Double Run Threshold Training

In the last few years, double threshold days have become a major talking point in endurance sport especially after the rise of Norwegian athletes dominating triathlon and middle-distance running.

Two sessions in one day, both at controlled medium-to-high intensity (usually around 2.0 to 3.5 mmol/L lactate). The idea is simple: by splitting the same work into two manageable sessions, athletes can accumulate more high-quality intensity across a week without destroying themselves in a single massive workout.

But does it really work? And more importantly, how does it affect fatigue and recovery compared to condensing all that work into one session? Before I do go further, I do want to make a note that you probably shouldn't just take a typical threshold workout that you have been doing and double it. That probably isn't the smartest idea, rather distrabuting the load in a way where you are breaking up the intenisty to accumulate more (DURATION) at intenisty without over doing it.

A recent 2024 study from the Norwegian University of Science and Technology (NTNU) took a hard look at this. Let’s break down what they found and how it applies to triathletes and endurance athletes chasing consistency and adaptation.

The Study

Kjøsen Talsnes R. et al., Frontiers in Physiology (2024). The participants were 14 national-level male endurance athletes (cross-country skiers and runners)

• Age: ~23 years

• VO₂max: ~70 ml/kg/min

  
  

Each athlete completed two different training days, matched for both total volume and intensity:

Single Session Day (SINGLE)

• 20-min warm-up

• 6 × 10-min intervals @ 90 % of the speed associated with 4 mmol/L blood lactate (high Zone 3 / low Zone 4)

• 2-min recoveries between reps

• 15-min cool-down

Double Threshold Day (DOUBLE)

Performed twice in one day morning and afternoon, separated by ~6.5 hours:

• 20-min warm-up

• 3 × 10-min intervals @ 90 % of speed at 4 mmol/L

• 2-min recoveries

• 15-min cool-down

Both options delivered the same total work. The only variable was distribution.

Physiological Stress Was Lower in the Double Day

During the workouts, the longer single session caused a greater drift in physiological load heart rate, ventilation, blood lactate, and perceived exertion all climbed higher in the second half.

• Heart Rate: +4.2 % higher

• Ventilation: +4.9 % higher

• Blood Lactate: +0.9 mmol/L higher

• Perceived Effort (RPE 1–10): +1.0 point higher

This means that, even though the total work was identical, the longer continuous session created more metabolic and cardiovascular strain.

Recovery Was Faster After the Double Day

The next morning, athletes reported significantly lower fatigue and soreness after performing the two shorter sessions.

• Perceived Fatigue: +1.0 point higher after SINGLE

• Muscle Soreness: +1.0 point higher after SINGLE

In Simple Terms

• Two shorter sessions created less acute physiological stress.

• The following day, athletes felt less fatigued and sore.

• Aerobic adaptations (VO₂max, mitochondrial density) were similar between the two approaches.

Essentially same gains, less strain.

Practical Takeaways for Triathletes

1. Splitting threshold work can increase quality.

   Two controlled sessions allow you to maintain a tighter lactate range without cardiac drift. That means cleaner Zone 3 to 4 work more minutes at the right physiological target.

2. It’s easier to recover from.

   Because each bout produces less metabolic stress, you can often stack more total high-intensity minutes across the week without digging yourself into a hole.

3. Single long threshold sessions still matter.

   They develop durability the ability to sustain power or pace as fatigue accumulates. For Ironman or marathon prep, this is a critical adaptation you don’t get from splitting sessions.

If we zoom out to a 7-day training microcycle:

Double Threshold Approach:

Use in controlled training blocks when you’re fresh, focusing on threshold development, aerobic power, and improving efficiency at sub-max efforts. Great in the build or pre-competition phase where the goal is to sharpen the engine without overreaching.

Single Long Session:

Use when you’re targeting race-specific durability ex, Ironman builds, long rides or tempo runs where the last hour simulates late-race fatigue. Perfect in specific prep or long-course phases where you want to condition the mind and body to handle sustained stress.

My Insight/Opinion

Double-threshold days aren’t magic they’re simply just a tool for distributing load. But it's safe to say, this isn't for everyone, I think the benefits from double threshold training could be very useful for matured/durable athletes in the sport. I think where potential problems with this is when new athletes rush themselves into training like (because the pros do it) which could raise the risk of injury. You need to know your body and have good intensity control to handle training days like this. But it's worth noting as age group athletes, this could be a useful tool to utilize when time crunched and recovery often is potentially limited.

For long-course athletes, the key is balance:

• Use doubles strategically to build more quality at threshold.

• Use long continuous sessions to train fatigue resistance.

• Layer both within a structured block where recovery is respected.

At the end of the day, it’s not about copying what the Norwegians do it’s about understanding why it works, and adapting it to your physiology, life schedule, and race demands.

References

Kjøsen Talsnes R, Torvik PØ, Skovereng K, Sandbakk Ø. (2024). Comparison of acute physiological responses between one long and two short sessions of moderate-intensity training in endurance athletes. Frontiers in Physiology, 15:1428536.