usable ATP. Aerobicdominant workouts include steady runs, cycling, and even circuits with weights in which your heart rate stays between 60 and 80 percent of your max, says trainer Joe Dowdell, the founder of Dowdell Fitness Systems programs. The more exercise minutes you put in, the more you can increase your aerobic capacity and the longer you’ll last in future activities. “Use a heart rate monitor to track how quickly your heart rate returns to normal after exercise,” Joe says. The better your aerobic fitness, the faster it should recover between sets or sprints.


“The beauty – and the confusion – is that the two systems are not mutually exclusive,” Gary says. “The more aerobically fit you are, the better your body can convert the by-products of anaerobic exercise—namely lactic acid—back into ATP, and anaerobic training would also benefit your aerobic capacity.” One way to train both systems is doing extended bouts of HIIT, Gary says: The sprints build anaerobic capacity; the accumulated work builds your aerobic system.