Did You Know That Cardio Uses 3 Different Energy Systems? Here’s How To Shape Your Workout To Maximize the Benefits of Each

Imagine you’re at the gym doing circuit training, or moving on the treadmill at home, cooking up a nice sweat. You can feel your heart rate picking up and you’re feeling good because this is what cardiovascular fitness and health is all about: good, solid activity to get the muscles moving and heart pumping. Cardio is cardio right?
Well…not exactly.
This monolithic view of cardiovascular training may be helpful for introducing cardio to fitness newbies because yes, any cardio is better for your heart and body than none. But in reality, not all cardiovascular training is created equal—and you could be missing out on serious health and fitness gains if you’re relying on the same type of training each time you work out.
Our cardiovascular system actually relies on three distinct energy systems—each with its own purpose and function. Those systems benefit from different kinds of activities that you should incorporate into your fitness routine if you want to truly maximize your overall cardiovascular health and fitness.
A primer on the three cardiovascular energy systems
During movement, our body relies on three distinct energy systems to fuel itself. There’s the aerobic energy system, anaerobic lactic system, and anaerobic alactic systems.
You may have heard the terms anaerobic and aerobic before, whether at the gym or in your high school bio classes. These terms refer to how your body creates energy: either with oxygen (aerobic) or without oxygen (anaerobic).
The aerobic energy system powers what we traditionally think of as cardio (think: running, walking cycling, swimming, etc.). According to the American Academy of Orthopedic Surgeons, your muscles “demand” more oxygen during aerobic exercise, which makes your heart beat faster in order to meet that demand (since your heart delivers oxygen to your body via the blood). Your cells convert oxygen (plus glucose, fat, and protein) into energy (called ATP) to help power and sustain this activity. This process makes a lot of ATP but tends to take some time.
The anaerobic lactic system works a little bit differently, and typically fuels more intense, temporary bursts of activity (like a rally during tennis). Your cells metabolize stored glucose—meaning turns it into energy—without using oxygen. A byproduct of this type of metabolism is lactate, which is why we call it anaerobic lactic: without oxygen, with lactate. This is a faster energy production process but is less efficient, so your body can’t sustain exercise levels at this rate for very long.
Last is anaerobic alactic, which means that it doesn’t use oxygen or create lactate to make energy. In this energy system, your cells make energy from creatine phosphate (CP), a compound that gets stored in your muscles. This is the quickest way to make energy, but provides the least amount of ATP compared to aerobic or anaerobic lactic energy systems. This means your body can only tap into this system to power super short bursts of exercise.
Why do these three energy systems matter for working out?
A simpler way to think about these systems comes via Andy Barr, DPT, a sports physiotherapist well versed in sports science. “We can break down and remember each system by intensity and duration,” he says. “The anaerobic alactic system is high intensity, short duration: think short, high energy bursts. The aerobic lactic system is medium intensity, medium duration: think medium, uptempo intervals. The aerobic system is low intensity, long duration: think long, endurance activities.”
Oftentimes, you will see these energy systems defined by a specific period of time. Anaerobic alactic provides energy for up to 10 seconds, anaerobic lactic works for 10-90 seconds, and aerobic powers 90+ seconds of activity. However, that definition really misses the mark in terms of understanding each system and how to truly train each system.
Rather, according to James Fitzgerald, a trainer and the owner of Opex Fitness, the way to know what system you’re in (and therefore training) comes from repeatability, how much rest you need before doing it again (“work to rest periods”), and how you feel during it.
“For the anaerobic alactic system, the activity should require a rest period that is between seven and 15 times the working period, which will typically max out around 20 seconds,” he says. “For the anaerobic lactic system, you should only be able to maintain that pace for the allotted amount of time or distance and then there’s a drop off. This zone is often quite uncomfortable. The rest period should be around five to seven times the work period to repeat the same length or time.” For the aerobic system, he says you should only need a rest period of one to two times the work period to repeat that same distance or time. “The feel here is typically comfortable and sustainable.”
To truly optimize your cardio and subsequently your overall health, fitness, and performance for the short- and long-term, we have to train all three of these systems. Even cooler is that there is some overlap between the systems, so you will see a cumulative positive effect across all three channels once you start training each. Here’s an easy-to-understand plan to do just that!
How to work out all three cardio energy systems, according to a trainer
This workout plan I created consists of three different workouts, each hitting one of the systems. Ideally you would complete three per week. If not, just cycle through each sequentially on your cardio days. I’d highly recommend not putting workouts A or B (see below) the day after any heavy lifting sessions because it may lead to potential overload. Lastly, precede each workout with a five-minute warm-up and a five-minute cool down.
Workout A (Anaerobic alactic; “high power”)
Choose your cardio equipment of choice. Treadmills can be the easiest here because running takes considerably more effort than ellipticals or bikes.
Following your warm-up, pick a pace for 20 seconds that requires you to rest for at least 140 seconds before you’re able to complete it again. You very likely won’t know that pace at first, so start conservatively and get a feel for wherever that is. Repeat three to five times.
Workout B (Anaerobic lactic; “uncomfortable”)
Choose your cardio equipment of choice. Rowers and cycles are preferred here because you can track your power output and distance easily.
Following your warm-up, pick a distance for one minute where your power and pace drops off significantly following that interval, and you need five to seven minutes before you’re able to complete it again. You very likely won’t know that pace or distance at first, so start conservatively and get a feel for wherever that is. Repeat three to five times.
Workout C (Aerobic; “sustain”)
For this, you can pick your cardio equipment of choice or do circuit training. I prefer the latter.
If using cardio equipment: Following your warm-up, pick a distance for four minutes that requires only a four-minute rest for you to be able to complete it again. You very likely won’t know that distance at first, so start conservative and get a feel for wherever that is. Repeat three to five times.
If doing circuit training: Following your warm-up, pick four exercises for one minute each (a total of four minutes) and keep track of the reps completed for each. Following a four-minute rest, complete the circuit again. If you’re in the “aerobic” system, you will be able to hit that same number of reps again. Repeat three to five times.