Most athletes who say they train Zone 2 actually train above it

The athletes who say they train mostly in Zone 2 almost always train above it. That is the uncomfortable conclusion endurance coach Matt Fitzgerald drew in a Strava Stories essay republished this week, leaning on a study of nine recreational triathletes whose own watches gave them away. Over an eight-week window around an Olympic-distance race, the group spent only 47 percent of training time at the easy end of the intensity spectrum. Just two of those eight weeks broke past 50 percent.

The number sits well below the 80 percent floor that the polarized model of endurance training calls for. Coaches and exercise physiologists have spent the better part of a decade telling weekend triathletes, marathoners and gravel cyclists to back off and accumulate more aerobic time. The data suggest the message lands while the legs go elsewhere.

What the 2021 paper actually measured

The study Fitzgerald cites was led by Joao Henrique Falk Neto and Margaret Kennedy at the University of Alberta and published in the journal Sports in June 2021. The team logged daily training across nine recreational triathletes, five men and four women, through six weeks before and two weeks after their goal race. Heart rate and pace files were stratified into three zones using each athletes individually tested thresholds, not generic age formulas.

Falk Neto and colleagues found weekly variability in load was high, but the intensity floor barely moved. What they describe is not 80 percent easy and 20 percent hard. It is closer to 47 percent easy and the rest split between tempo grinding and outright threshold work. The group also showed week-to-week load spikes that pushed past markers linked to overuse injury, although the authors stopped short of pinning any specific injury to the pattern.

Why athletes drift up

Fitzgerald argues that recreational endurance athletes drift up out of Zone 2 for predictable reasons. They run by feel rather than by physiology. They tolerate average heart rates a few beats above target because the average looks correct on the watch. They round small overages down. He puts a single, mechanical line on the boundary. “If your heart rate is above 80 percent of maximum, give or take, then you are above Zone 2,” he writes.

The pace equivalent is similar. Fitzgerald defines easy running as anything below 65 percent of maximal aerobic speed, the fastest pace an athlete can hold for six minutes. Most jogs by amateur runners drift 5 to 10 percent above that line on a flat day, and further on hills.

His sharpest framing is about cumulative dose. “The body does not experience averages. It experiences actual time spent at certain intensities,” Fitzgerald writes. Twenty minutes at 84 percent of max heart rate inside a 60-minute run does not become Zone 2 just because the rest of the run sat at 76. The mitochondria respond to the actual minutes, not the rolling average on a wrist.

How the polarized model got there

The Zone 2 idea is not a coaching fad. A 2015 review in Frontiers in Physiology by Thomas Stoggl and Billy Sperlich pulled training logs from elite endurance athletes in running, cycling, rowing and cross-country skiing. Across all four sports, elites logged 75 to 80 percent of their volume below the first lactate threshold. The other 20 to 25 percent split between threshold work and frank intervals. Coaches now call the shape polarized when the high end is concentrated at VO2max work, pyramidal when more of it sits at threshold.

The same authors ran a randomized trial pitting polarized distribution against three other shapes over nine weeks. The polarized group added 6.8 ml per kilogram per minute on VO2peak. The threshold group, the high-volume group and the high-intensity-only group all gained less. The reasoning is not exotic. Long aerobic work expands capillary density and mitochondrial mass without piling on fatigue. Hard intervals lift the ceiling. The middle ground does neither efficiently and costs more in recovery.

What recalibration actually looks like

The fix Fitzgerald describes is unglamorous. Trust the threshold tests, not the feel. Recreational athletes can confirm their Zone 2 ceiling with a 30-minute time-trial heart rate average or a lab-based lactate test, and then set the upper boundary a few beats below that figure. The cost is jogging slower than is socially comfortable, walking up steeper hills and accepting that the watch will show a pace that looks soft.

The bike makes the discipline harder. Heart rate creeps up on a climb in a way that feels effortless on the legs but registers as moderate on the cardiovascular system. Falk Netos cohort logged the largest deviations during cycling sessions specifically.

The Falk Neto paper carries one further methodological lesson that coaches have begun repeating. Self-reported intensity correlates poorly with measured intensity. Athletes who say they ran easy rarely produce a file that reads as easy. Any honest audit of training distribution needs the watch data, not the training-log narrative.

What to take from it

Zone 2 training is not broken. Two decades of work back low-intensity volume as the foundation of aerobic capacity. What is broken, on this reading, is the gap between intent and execution. Athletes who plan to train easy 80 percent of the time end up training easy closer to half the time. Closing that gap takes either a coach willing to be unpopular or a willingness to ignore the social pull of the group run.

For most amateur endurance athletes the cleanest correction is to let one number rule the easy days. Below 80 percent of max heart rate, no matter how slow the pace gets. Anything else is a different workout, with different adaptations and different costs.

References

1. Falk Neto JH, Parent EC, Vleck V, Kennedy MD. The training characteristics of recreational-level triathletes: influence on fatigue and health. Sports 9(7):94. 2021. https://doi.org/10.3390/sports9070094
2. Stoggl TL, Sperlich B. The training intensity distribution among well-trained and elite endurance athletes. Frontiers in Physiology 6:295. 2015. https://doi.org/10.3389/fphys.2015.00295