What’s the Difference Between Aerobic and Anaerobic?

Who knew that two simple letters—an—could make all the difference in triathlon training? Frequently referenced, but often misunderstood, the difference between the terms aerobic and anaerobic seems small, but is actually big (and important). Incorporating both anaerobic and aerobic efforts into your training is crucial to your development as a well-rounded triathlete. That’s why it’s important to make sure you know the difference and why it matters.

Aerobic training

The word aerobic literally means “with oxygen.” When an athlete exercises in an aerobic zone, they’re going easy enough that the heart is able to continue to deliver plenty of oxygenated blood to working muscles. The cells of the body require this oxygen to generate energy by breaking down carbohydrates and fat to serve as fuel. The waste products of this process are carbon dioxide and water, which are expelled by breathing.

This type of steady-state exercise can sometimes be referred to as “cardio,” because of its reliance on the cardiovascular system. It’s an easier effort, in a lower training zone or at the lower end of the rate of perceived exertion scale, but is one that can be sustained for hours, which is essential for the endurance athlete. Where effort level is concerned, you might hear a coach refer to this as your “all-day pace.” Having a strong aerobic base will ultimately allow an athlete to go longer, and faster, at a sustainable effort.

Anaerobic training

Anaerobic means “without oxygen.” During this type of shorter, intense activity, the body cannot keep up with the demand for oxygen required by the muscles. Therefore, it must utilize a different pathway for creating energy by drawing fuel from sources within the muscles. Examples of an anaerobic effort might include doing fast track intervals or sprinting to the finish line. The benefits of this type of exercise include increased stamina, building your top end tolerance, and developing fast twitch muscle fibers.

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“When people talk about being anaerobic, they sometimes use the term, lactate threshold, which is easy for them to relate to the feeling of their legs burning,” said Marni Sumbal, USAT-certified coach, board certified sports dietician, and 17-time Ironman. “The heart is beating quickly, the breathing rate is high, and the anaerobic point is reached when the body goes into an oxygen deficit. At that point, the body needs to use physical resources in order to be able to clear lactate buildup and carbon dioxide.”

As a result, an anaerobic effort is not one that an athlete can sustain for a long period of time. Some research suggests this system can solely fuel the body for 2-3 minutes, tops.

How to determine aerobic vs. anaerobic efforts

Most athletes rely on training zones to delineate the differences between aerobic and anaerobic training—with the line between those two often referred to as “threshold,” meaning your lactate threshold or the point at which your body can no longer replace the oxygen it’s using and lactate begins to accumulate in the bloodstream. To determine where this point is for you requires you to do a test, such as a 30-minute time trial effort, and then establish a lactate threshold heart rate (LTHR) and zones based on that test.

Use this resource for multiple kinds of tests and common calculations: How to Establish Triathlon Training Zones

A common calculation for training zones is based on TrainingPeaks’ definition of lactate threshold heart rate (LTHR):

• Zone 1: Recovery – Less than 85% of LTHR
• Zone 2: Aerobic – 85-89%
• Zone 3: Tempo – 90-94%
• Zone 4: Sub-Threshold – 95-99%
• Zone 5A: Super-Threshold – 100-102%
• Zone 5B: Aerobic Capacity – 103-106%
• Zone 5C: Anaerobic Capacity – More than 106%

Once you understand your zones, then determining the difference between an aerobic and anaerobic effort while swimming, cycling, and running can be done with use of a heart-rate monitor, pace or power, or paying attention to the rate of perceived exertion.

A good tip to determine aerobic effort while running is thinking about it as a “conversational pace.” Try talking out loud while you’re running. If you can’t talk without gasping for breath, you’re running too fast for it to be a true aerobic effort. Studies have shown this is as effective as any metric.

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Unlike running or riding a bike, determining the point at which the body switches from an aerobic to anaerobic state while swimming can be challenging. This is partially because the act of swimming is more of an anaerobic sport to begin with—again, think “without oxygen.”

While a heart-rate monitor is a useful tool, it’s not a practical reference point for use in the water. Instead, it’s more helpful to focus on rate of perceived exertion (RPE), which can be assigned with number from 1-10 on a scale of easy to hard.

“In the pool, determining zones can be rather challenging, because a lot of triathletes are one- or two-speed swimmers,” she said. “The majority of triathletes aren’t coming from a swimming background. The effort is either easy or hard, but there’s no in between.”

For the beginner, Sumbal finds little benefit in doing 1,000-yard sets to determine heart-rate threshold values. Instead, she advises athletes to do shorter 25- to 50-yard intervals to determine what an easy, moderate, and hard pace feel like. Once an athlete has progressed to the next level, a good test can include a 400-yard and 200-yard best effort swim.

“Ideally, the time for the 200-yard best effort swim should be faster than the 400-yard swim,” she said. “For example, if an athlete did the 400y at a 1:30 per 100y pace, then a good 200y pace might be 1:25 per 100y.” Once a pace is determined for each distance set, it can be used as a baseline for future workouts. For example, if an athlete can hold 1:25 per 100 for a 200-yard best effort swim, then he or she should be able to swim 25s, 50s, or 100s at a little faster than that 1:25 pace.

If you’re more numbers driven, think about an aerobic effort as being 50-70% of max heart rate, so Zone 1 and Zone 2. An aerobic swim set might include sustained swimming at a comfortable pace, with short rest, in an effort to build cardiovascular fitness. An anaerobic swim set could include short intervals, like 25s to 100s at 90-95% of max heart-rate, with longer rest. The goal here is to train the muscles to buffer lactic acid, while allowing plenty of time to regulate breathing from the hard effort.

Why do we need both aerobic and anaerobic training?

Why is both aerobic and anaerobic training important? You’ve probably heard of the 80/20 rule as a framework for balancing intensity in triathlon training. Research has shown that endurance athletes gain the most fitness when training about 80% of the time at lower intensity (ie. aerobic) and 20% at moderate to high intensity (ie. anaerobic). The trap that most age-group athletes fall into is doing the bulk of their training at a moderate level. They don’t train easy enough to benefit from consistent aerobic effort, and they’re never rested enough to train at a hard, anaerobic effort to gain maximum benefit.

For the majority of athletes, a long-distance triathlon will primarily be an aerobic effort. So, this begs the question: If we’re going to be racing at an aerobic effort, what’s the value of anaerobic training?

“If you’re an athlete who really wants to get faster on race day, you want to be able to improve your steady-state effort,” Sumbal said. “You want to be able to hold this effort for longer, while going faster, without it coming at such a cost. For example, let’s say, for the bike, an athlete’s threshold might be 150bpm [heart-rate]. Once he goes over that value, he starts to fatigue. What you want to do is train both above that value and below that value. You want to stretch and raise the aerobic threshold.”

As an example, Sumbal describes an athlete whose aerobic threshold is in the 150-160bpm range. She will instruct the athlete to spend a few minutes running above the threshold, say at 170bmp, and then run under it, at 130bpm, as a way to target both ends of the spectrum.

But, the key to knowing what an athlete really needs to work on is to test them and find out. Are they an athlete who needs to develop that aerobic engine or who needs to focus on top-end efforts?

This is why Sumbal sometimes prefers an alternative method of testing: a heart-rate build test.

“It’s 40 minutes long and, for each 10-minute section, an athlete should run as fast as he can while keeping his heart rate under a certain number. For example, for the first 10 minutes, an athlete might run as fast as he can, but keep his heart rate under 140. The next 10 minutes would be as fast as he can with his heart rate under 150. The next 10 minutes would be under 160 and the last would be under 170. It really gives us a good indication of an athlete’s physiology and which zones they need to train more to improve.”

She also likes to mix up what kinds of training her athletes do when, instead of falling into the trap of thinking they only need to do easy aerobic base-building in the early season.

“Doing a bit of intensity and focusing on strength in the early part of the season is the best way to really train threshold, to become strong, and to have good skills. In order to be efficient, you need to spend some time building a strong foundation and working on threshold.”

She cautions against thinking that certain workouts should only be reserved for certain times of the season. You can throw some microbursts in on the bike after big gear work, or a couple of hard 25s at the end of a swim to work on turnover. While early season doesn’t have to be only aerobic and faster anaerobic work isn’t only for short-course athletes, the key is to make sure the easy days are easy and the hard days are hard.

“It’s not so black or white,” she said. “We come from these periodized types of training blocks where we only do this type of workout during this time of the year. Of course, we’re not doing race pace work in December, because that doesn’t make sense, but to think you can’t include intensity is causing your body to miss out on utilizing different energy systems. It’s good to throw that in sometimes to stress the body, but do it in a safe way.”

“You can throw a little spicy effort into long runs, bricks, or swims,” she said, but keep it easy on easy days—that’s the biggest mistake athletes make. “Easy should be really easy, or it will come back to haunt you. Even if you go a little too hard on that easy day, you’re missing out on training that energy system, because you won’t be able to go really hard when you need to. Sometimes, you have to push ego aside, don’t worry about what Strava says, and just be honest with yourself.”