VO2 max intervals outside: hill or flat road, and how to ride 5x4 with no hills nearby

Uphill or flat: what the research says

Gradient changes the effort even when the average watts match. Four- and 10-minute power were not significantly different on a 9.8% climb and a 0.6% road, but 1-minute power and the work available above critical power were higher uphill [Hovorka et al. 2022]. Riders also train toward the terrain they train on [Nimmerichter et al. 2012].

The Hovorka study is the closest answer to the hill-or-flat question for VO2 work. Thirteen well-trained endurance athletes rode 10-, 4- and 1-minute trials on both a flat road and a steep climb. The 4-minute trial is the one that matters for a 5x4 session, and the group average did not change with the grade. The limits of agreement were wide, though, so an individual rider could be noticeably stronger on one terrain than the other. The authors' conclusion is plain: flat and uphill cycling are not equivalent [Hovorka et al. 2022].

Specificity is the second finding. Eighteen cyclists did six 5-minute reps twice a week for four weeks. The low-cadence uphill group raised uphill time-trial power 4.4% and flat power 1.5%; the high-cadence flat group raised flat power 2.6% and lost 1.3% uphill. Across the study, riders produced 4.4 to 6.3% more power uphill than on the flat [Nimmerichter et al. 2012]. Those reps were at threshold, not VO2 max, so read this as evidence that terrain shapes the adaptation, not as a VO2 result.

The cost of a watt also shifts. In 13 trained cyclists riding 6 minutes at the same controlled power, gross efficiency was 20.0% on a 1.1% road and 18.7% on a 5.1% climb [Nimmerichter et al. 2015]. The same number is slightly different work on the hill. So a rider who races on climbs should do some VO2 reps on climbs, and a rider who races flat crits or time trials should not do all of them uphill.

Why a hill makes VO2 reps easier to run

A hill gives resistance that holds 106 to 120% of FTP without a sprint to get there, keeps the speed near 15 to 20 km/h instead of near 40, and needs about half the distance per rep. A flat VO2 rep at the same watts covers roughly 2.5 km in 4 minutes, and the position the rider holds changes the power.

Reaching the target is the first problem on the flat. A rider at 300 watts on a calm flat road is moving at roughly 38 to 40 km/h, and getting there takes a hard acceleration that overshoots the first 20 seconds. On a 6% grade, the same 300 watts arrives at about 17 to 19 km/h with almost no surge. The higher 1-minute power uphill in the Hovorka trials fits what most riders feel: the grade makes it easier to push hard early [Hovorka et al. 2022].

Distance is the second problem. Four minutes at 18 km/h is 1.2 km; at 38 km/h it is 2.5 km. Five reps on the flat need a 2.5 km stretch with no lights, no stop signs and a safe turnaround, ridden five times. That stretch is much harder to find than a 1.2 km climb. Speed is also a safety matter, and this part is reasoning rather than a study result: a rider at VO2 max effort has less attention to spare, and a junction or a car door at 40 km/h leaves half the stopping time that it leaves at 20 km/h.

Position is the third. A lower torso cuts drag but limits power output; in 19 time-trial cyclists, the aerodynamic gain only outweighed the power loss above 46 km/h, and below 30 km/h a more upright position was better [Fintelman et al. 2014]. The model used time-trial bikes, so apply it loosely. The practical point holds: low on the drops on the flat, the watts can fall while the speed rises. Ride flat VO2 reps on the hoods or in the position you race in, and judge the rep by power, not speed.

How to ride 5x4 on a short hill, rolling roads or the flat

Choose the format before the road: 4x4 to 5x4, 4x8, or 30-second reps with 15 seconds easy all have trial support [Seiler et al. 2013, Rønnestad et al. 2015]. Then fit it. On a short hill, start at the bottom and carry the rep over the top. On rolling roads, start each rep at the base of a rise. On the flat, use a headwind leg.

The formats come first because the road should serve them. In 35 cyclists on about six hours a week, 4x8 minutes at 90% of peak heart rate improved VO2peak and power 11.4%, against 4 to 6% for 4x4 and 4x16 [Seiler et al. 2013]. Short intervals of 30 seconds hard and 15 seconds easy raised VO2max 8.7% against 2.6% for effort-matched 4x5 minutes [Rønnestad et al. 2015]. A 30-second rep at 18 km/h covers 150 m, so the short format fits a very small hill that a 4-minute rep never could.

A short hill can still carry a 4-minute rep. Start at the bottom of a 1- to 2-minute climb, reach the target range on the grade, then crest and keep the power on the flat in a bigger gear and a higher cadence. Watch the power for 30 seconds after the crest; most riders ease off there. Turn around or loop back during the recovery, and start the next rep at the same point. How long a hill each rep length needs is covered in the sibling guide on hill repeats.

Rolling roads work if the rep starts at the base of a rise, so the first and hardest minute goes uphill. Keep pedalling over each crest and down the next dip; the power will sag, and that is acceptable if the rep stays inside a range such as 106 to 120% of FTP rather than one number. Do not shorten reps to match the risers: ten 2-minute efforts are a different session. If the risers are short, switch the day to 30/15 blocks instead.

On a flat road, ride each rep into the wind. A headwind adds resistance, lowers the speed and shortens the distance each rep covers, which brings back some of what a hill gives. Use the big ring at 90 to 100 rpm, stay seated, and recover on the tailwind leg back. The best road for intervals, with no lights and a turnaround, is the subject of its own guide in this cluster. Finding that road is the other half of doing intervals outside: the routes and hills have to fit the workout, not the other way round. Gritmaps, a free beta, lines up climbs and headwinds with effort blocks. AdaptCycling's route suggestions (paid) pick a loop sized to today's planned time and check for 3 to 8% climbs of 200 m or more near the start; they pick a loop where the work fits, not where each rep starts.

When to take VO2 intervals inside

Move the session indoors when the only flat stretch has a junction every 500 m, the roads are wet or dark, or the format is 4x8 and no road holds 3 km without a stop. On a trainer in ERG mode, the watts stay in range for the whole rep, which no hill or wind leg can promise.

The trainer solves exactly the problems this page is about. It needs no distance, no grade and no turnaround, and the first 20 seconds do not overshoot because ERG mode sets the resistance for the target. The 30/15 format is the clearest case: 39 short reps per session on an open road is a lot of starts and stops, and on a trainer it is just a file. The indoor cycling training guide covers the setup, and the heat and fan problem that comes with it.

The cost is specificity. Indoor reps teach the body to push at a fixed cadence and a fixed resistance on a flat-feeling road, which is the opposite of the climbing work the Nimmerichter data favour for a climber [Nimmerichter et al. 2012]. A reasonable split for a rider with a choice is one VO2 session outside on a climb when the weather allows, and indoors otherwise. A Wahoo ELEMNT or Hammerhead Karoo can drive a smart trainer in ERG with no app; Zwift is the best full indoor experience; AdaptCycling's Ride Live (beta) runs the planned session in Chrome or Edge on a laptop or desktop.

Quick answers

Sources cited in this guide

1. 01
   Hovorka et al. 2022. Effects of Flat and Uphill Cycling on the Power-duration Relationship. International Journal of Sports Medicine.
2. 02
   Nimmerichter et al. 2012. Effects of low and high cadence interval training on power output in flat and uphill cycling time-trials. European Journal of Applied Physiology.
3. 03
   Nimmerichter et al. 2015. Gross efficiency during flat and uphill cycling in field conditions. International Journal of Sports Physiology and Performance.
4. 04
   Fintelman et al. 2014. Optimal cycling time trial position models: aerodynamics versus power output and metabolic energy. Journal of Biomechanics.
5. 05
   Seiler et al. 2013. Adaptations to aerobic interval training: interactive effects of exercise intensity and total work duration. Scandinavian Journal of Medicine & Science in Sports.
6. 06
   Rønnestad et al. 2015. Short intervals induce superior training adaptations compared with long intervals in cyclists — an effort-matched approach. Scandinavian Journal of Medicine & Science in Sports.

More inside How to do intervals outside

Other articles in this series

1. 01

   How long should a hill be for hill repeats?

   Rep length sets the hill. A table from 30-second to 10-minute repeats: how long, how steep, and which workout each hill serves.

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2. 02

   The best road for intervals: what to look for

   Long enough for the rep, no lights, a safe turnaround. How to find a stretch of road that lets an interval session run as written.

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3. 03

   Threshold intervals without hills: rolling and flat roads

   No long climb nearby? How to hold threshold and sweet spot on flat or rolling roads: pace by range, use the wind, fit reps to the road.

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4. 04

   Zone 2 outside: how to plan a ride without stops

   Lights and junctions break an easy ride into pieces. How to plan a Zone 2 loop that stays steady, and what to do when you cannot.

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