Physiology Friday #231: Is Double Threshold Training the Fast-track to Fitness and Performance?
Embrace the Norwegian method.
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This is not a post about zone 2 training.
We’ve heard enough about the merits of performing a lot of lower-intensity training for improving cardiorespiratory fitness, mitochondrial health, and longevity.
In fact, endurance athletes have long recognized the importance — rather the necessity — of low-intensity training. When exercise volume creeps high enough, there’s only so much high-intensity work you can do without risking injury, burnout, or the dreaded “overtraining” (a condition that we still can’t accurately characterize, measure, or define).
It’s time we started talking (again) about the performance and health benefits of high-intensity training. Specifically, I want to discuss a unique training strategy referred to as double threshold training — which some are hailing as the fast track to fitness.
The origins of double threshold training can be traced back to Norway, where coaches and sports scientists began experimenting with this approach to maximize the performance of endurance athletes (does the name Jakob Ingebrigtsen ring a bell?)
Is double threshold training something you should implement into your exercise routine?
Double threshold training, often referred to as the Norwegian method, revolves around the concept of performing two high-intensity training sessions in a single day, both of which are structured around the lactate threshold or an intensity between the first and second ventilatory thresholds, also known as VT1 and VT2, respectively.
The lactate threshold is a critical intensity marker in endurance sports, representing the point at which lactate begins to accumulate in the blood at a faster rate than it can be cleared. Below the lactate threshold, we can continue exercise “indefinitely” and above it, we slowly fatigue (but not because of lactate, as was once incorrectly believed).
VT1, also known as the aerobic threshold, occurs at a relatively low to moderate intensity of exercise. At this point, the body relies primarily on aerobic metabolism, with a steady-state balance between aerobic energy production and energy demand. The increase in ventilation (breathing) is proportional to the increase in carbon dioxide production due to the oxidation of carbohydrates and fats — our body’s energy sources. Training at or just below VT1 is beneficial for enhancing the aerobic base, improving fat oxidation, and increasing overall endurance capacity. This is what we’d refer to as zone 2 training.
VT2, also known as the anaerobic threshold or respiratory compensation point, occurs at a higher intensity, closely aligned with the lactate threshold. At VT2, the body experiences a more rapid increase in ventilation relative to the intensity of exercise, primarily due to the increased production of carbon dioxide. Training at or just below VT2 is crucial for improving the body's ability to sustain high-intensity efforts and delay the onset of fatigue, as it helps in increasing the lactate threshold.
Threshold training focuses on working at or near the lactate threshold or between VT1 and VT2. This intensity is often sustainable for 20–60 minutes with the goal of improving aerobic efficiency.
Scientifically, the double threshold method is grounded in the idea that by targeting the lactate threshold in two separate sessions, rather than one longer session, athletes can enhance their aerobic capacity and improve their body's ability to sustain high-intensity efforts over longer periods. This approach also allows for a high volume of quality training without the excessive strain that might accompany continuous high-intensity sessions, thereby reducing the risk of overtraining. The 6 or more hours between the two threshold sessions is a time to rest and recover. Double threshold training — because it could allow for a higher training volume — may also have benefits for those looking to optimize their VO2 max.
Another key benefit is the focus on quality over quantity. Double threshold sessions are designed to be intense but controlled. They’re hard but don’t carry the degree of physical wear and tear associated with more traditional high-volume training methods. This can lead to better recovery and a reduced risk of injury.
However, double threshold training is not without its drawbacks. For one, it requires that you’ve got the time to set aside to workout twice in one day. Second, I think there are advantages to having a more prolonged threshold session, especially if you’re training for an (ultra)endurance event like a marathon or longer. Forcing the body into fatigue isn’t always a bad thing.
One thing that’s not been well characterized is how the body responds to double threshold training compared to a single longer session. Does breaking up a session into two equal parts lead to better performance during both workouts? How is recovery affected? And how is training stress different between a single long session and a double threshold session?
These questions are intriguing and have relevance whether you’re a recreational endurance athlete, age-group competitor, or even if you’re just training for overall health. And thanks to a new study published in Frontiers in Physiology, we’ve got some answers.1
For the study, 14 male endurance athletes (11 cross-country skiers and 3 runners) completed two different time- and intensity-matched training sessions in the laboratory on a treadmill.
Single: the participants performed 6 x 10-minute threshold intervals at an intensity that corresponded to 90% of the treadmill speed where they reached their lactate threshold (determined on a prior visit). A 2-minute recovery period separated each interval.
Double: the participants performed one 3 x 10-minute interval session in the morning and another 3 x 10-minute interval session in the afternoon (~6.5 hours after the first session). A 2-minute recovery period separated each interval.
The primary outcomes were related to the training stress and physiological variables during the workouts and the recovery status of participants after the workouts.
Results
During the single threshold session, average heart rate was ~3% higher during the first 3 intervals compared to the second 3 intervals. This isn’t surprising and characterizes cardiovascular drift that can occur due to fatigue and/or dehydration during a longer workout.
But this heart rate increase wasn’t present on the double training day: heart rate was ~3% lower during the second (afternoon) workout of the double threshold session compared to the morning session and was 4% lower during the second double threshold session compared to the last 3 intervals of the single threshold session.
Thus, doing two shorter workouts prevents cardiovascular drift and allows athletes to maintain a consistent heart rate across a given workload (important for the sticklers about controlling heart rate during a workout!)
Oxygen consumption (VO2), however, didn’t differ between the conditions. An important finding, I believe, as it might suggest that double threshold sessions — at least if they’re the same intensity and duration as a single threshold session — shouldn’t theoretically be any better for improving maximal oxygen consumption.
Blood glucose and blood lactate also increased during the single threshold session — they were higher during the second half compared to the first. But blood glucose and blood lactate were lower during the second double threshold session compared to the first and in the second threshold session compared to the last half of the single session.
The higher glucose and lactate during the single threshold workout were accompanied by a higher rating of perceived exertion (RPE) — the participants rated the single threshold session as being harder than the double threshold session. Their RPE also increased from the first half to the second half of the single session but wasn’t different from the first to the second double session.
The researcher also measured indices of autonomic nervous system recovery, finding that resting (supine) heart rate in the hour after exercise was 7–13% higher after the single threshold session compared to either double threshold session. Heart rate recovery after exercise was similar when comparing the first and second double threshold sessions.
The single threshold session also affected several markers of perceived training stress including a higher session RPE and training load and increased fatigue and muscle soreness the next day compared to the double threshold session.
On the other hand, participants reported feeling equally motivated and “ready” for each session (though they were slightly less motivated to perform the second double threshold session compared to the first), felt that both workouts produced a similar “training quality,” and had a similar sleep quality after both workouts (this latter finding is somewhat surprising, as I find that an afternoon high-intensity workout can sometimes wreck my sleep. But I digress).
To me, all of this indicates that objectively and subjectively, a single threshold session is a harder physiological stress than a work-matched double threshold session. Even though the external work done was equal (both sessions were 60 minutes of total work) the internal work of the single threshold session was higher, leading to greater physical, mental, and autonomic stress and poorer next-day recovery.
Is this all a bad thing? And would it suggest you should avoid the overly stressful long single threshold workout? Not so fast.
As I mentioned in the introduction, sometimes we want a greater physiological stress (perceived or otherwise) so that the body adapts to exercising while physically and mentally fatigued. The only way to do this is to slog through a long session, cardiovascular drift be damned.
Furthermore, the molecular adaptations that happen with a single long workout sometimes can’t be replicated by just splitting that workout in half. For example, the higher heart rate, increased core body temperature, and recruitment of type II muscle fibers (which are engaged even during moderate-intensity training when type I muscle fibers become fatigued) trigger beneficial adaptations.
There is a time and a place for the long threshold session — just make sure you’re well recovered beforehand and allow adequate recovery after.
But there’s also a time and a place for the double threshold!
Indeed, the ability to perform the same amount of work with less “stress” is a major benefit if you’re trying to ramp up training volume or if you’re trying to taper for a race while maintaining intensity. It seems as if a double threshold session might actually reduce injury and burnout risk (provided equal volume) because you are more recovered for each session! That’s a huge asset in a training toolkit.
Of course, the lower stress per session also means that you might be able to perform more work per session (for example, the participants could add 7 10-minute intervals to each session instead of 6) with the same physiological stress as the shorter single session.
That’s actually one of the main benefits of double threshold training…and one that I actually experimented with for the sake of this post. If you ever questioned my love for my readers.
Yesterday, I performed my own double threshold session — running 6 miles of tempo work in the morning and another 3 miles in the afternoon. You can take a look at the workout data below.
I certainly couldn’t have done 9 miles of tempo running at once (at the same pace and heart rate) in a single session. Breaking up the workout allowed me to accumulate quality running at a somewhat controlled heart rate and rack up more volume than I could have in one longer workout. You’ll notice that in the first workout my heart rate drifted close to 174 beats per minute by the end — imagine what it would have done had I continued another 3 miles.
By the afternoon session, I was able to run 3 miles at a similar (though somewhat slower) pace at a lower heart rate than the morning session. Take into consideration that it was much hotter for the second session.
So, are you convinced you should try a double threshold session? Should you try a double threshold session?
I see this “unique” training method as a way to mix up training if you’re in a rut and try something new. As I begin a 13-week training block for my second marathon, I’m excited to integrate a few double threshold sessions to see the fitness they might bring.
And you don’t need to use a double threshold to ramp up volume and intensity. You can simply cut your normal workout in half and perform it in two separate sessions — you might feel better during both and experience better recovery and less injury risk.
It’s worth a shot. If you try it out, I’d love to hear about it in the comments below or elsewhere.
Thanks for reading. See you next Friday.
~Brady~
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Thanks a lot for the article, I'll try this for some weeks
You’ve got me thinking if I can fit in another workout before I head to work - that’s the difficulty, finding another block of time without getting up at 3AM.