Can the Turbo Replace the Long Ride? Indoor Volume and Winter Durability

Every winter, the long Sunday ride becomes a negotiation. It's dark by late afternoon, the roads are wet, and the trainer in the spare room is warm and safe. But few triathletes can face three or four hours on a turbo, and many worry that swapping long rides for shorter indoor sessions will leave them fading late in next season's races.

This article looks at what durability research says about that trade-off. It builds on TriForward's "How Long Does the Long Ride Actually Need to Be?" and "Fresh Watts vs. Hour-Four Watts", and focuses on the winter question: can a shorter, denser indoor week stand in for the long outdoor ride? There's no direct trial in triathletes, so much of what follows is labelled inference.

What durability means

Durability is your ability to resist the decline in physiological characteristics that happens during prolonged exercise. A 2021 paper by Maunder, Seiler, Mildenhall, Kilding and Plews in Sports Medicine defined it as the time of onset and the magnitude of deterioration in physiological profiling characteristics, such as economy or threshold power, over a long effort.

In practice, two riders can have the same fresh threshold power, and one can hold far more of it after three hours. For a half or full Ironman, that later power matters more than the fresh number. TriForward's "Durability, Resilience, Fatigability" covers the terminology in more detail.

How long before fatigue shows?

Power at the first threshold falls after two hours

A 2022 study by Stevenson, Kilding, Plews and Maunder in the European Journal of Applied Physiology tested 14 endurance-trained cyclists and triathletes (13 men, 1 woman). They did an incremental test before and after two hours of cycling at 90% of their first ventilatory threshold (VT1), which is a steady, moderate effort.

• Power at VT1 fell from 217 W to 196 W.
• Power at the lactate-based threshold fell from 212 W to 190 W.
• Heart rate at VT1 rose from 142 to 151 beats per minute.

Inference: that's about a 10% drop in the power at which easy riding becomes moderate, after just two hours of riding below it. The authors attributed it to reduced efficiency and changes in energy use.

Hard-effort capacity holds for about an hour, then slips

A 2019 study by Clark and colleagues in the Journal of Applied Physiology looked at the same question from the top end. Sixteen participants (sex not stated in the abstract) did a 3-minute all-out cycling test fresh, and immediately after 40 minutes, 80 minutes and 2 hours of heavy-intensity cycling.

• End-test power, a marker of critical power, didn't change after 40 or 80 minutes: 260 W fresh, 268 W and 260 W.
• After 2 hours it was 9% lower, at 236 W, with a placebo drink.
• With 60 g of carbohydrate an hour it was preserved, at 254 W.
• Work capacity above that power fell earlier, from 17.9 kJ fresh to 14.7 kJ after 80 minutes and 13.8 kJ after 2 hours, and carbohydrate didn't protect it.

Together these studies suggest that, for trained riders, the kind of fatigue durability describes mostly appears after about two hours. Inference: a typical 60 to 75-minute turbo session ends before that point, so it trains fresh fitness but rarely exposes you to late-ride fatigue.

Does it matter how you train?

The closest thing to a direct trial

The most relevant study is a 2023 trial by Matomäki, Heinonen, Nummela and colleagues in Frontiers in Physiology. It compared two very different ways of training, one of which happened to be indoors.

• Participants: 35 sedentary or recreationally active adults aged 18 to 40, 16 men and 19 women, all doing less than six hours of endurance exercise a week before the study.
• Low-intensity group: 6.8 hours a week of cycling below the first lactate threshold, mostly outdoors on their own bikes, including long sessions of 1.5 to 4 hours.
• High-intensity group: 1.6 hours a week of interval sessions on indoor Wattbike trainers, two or three times a week, with 15 to 30 minutes of hard work per session in intervals of 3 to 7 minutes.
• Durability test: before and after 10 weeks, a 3-hour ride at about 48% of VO2max, measuring how much heart rate, ventilation, energy use, perceived effort and other markers drifted.

The results:

• Both groups improved durability overall: Hedges' g 0.49 for the low-intensity group and 0.62 for the interval group.
• There was no significant difference between the groups (p = 0.42).
• In the interval group, drift fell from 8.8% to 5.4% and started later, at 137 minutes instead of 108. The low-intensity group's changes in drift (7.7% to 6.3%, 106 to 131 minutes) weren't statistically significant.
• Only the interval group increased VO2max.

So 1.6 hours a week of indoor intervals improved durability about as much as more than four times the training time of mostly outdoor easy riding. The authors concluded that both approaches improve durability, but that ten weeks didn't change drift very much in either group.

Why might short intervals work? The authors didn't test mechanisms, but the interval group also improved VO2max, which means a fixed easy pace becomes a smaller fraction of maximum. Inference: if the same ride takes a smaller share of your capacity, it should cause less drift, whichever way you raised that capacity. It's also worth noting that the low-intensity group did improve; its changes in drift on their own just weren't large enough to be statistically clear in a group of 16.

The catch is the population. These were sedentary or recreationally active adults, for whom almost any training helps. Their durability test was a 3-hour ride at a low intensity, not a race effort. Inference: in trained triathletes who already have a big aerobic base, the extra gain from short intervals is likely to be smaller.

What professionals' training logs show

A 2023 study by Spragg, Leo and Swart in the European Journal of Sport Science looked at 30 under-23 professional cyclists across a season (sex not stated in the abstract). They compared the best 2, 5 and 12-minute power each rider produced fresh, before 2,000 kJ of work, with the best they produced after 2,000 kJ.

• Fatigued power wasn't fixed. Fatigued 5 and 12-minute power was lower late in the season than early or mid-season.
• More time below VT1 was linked to better fatigued 2-minute power (r = 0.43).
• A shift towards a more polarised intensity distribution was linked to better fatigued 12-minute power (r = 0.414).

These are correlations in young professionals riding far more than any age-grouper, not a trial. But they suggest that easy volume and the overall intensity mix both relate to durability. TriForward's "Polarised, Pyramidal or Threshold: How to Spread Your Winter Intensity" covers that mix in more detail.

What indoor riding can't replicate

Even if shorter indoor sessions build fitness efficiently, some things about long outdoor rides are hard to copy:

• Hours in the fatigued state. Based on Stevenson and Clark, the fatigue that matters for long races starts to show after around two hours. A one-hour session doesn't get there.
• Fuelling practice. Clark found that carbohydrate preserved power late in a long ride. Long rides are where you practise eating, as TriForward's "Feeding the Fade" explains. A short turbo session rarely needs much fuel; see "Do You Need Carbs on a 60-Minute Turbo Session?".
• Variable power. A 2019 study by Jeffries and colleagues in 20 male competitive cyclists found that power during 20-minute time trials was about twice as variable outdoors (within-person variation 69 W) as indoors (33 W). Riding a hilly course is different from holding a steady number.
• Position and contact points. Hours in the aero position, on the saddle, with your hands and neck in place, are their own kind of training.

TriForward's "Why Your Indoor Watts Are Lower: Heat, Fans and the Smart Trainer" explains why indoor numbers can also differ from outdoor ones.

Planning a winter around the trainer

The following combines the evidence above. It's inference for triathletes, not a tested plan:

1. Use indoor sessions for what they do well. Structured intervals are efficient on a trainer, and the Matomäki trial suggests they can improve durability as well as fitness, at least in less-trained riders.
2. Keep some easy volume. In professionals, more time below VT1 was linked to better fatigued power. Easy indoor rides of 60 to 90 minutes, or easy outdoor rides when the weather allows, still count.
3. Include an occasional longer ride. Every two or three weeks, a ride of around two hours, indoors with good cooling or outdoors, keeps some exposure to the fatigue that long races demand.
4. Practise fuelling on longer rides, not short ones.
5. Build towards race-length rides in spring. As the race gets closer, long outdoor rides matter more, because they're the most specific preparation. TriForward's "How Long Does the Long Ride Actually Need to Be?" covers how long.
6. Don't replace all easy riding with intervals. More hard sessions mean more recovery cost, and the trial's interval group did only two or three short sessions a week.

A triathlon scenario

This example is illustrative, not drawn from a study. A 45-year-old triathlete is training for a half-distance race in June. Through the summer he rode three to four hours every Sunday. From November to February, he has two weekday indoor slots of an hour and a weekend slot of 90 minutes to two hours.

A reasonable winter plan, based on the evidence above:

• Weekday 1: an indoor interval session of about an hour, with 20 to 30 minutes of hard work.
• Weekday 2: an easy indoor ride of about an hour, below his first threshold.
• Weekend: most weeks, 90 minutes indoors or outdoors with some steady tempo. Every second or third week, a two-hour ride, practising fuelling.
• From March: extend the weekend ride outdoors by about 30 minutes every couple of weeks, aiming for two or three rides of three hours or more before race day.

He loses some long-ride hours over winter, but the evidence suggests he's unlikely to lose much durability if the indoor work is consistent, and the spring build restores the specific long-ride exposure.

Common mistakes

• Treating every indoor session as a hard session. Easy volume still matters.
• Stopping long rides completely for four months, then jumping straight to a five-hour ride in March.
• Riding long indoors in a hot room without a fan, then blaming low numbers on lost fitness.
• Skipping fuelling practice because every session is short.
• Copying a professional's winter volume. Durability gains seem possible with much less, at least in less-trained riders.
• Testing durability with fresh tests. A ramp or 20-minute test tells you nothing about hour four.

Checking whether your durability is holding

If you're worried that a winter of shorter rides is costing you, you can check rather than guess. TriForward's "Your Durability Test Is Mostly Noise" explains how to make a durability test meaningful, but a simple version is to compare a short effort ridden fresh with the same effort ridden late in a longer session.

• Pick one effort, such as 5 or 12 minutes, the durations Spragg's study used.
• Ride it fresh on one day and at the end of a two-hour ride on another, in similar conditions and with similar fuelling.
• Compare the gap, not the absolute numbers. A shrinking gap over a few months suggests durability is holding or improving.
• Watch heart rate at a fixed easy power during long rides. Stevenson found heart rate at the first threshold rose about 9 beats per minute after two hours, so a smaller rise on a familiar ride is an encouraging sign.

Inference: indoor and outdoor tests can differ, so compare indoor with indoor and outdoor with outdoor. A single test is noisy; look for a trend across two or three.

How this fits with the rest of the week

Swimming and running also add to your aerobic base and to your overall fatigue, which the bike-only research doesn't account for. TriForward's "Two Sessions a Week" explains how strength training may also support durability, and "Lifting and Endurance in the Same Week" covers how to fit it alongside bike sessions. For athletes with very little time, TriForward's "Raising FTP on 8 Hours a Week" looks at which intervals give the most return.

What we don't know

• Trained triathletes. No trial has compared indoor intervals with long outdoor rides for durability in trained athletes.
• Race-relevant durability. The main trial measured drift during a low-intensity 3-hour ride, not power late in a race.
• The minimum long-ride dose. How many rides over two hours a winter needs isn't known.
• Women. The main trial included 19 women, but the fatigue studies were almost all in men.
• Running. Whether bike-only durability training carries over to the run off the bike is untested.

References

• Maunder E, Seiler S, Mildenhall MJ, Kilding AE, Plews DJ. The importance of 'durability' in the physiological profiling of endurance athletes. Sports Medicine, 2021;51(8):1619–1628. doi:10.1007/s40279-021-01459-0
• Stevenson JD, Kilding AE, Plews DJ, Maunder E. Prolonged cycling reduces power output at the moderate-to-heavy intensity transition. European Journal of Applied Physiology, 2022;122(12):2673–2682. doi:10.1007/s00421-022-05036-9
• Clark IE, Vanhatalo A, Thompson C, Joseph C, Black MI, Blackwell JR, et al. Dynamics of the power-duration relationship during prolonged endurance exercise and influence of carbohydrate ingestion. Journal of Applied Physiology, 2019;127(3):726–736. doi:10.1152/japplphysiol.00207.2019
• Matomäki P, Heinonen OJ, Nummela A, Laukkanen J, Auvinen EP, Pirkola L, Kyröläinen H. Durability is improved by both low and high intensity endurance training. Frontiers in Physiology, 2023;14:1128111. doi:10.3389/fphys.2023.1128111
• Spragg J, Leo P, Swart J. The relationship between training characteristics and durability in professional cyclists across a competitive season. European Journal of Sport Science, 2023;23(4):489–498. doi:10.1080/17461391.2022.2049886
• Jeffries O, Waldron M, Patterson SD, Galna B. An analysis of variability in power output during indoor and outdoor cycling time trials. International Journal of Sports Physiology and Performance, 2019;14(9):1273–1279. doi:10.1123/ijspp.2018-0539