The Science of suffering: 4X4 vs 30:30 vs 15:15 intervals
What actually works for VO2 max in the hills?
There is a common myth in trail and fell running, “VO2 max is for road runners”.
It isn’t!! If anything, VO2 max, your maximal aerobic power matters more when running off road, because racing off road isn’t steady, it’s repeated climbs, surges over crests of hills, down hill ‘races’, responding to someone’s attack to overtake. It’s digging in late race when the legs are full of lactate but the hill doesn’t care.
VO2 max is your aerobic ceiling, raise that and everything underneath becomes more sustainable.
The question isn’t whether to train it, the question is how!
I’m going to break down three of the most effective protocols in the literature:
• Norwegian 4×4
• Billat 30/30
• 15:15 (Buchheit / HIIT Science short intervals)
And more importantly, when would we use each in real, world trail and fell running.
Norwegian 4×4 (Helgerud Protocol)
What It Is?
4 minutes hard (RPE 7 to 8)
3 minutes active recovery
Repeat x4
Target intensity: 90–95% HR max during work.
Where did it come from?
The protocol became well known after Helgerud et al. (2007) compared four 8 week endurance programs in runners:
• 70% HRmax long slow distance
• 85% HRmax “threshold”
• 15/15 intervals
• 4×4 at 90–95% HRmax
The 4×4 group improved VO₂max by 7.2% significantly more than threshold or LSD (Helgerud et al., 2007, Med Sci Sports Exerc).
How It Works Physiologically?
The key adaptation appears largely central:
• Increased stroke volume
• Increased cardiac output
• Improved oxygen delivery
Helgerud’s data showed VO₂max changes strongly linked to stroke volume increases, suggesting improved heart pumping capacity.
You are essentially working the heart at near maximal aerobic demand for repeated controlled intervals.
Pros
• Strong evidence base
• Reliable VO₂max improvement
• Repeatable structure
• Easy to control intensity
Limitations
• Mentally demanding
• Can be mechanically stressful if done too fast downhill
• Less “speed change” stimulus than shorter reps
Seiler et al. (2013) showed that longer intervals (for example 4×8) may produce even greater improvements in some contexts, reminding us 4×4 is powerful, but not sacred.
Who It Suits?
• Experienced runners
• Athletes early in a VO₂max block
• Trail runners preparing for sustained climbs, on long trails and fells.
• Masters athletes who tolerate longer efforts better than aggressive short bursts so less risk of injury.
Trail Application
Use a steady 5–8% climb. A road would be better if possible. Treadmill can be utilised.
Rep 1: HR won’t hit 90% immediately, but that is fine.
Reps 2–4: you should accumulate significant time ≥90% HRmax.
Billat 30/30
What It Is?
30 seconds hard (RPE 8)
30 seconds easy or static
Typically 10 to 20 repetitions
Run at approximately vVO₂max, the minimal speed that elicits VO₂max.
Where It Came From?
Developed by Véronique Billat in the late 1990s, early 2000s.
Billat et al. (2000) showed that intermittent intervals at vVO₂max allowed runners to accumulate more time at VO₂max than continuous running at the same speed.
How It Works Physiologically?
Short recoveries prevent VO₂ from dropping fully.
You don’t reset, and your heart rate remains high.
This allows accumulation of time near VO₂max without the premature exhaustion of a continuous effort.
However, and this is important, later research shows 30 second intervals do not automatically produce more time >90% VO₂max than longer intervals if intensity is poorly managed. Execution matters.
Short intervals often feel brutal, and Lactate can spike, but high lactate does not equal optimal VO₂ stimulus.
Recent comparisons suggest 30 second intervals can sometimes produce less time above 90% VO₂max than 2–4 minute intervals when poorly paced.
Pros
• Develops change of pace ability
• Mentally engaging
• Lower mental monotony than 4×4
• Good race specific surging stimulus
Limitations
• Easy to turn into sprint training
• Intensity drift common
• VO₂ dose highly execution dependent
Who It Suits?
• Fell and trail runners dealing with rolling terrain
• Athletes needing to be able to gear change in race conditions.
• Runners who struggle mentally with longer reps
Trail Application
Perfect for
• Undulating trails
• Rolling hilly roads
• Repeated 30 second steep banks
You need to keep “hard” controlled.
15:15 Protocol (Buchheit / HIIT Science)
What It Is?
15 seconds hard (RPE 8+)
15 seconds easy
Typically 2 sets of 15 repetitions
Where It Came From?
Popularised through work by Martin Buchheit and the Science and application of HIIT.
Research on short intermittent intervals shows they can produce significant time >90% VO₂max when intensity is properly calibrated (Buchheit & Laursen, 2013).
How It Works Physiologically?
Shorter reps = faster VO₂ kinetics accumulation. Because recoveries are so short, VO₂ doesn’t drop much therefore you build a “VO₂ plateau” across the set.
There is a less dramatic lactate accumulation than 30/30 if controlled properly.
Pros
• Lower psychological barrier
• Good for masters athletes
• Useful in-season and start of season
• Lower eccentric load than longer reps
Limitations
• Hard to pace without discipline
• Less research in pure running populations compared to cycling
• Requires tight intensity control
Who It Suits?
• Masters athletes (40+)
• Athletes balancing heavy strength training
• In season fell and trail runners
• Return to intensity blocks
Trail Application
Ideal on
• Moderate incline
• Firm terrain
When To Use Each in a Training Cycle
This is how a simple progression for trail/fell runners may look like:
Base phase
15:15 intervals
This will introduce VO2 max training and start improving leg turn over.
Build phase
30:30 intervals
This will increase time at VO2 and also start a conditioning phase.
Peak phase
4x4 intervals
Maximising your climbing engine and maintaining speed for a longer timed duration.
This progression gradually increases metabolic stress while maintaining durability.
Practical Application for Trail Runners
Terrain matters. These sessions should mimic the demands of the hills that YOU are running on.
Treadmill gradients (6–10%) work brilliantly for controlled sessions and if you live in flatter areas.
Outdoor preference’s:
• steady hill climbs
• long fell paths
• rolling trails
Key cues:
• Run by effort not pace! This is important.
• Maintain upright posture and strong arm drive
• Focus on controlled breathing!
Remember:
The goal is oxygen demand, not exhaustion. You shouldn’t come away with DOM’s or unable to train the next day.
Final Thought
Firstly every person will require different protocols depending on training age, experience and events that they are training for.
Also every coach will have a different approach to training their individual athletes.
The truth is simple. Getting stronger in the hills requires moments of discomfort.
Structured suffering.
But when applied intelligently, these sessions create something powerful, a bigger engine for the mountains, and when the race climbs begin, that engine makes all the difference.