What the double threshold actually is
The central idea is simple to describe and hard to execute. Instead of a single hard session per day, the athlete does two interval sessions on the same day, one in the morning and one in the afternoon, both controlled to keep lactate in a moderate range, typically between 2.5 and 4.0 mmol/L. It is not running until you vomit. It is the opposite: each rep is fast enough to generate stimulus, but contained enough that you can repeat it all again hours later, and the next day.
The physiological trick is accumulating a lot of time at threshold intensity without blowing up fatigue. By running each session a little below the ceiling, the athlete builds up a weekly quality volume that would be impossible if every workout were taken to failure. The rest of the week is filled with true, easy running, the classic high volume, low intensity model.
The scientific controversy: potent, but not copyable
The 2023 systematic review concludes that the lactate-guided threshold model, within a high volume context, is physiologically coherent and has empirical support for the elite. So far, consensus. The problem starts when the recipe comes down from the podium to the weekend runner.
The double threshold was designed for those who train 10 to 14 times per week and log a very high volume. The logistics of two controlled sessions per day, with hours of recovery between them, do not fit the life of someone who runs 5 hours per week between meetings. In addition, fine control depends on measuring lactate, which in practice means pricking your finger several times per session. Marius Bakken, who documented the model, reports having done more than 5,500 lactate tests over two decades to calibrate his own training. That is an elite laboratory routine, not amateur training.
In response, a leaner variant gained traction from 2025, the single threshold, popularized by Kristoffer Ingebrigtsen, the eldest brother of the family. The logic: a single sub-threshold session per day, anchored more on the first lactate inflection point (LT1, near 2 mmol/L) than on the second (LT2, in the range of 3.5 to 4.5 mmol/L), alternated with easy running. It is the same philosophy of controlling intensity, adapted for someone with a full-time job.
The authorship controversy: "this system is mine and mine alone"
The second fight is not in the laboratory, it is in court. Marius Bakken, former Norwegian record holder in the 5,000 m with 13:06.39 and two-time Olympian (Sydney 2000 and Athens 2004), is widely credited with structuring and popularizing the model between the late 1990s and the 2000s, measuring his own lactate workout after workout. He observed that he got the greatest return by keeping lactate in a low range, around 3.0 mmol/L, pushing the threshold up without destroying himself for the next session.
On the other side, Gjert Ingebrigtsen, father and former coach of Jakob, publicly downplayed Bakken's influence. During the trial in which he answered for charges unrelated to training (violence against his children, denied by him), Gjert stated in testimony a sentence that became a symbol of the dispute.
The honest angle here is this: the science of controlling intensity by lactate and the dose-response at threshold are well established and bigger than any name. Who "invented" the Norwegian version is a folklore and ego dispute that does not change a single line of what physiology says. You can ignore the authorship fight and still take advantage of the method.
What the amateur can steal (without pricking a finger)
The biggest gain of the method for the common runner is not the double session or the lactate meter. It is intensity discipline. The amateur's classic mistake, documented for years by Stephen Seiler, is running the easy days too fast and the hard days too slow, living in the "gray zone" between truly easy and threshold. This zone accumulates fatigue faster than it generates adaptation.
Translating to practice without prescribing anything: evidence indicates that turning the threshold into a VO2max session is what breaks the amateur. If every rep becomes a race, quality volume plummets and fatigue rises. Studies suggest that holding the interval pace at a "comfortably hard" level, where you can talk in short sentences, delivers more stimulus added up over the weeks than two heroic workouts that force three days of recovery. Before changing the structure of your week, talk to an exercise professional or coach who knows your history.
How this informs the running and triathlon core
It is exactly this control that Darwin uses when distributing threshold sessions in your medium and long distance running week or your triathlon week. Instead of pushing you into the gray zone, the app anchors the hard sessions at a sustainable threshold intensity and protects the easy days, so that they are truly easy. You do not need a lactate meter or two workouts a day to benefit from the principle: what matters is not letting the threshold become a sprint and not letting the easy become medium. The athlete operates the machine, science just organizes the dose.
