Aerobic base training doesn’t have to mean 8 to 12 weeks of long, easy sessions. I use 2 to 4 weeks of easy, structured training to bring athletes back after their off-season break. After that, their winter is shaped by what’s actually holding them back. For most age-groupers that’s VO2max, and short, sharp work develops it far more efficiently than easy volume alone.
What is traditional aerobic base training?
Traditional aerobic base training is a long block, usually 8 to 12 weeks or more, where nearly all sessions sit below the aerobic threshold. That’s the intensity where blood lactate first starts to rise, often called Zones 1 or 2. The aim is to build mitochondria, capillaries and fat-burning capacity before adding intensity in spring.
Those adaptations are real, proven and easy training will always be a big part of an endurance athlete’s week. The problem is the assumption that everyone needs the same long easy block before any intensity, whatever their limiter. If you train 15 or more hours a week, easy volume can do much of the work. On 6 to 10 hours, it’s a slow route to your biggest gains.
How long should base training be for a triathlete?
For most experienced age-groupers, 2 to 4 weeks of easy, structured training is enough to return from an off-season break. It rebuilds routine, frequency and tissue tolerance. After that, base training should target the athlete’s specific limiter rather than simply adding more easy hours for the sake of it or just because that's how it's always been done.
In those first weeks I’m after consistency, not fitness records: short, frequent, easy sessions in all three sports, two strength sessions a week (see my guide to off-season strength training for triathletes) and lots of swim technique. A small dose of short sprints helps even here. In a 2020 study of elite cyclists in their transition period, riders who added 3 × 3 × 30-second sprints to one easy weekly ride improved their sprint performance. They also held their 20-minute power, which fell in the group that rode easy only. It’s a small study, but the message fits what I see: you don’t need to strip out intensity completely to recover.
The exception is athletes who are new to the sport. If you’ve been training for less than a year, a longer block of mostly easy, consistent training is still the right foundation for your winter.
What is a needs-based approach to base training?
A needs-based approach starts by finding out what limits your performance, then builds winter training around improving it. That limiter might be VO2max, your aerobic threshold, fatigue resistance or swim technique. Testing, or a careful look at your training data, tells you which one it is.
When I test athletes in autumn, the most common limiter by far is VO2max: the ceiling on how much oxygen they can use. Let's be honest here - the bigger the oxidative capacity of an athlete the better and, as VO2 takes longer to improve than VLamax or percentage of VO2 utilisation (where threshold sists), for example, it almost always makes sense to work on that in the early months of training. Easy volume nudges VO2max up slowly. Well-designed high-intensity work moves it much faster.
The research supports this. In Stöggl and Sperlich’s 2014 study of 48 well-trained runners, cyclists, triathletes and skiers, the group that did only high-volume, low-intensity training made the smallest gains in VO2peak. A polarised plan with two high-intensity sessions a week made the largest gains, at +11.7% over nine weeks.
Testing makes this precise. A lactate test or INSCYD metabolic profile shows your VO2max, your aerobic and anaerobic thresholds and how they relate to each other. I offer both in person in the Netherlands and Belgium. If you’re setting zones yourself, start with my free training zones guide.
How do you improve VO2max in the off-season?
The most time-efficient way to raise VO2max is short, repeated high-intensity efforts. Examples are microbursts such as 30 seconds hard with 15 seconds easy, and brief surges added to longer easy sessions. Both keep you at a high percentage of VO2max for longer than steady efforts do, for a similar perceived effort.
I use two main tools:
- Microbursts (short-interval HIT). The best-known format comes from Bent Rønnestad’s research group: 3 sets of 13 × 30 seconds hard / 15 seconds easy, with 3 minutes between sets. In their 2015 study, cyclists doing this twice a week for 10 weeks improved VO2max by 8.7%, compared with 2.6% for an effort-matched 4 × 5-minute interval group. A 2020 follow-up in elite cyclists found bigger gains in peak aerobic power and 20-minute power after just three weeks.
- 10-second surges inside longer sessions. On a 90 to 120-minute ride, I add 10 seconds at around 110% of threshold power every 10 minutes or so. On long runs it’s 10-second pickups at faster than threshold pace, or simply 10 seconds at higher cadence. The result is that the athlete spends more time at a high percentage of VO2max without any additional stress or effort.
That second tool also trains durability, which is how well you hold your power or pace late in a long session (Maunder et al., 2021). It matters hugely in 70.3 and Ironman racing. Easy hours aren’t the only way to build it: a 2023 study in Frontiers in Physiology found durability improved after both low- and high-intensity training.
Why should winter be your swim technique season?
For most age-group triathletes, swim speed is limited more by technique than by fitness. Winter, with no races and indoor pools, is the best time to change technique, in my experience. Short, frequent sessions with plenty of drills and focused feedback pay off more now than extra metres at steady pace. Carrying less fatigue and soreness from bike and run also means that athletes can focus more on good body position and stroke mechanics.
This is one of the most consistent patterns I see. Athletes who spend winter grinding out longer, harder swim sets usually arrive in spring at the same pace. Athletes who spend it on body position, catch and breathing usually arrive faster, and fresher too. In practice that means two or three sessions a week of 30 to 45 minutes, heavy on drills, with a filmed stroke review if you can get one.
What does a needs-based winter week look like?
On about 8 hours a week, a sample VO2max-focused winter week might have one microburst session, longer easy sessions with short surges, two technique-led swims and two short strength sessions. Most training time is still easy, but every session has a clear purpose.
| Day | Session |
|---|---|
| Mon | Strength, 30 min |
| Tue | Bike microbursts: 60 min including 3 × 13 × 30s/15s |
| Wed | Swim technique, 45 min · Easy run, 40 min |
| Thu | Run, 50 min including 10 × 30s fast / 30s easy |
| Fri | Swim technique, 45 min · Strength, 30 min |
| Sat | Long ride, 2 h, with 10-second surges at ~110% threshold every few minutes in the middle hour |
| Sun | Long run, 65 min, with 10-second pickups |
The exact mix changes with the athlete. Someone whose testing shows a low aerobic threshold relative to their VO2max would get more steady work and less HIT. That’s the point of a needs-based plan.
FAQ
Is long slow distance base training outdated?
Not outdated, but often not the best fit. Long easy training builds real adaptations and suits athletes who are new to the sport or have plenty of time. For experienced age-groupers on limited hours, a short re-entry block followed by targeted work usually gives a better return.
Can I do high-intensity training in the off-season?
Yes. After a short off-season break and 2 to 4 weeks of easy re-entry training, short-interval work is one of the most effective ways to raise VO2max. Keep it to one or two sessions a week, and keep the rest of your training easy.
What are microbursts in cycling?
Microbursts are very short, repeated hard efforts with short recoveries, such as 30 seconds on and 15 seconds off. Research by Rønnestad and colleagues found this format improved VO2max and performance more than effort-matched longer intervals in trained cyclists.
Who should still do a long, traditional base phase?
Athletes new to endurance sport, those returning from injury, and athletes whose testing shows aerobic threshold or fatigue resistance as their main limiter. For them, more consistent easy volume is exactly the right need.
Want a winter plan built around your limiter?
Most winter plans give everyone the same 12 weeks of easy base. If you’d rather know what’s actually holding you back and train that, book a free intro call. You can also read more about how my online triathlon coaching works. Testing is available in person in the Netherlands and Belgium.
Matt Warnock is a British Triathlon-qualified coach (also ESCI, Ironman University and IUSCA certified). Based in the Netherlands, he coaches triathletes, runners, cyclists and swimmers online across the UK, Ireland, the Netherlands and Belgium, and is the official training partner of ATAC.
