In a few short years, the concept of “threshold” has gone from niche sports science jargon to mainstream obsession. You want to follow Peter Attia’s advice and maximize your longevity by exercising in Zone 2? Cycle like Tadej Pogačar? Smash your running PRs by adopting Norwegian miler Jakob Ingebrigtsen’s double-threshold workout routine? Then you’ll need to learn how to identify your threshold, that quasi-mystical point where exercise shifts from easy to hard.
That’s the theory, at least. But scientists have been debating how to define threshold and what it means for half a century now. Two new studies suggest that the concept really is important, but not necessarily in the ways we expect. Having a higher threshold doesn’t automatically mean you’ll be faster or healthier—but understanding where your threshold is and recognizing what it feels like to cross it might be the key to training smarter and more effectively.
The Problem with Max Heart Rate
The first study, published in Sports Medicine by Dominique Hansen of Hasselt University in Belgium, deals with training prescription. When you advise someone to run or bike, how do you specify the appropriate intensity? Public health guidelines generally offer suggestions based on percentage of maximum heart rate. A “moderate” effort, according to the American College of Sports Medicine, is between 64 and 76 percent of your max heart rate—or, equivalently, between 46 and 63 percent of your VO2 max, a fitness measure that can be assessed in a lab or estimated by your watch.
The problem, Hansen explains, is that we all respond differently to this type of prescription. A workout at 70 percent of max heart rate is moderate for many people, but will be too easy for some and too hard for others. This is one of the big reasons why exercise studies produce conflicting results, and why some people following generic max-based training plans get disappointing results: the prescription is off.
The alternative is to gauge your training based on threshold—or rather, thresholds. The original threshold concept, formulated in the 1960s, distinguished only between easy “aerobic” and hard “anaerobic” exercise, based on whether your muscles get enough oxygen. Physiologists now recognize two transitions: one when anaerobic (meaning oxygen-free) energy starts to kick in, and a second when the effort becomes unsustainable even with the help of anaerobic energy. Once you pass that second threshold, the clock is ticking and you’ll soon have to slow down or stop.
Hansen and his colleagues pool the results of 45 studies with more than 1,500 participants to show that prescribing training efforts based on thresholds rather than max produces larger and more consistent improvements in fitness and health. Having two thresholds rather than one means there are three distinct zones—call them easy, medium, and hard—and workouts can be described by how far above, between, or below the thresholds they are. For example, Attia’s Zone 2 training (don’t get hung up on the nomenclature—there are many different zone definitions) involves exercising just below the first threshold, at the very upper end of the easy zone. Ingebrigtsen’s double-threshold workouts aim to stay in the medium zone between the first and second thresholds.
What Threshold Training Does (and Doesn’t Do)
The second study makes a subtle but important point about the goal of threshold-based training. Physiologists have long assumed that endurance is a function of three key variables: VO2 max (the size of your engine), economy (the efficiency of your engine), and first threshold (which dictates how much of your engine’s power you can sustain during a workout or race). Improve any one of these three parameters, the thinking goes, and you’ll get faster. Training at or near your first threshold, then, is a way of raising that threshold and thus improving your performance.
In the International Journal of Sports Physiology and Performance, a research team led by Jan-Michael Johansen of the University of South-Eastern Norway punctures this myth. Thresholds are usually expressed as a percentage of VO2 max: one athlete might hit their first threshold at 75 percent of VO2 max, while another might hit it at 90 percent. But that doesn’t mean the second athlete is better than the first one. In tests of nearly 300 athletes ranging from regional level to international elites, Johansen found no link between where an athlete’s threshold lies and how good they are.
The real magic of training near your thresholds, Johansen says, is that it improves the other two parameters, VO2 max and economy. If your first threshold is at 80 percent of your VO2 max, following Ingebrigtsen’s double-threshold approach is unlikely to change that. But it will increase your VO2 max, so your threshold will increase to 80 percent of a bigger number, meaning you can run faster without fatiguing.
None of this tells you exactly how to train. Real-life training programs, whether for fitness or elite performance, generally incorporate a full range of intensities. The art lies in finding a balance between easy and hard, pushing yourself enough to trigger fitness gains without burying yourself with so much fatigue that you can’t recover for the next workout. Exercising at a heart rate calculated from your max—if you even know your max, as opposed to calculating it from a wildly inaccurate rule of thumb like 220 minus age—is too blunt an instrument to get that balance right.
By contrast, understanding where your thresholds are helps you ensure that easy days stay easy, medium days stay medium, and hard days are truly hard. You can measure your thresholds in a lab or, like Attia and Ingebrigtsen, self-test your lactate levels by pricking an earlobe mid-workout. But there’s a simpler option called the Talk Test: speaking in full sentences means you’re in the easy zone; short phrases are medium; single words means you’re going hard. Learn to feel those gear changes, and you’ll have a better handle on how to train than any watch or wearable tech can give you.
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