There are two key pieces of information an athlete needs before starting a training session: how long the session will be and how hard to push. The first is simple time or distance. The second is more elusive. My preference is to give athletes a heart rate training zone. In this article, I explain heart rate training, the different ways to determine training zones, my preferred method, and the zone I believe is most important for triathlon.

Traditional Ways Coaches Set Training Intensity

Before heart rate monitors, coaches prescribed training intensity using speed, pace, or perceived effort. Runners and swimmers were given pace targets. Some coaches simply said “go easy” or “go all out.” More recently, cycling coaches rely on power meters, using watts to quantify effort.

My favorite method, which began in Finland in the 1970s, is using a heart rate monitor. Coaches assign a target heart rate zone for most of the workout. Low zones indicate recovery, middle zones build endurance, and high zones develop speed and strength.

How Heart Rate Zones Are Determined

Coaches use several methods to establish heart rate zones. Some use percentages of maximum heart rate, often estimated with the “220 minus age” formula. Others incorporate resting heart rate to refine the lower boundary of each zone. Some coaches have athletes perform maximal effort tests to determine their true ceiling. Others rely on lactate threshold testing, using blood samples taken during exercise.

I prefer the Phil Mafetone method, which uses 180 minus the athlete’s age, with adjustments based on fitness. This produces the Maximum Aerobic Heart Rate (MAHR), not to be confused with maximum heart rate. MAHR marks the line between highly efficient aerobic training and less efficient aerobic training.

Why Aerobic Development Matters

During exercise, the body relies mainly on two fuel sources: glycogen and fatty acids. Glycogen is limited to about two hours of supply and is used heavily at higher intensities. Fatty acids, on the other hand, are essentially unlimited and used more when intensity and heart rate are low.

Before heart rate monitors, coaches often told athletes to train at a conversational pace. This naturally kept intensity low and encouraged the body to become better at burning fat. Over time, athletes can run faster or produce more power at the same low heart rate. This is aerobic development, or base work.

Many athletes fear that training easy will make them race slow. But the goal isn’t to race easy, the goal is to build a massive aerobic engine. When combined later with high-intensity work, this foundation allows athletes to reach their full potential.

Arthur Lydiard, the legendary New Zealand coach, pioneered this concept in the 1960s and 70s. His athletes ran high mileage to build aerobic fitness and went on to win Olympic medals. The same principle applies beautifully to triathlon, where races can last up to 17 hours.

What Counts as Aerobic Development

This brings us back to MAHR. Training below MAHR keeps stress low, avoids triggering stress hormones, and allows for quick recovery. Heart rate reflects more than just training intensity — dehydration, lack of sleep, and life stress all elevate heart rate. What feels easy may still be too stressful for optimal aerobic development. The heart rate monitor keeps you honest.

My Experience With MAHR Training

I began using a heart rate monitor in the 1990s, including during the Hawaii Ironman. Looking back, my run training was too intense, and I never developed a strong aerobic base. As a result, I struggled in the marathon during my early Ironman races. I performed well because I was young and fit, but the marathon was always a battle.

Everything changed in 1996, when I raced Hawaii at age 45. That year, I ran the entire marathon, and my final six miles were my fastest. I credit this to the deep aerobic base I built by training with MAHR as my upper limit.

The Challenge and Reward of MAHR Training

This method is not easy at first. I had to swallow my pride, slow down, and even walk to keep my heart rate low. But with patience and consistency, my pace improved dramatically at the same low heart rate. Eventually, I felt strong, efficient, and energized.

This doesn’t mean avoiding hard training altogether. A well-rounded triathlon plan includes high-intensity work. I’ve spent plenty of time in the pain cave across all three sports. Even elite athletes use this approach. Five-time ITU World Champion Francisco Javier Gómez Noya has said that his base training is capped at a heart rate of 155, which matches the MAHR for a newly turned 30-year-old under this method.