Sweet spot vs zone 2 for the indoor winter: what the trials found and where each belongs

What zone 2 and sweet spot are each for

Zone 2 builds the aerobic engine that everything else runs on; sweet spot buys a threshold stimulus in less time. Professional endurance athletes oxidise far more fat at a given power and produce less lactate than moderately active people, and the two measures track each other almost perfectly [San-Millán & Brooks 2018]. That capacity is what easy volume trains.

Zone 2 is 56 to 75% of FTP, and its job is metabolic rather than mechanical. In San-Millán and Brooks' comparison of professional cyclists with moderately active and metabolically unhealthy adults, fat oxidation and blood lactate were inversely correlated in every group, at r = −0.97 in the professionals [San-Millán & Brooks 2018]. That capacity comes from hours below the first threshold, not from riding at it. It is slow to build and slow to lose, which is why winter is where it gets built.

Sweet spot is 88 to 94% of FTP: hard enough to load the threshold system, easy enough to repeat. Its case is arithmetic: a 2×20 at 90% of FTP delivers roughly the training load of 90 minutes of endurance riding in one 60-minute slot, and a rider with four hours a week cannot spend two of them on one easy ride. That is the honest use of sweet spot — a compromise for the time-limited.

The trainer tilts the choice toward sweet spot for a reason unrelated to physiology. At matched intensity, riders rate the same work as harder indoors — by 1.4 to 1.6 RPE units for breathing and 2.5 to 2.7 for the legs [Olsson et al. 2024] — and at matched perceived effort they produce 21% fewer watts on the trainer than outside [Mieras et al. 2014]. Zone 2 feels like nothing on a road and like work on a trainer, so the target creeps up to make the hour pass. That is how a zone 2 winter becomes a sweet spot winter without anyone deciding it should.

What happened when polarized met threshold on equal hours

Twelve trained cyclists rode six weeks polarized — 80% low, 20% high, nothing in the middle — and six weeks threshold-heavy, 57% low and 43% moderate, in crossover. Polarized won on peak power (8% vs 3%), lactate threshold (9% vs 2%) and high-intensity capacity (85% vs 37%), on 6.4 hours a week against the threshold model's 7.5 [Neal et al. 2013].

The threshold model improved every measure; it just improved them less, and on more hours. Muscle enzyme activity did not change in either period, so the difference was systemic [Neal et al. 2013]. The threshold group's 43% of time between the thresholds is, in practice, a sweet spot winter — three or four sessions a week at 85 to 95% of FTP with easy riding around them. It is exactly the week the trainer makes convenient.

The second trial widened the field. Stöggl and Sperlich put 48 well-trained runners, cyclists, triathletes and skiers through nine weeks of one of four distributions. The polarized group gained 11.7% in VO2peak and 17.4% in time to exhaustion; the threshold group's VO2peak did not change and its time to exhaustion rose a non-significant 6.2%; the high-volume group's VO2peak did not change either [Stöggl & Sperlich 2014]. In runners, the same pattern held over five months when the easy share was pushed from 67% to 80% with high-intensity work held constant — the low-intensity group improved a 10.4 km race by 157 seconds against 121 [Esteve-Lanao et al. 2007].

None of this says sweet spot does not work; every threshold group got fitter. What the trials say is narrower: on the same hours, easy volume plus a small dose of genuinely hard work beats a week built around the middle. The middle is the less efficient place to spend a limited hour, and on a trainer it is the place riders drift to without noticing.

The honest case for sweet spot on four hours a week

Below roughly five hours a week the polarized arithmetic gets tight, because 80% easy of four hours is three hours of zone 2 that the rider may not have in one or two sittings. One sweet spot session covers the threshold stimulus and part of the load in a single hour. The cap is what makes it honest: one, at most two, never the whole week.

The reason the cap exists is Hickson's intensity study. Twelve subjects trained 40 minutes a day, six days a week, for ten weeks, then kept the frequency and duration but cut the work rate by a third or two-thirds for 15 more weeks. VO2max was not maintained at either reduction, and long endurance fell 21% in the one-third group and 30% in the two-thirds group [Hickson et al. 1985]. A winter of nothing but sweet spot is reduced intensity with the frequency kept — the protocol that lost fitness in that trial.

So the four-hour week keeps its hard day hard and lets sweet spot replace part of the easy volume, not the intensity. A workable shape: one 60-minute session carrying the week's high-intensity work — 4×8 minutes at 90% of peak heart rate, or 30/15s — one 60-minute 2×20 at 88 to 94% of FTP, and one 90 to 120 minute endurance ride at 60 to 70%. The endurance ride is the one riders cut when the week gets short, and the one that should survive: it is the only session training the capacity San-Millán and Brooks measured.

The failure mode is a third sweet spot session because it is convenient in ERG. Two sessions at 90% of FTP plus one at 94% of peak heart rate and no easy riding is the threshold model from Neal's trial with the easy volume removed [Neal et al. 2013]. The rider gets fitter for six to eight weeks, plateaus around the new year, and reads the plateau as a need for more intensity. The week that holds through February keeps the hardest session very hard, the sweet spot session capped, and the easy ride genuinely easy.

A winter week that uses both

Mostly easy, one hard, sweet spot as the third lever that scales with how few hours there are. Four hours: hard, sweet spot, endurance. Six hours: hard, sweet spot, two endurance rides. Eight and up: two hard, one long ride, the rest easy, and sweet spot drops out because the volume now does its job.

The tiers follow the trials. Above six hours the polarized shape is affordable — 80% easy of eight hours is more than six hours of zone 2, enough to build the base without borrowing from the middle — and the second hard session adds more than a sweet spot session would [Stöggl & Sperlich 2014]. Below six, the sweet spot session is the compromise that keeps the threshold system loaded while the easy volume shrinks. At four it is doing real work; at three the rider is in maintenance and the hard session alone holds VO2max [Hickson & Rosenkoetter 1981].

Two indoor rules keep the easy rides easy. The endurance session is prescribed by power and a heart-rate ceiling, and the ceiling wins: when drift pushes heart rate out of zone 2 in minute fifty, the watts come down. And the fan runs from the first minute, because a fan switched on after drift has set in does not reverse it — the indoor cycling training guide covers the heat mechanism and the airflow numbers, and the sibling spoke on session length covers how long each of these rides should be on a trainer. Zone 2 on a trainer is a cooling problem before it is a discipline problem.

This is also the week that survives winter's interruptions best, and it is the shape we build around. When a cold takes ten days out of December, the sweet spot session is the first thing to drop on the return week and the last thing to add back; the hard session comes back first at reduced volume, the easy ride never really left. AdaptCycling reads the rides that actually happened — from a Wahoo, a Hammerhead, a Polar or intervals.icu — and rebuilds the week that way instead of scheduling the next 2×20 on the next free Tuesday. The distribution is easy to write down. Keeping it intact through an indoor winter needs a plan that reads the calendar.

Quick answers

Sources cited in this guide

1. 01
   Neal et al. 2013. Six weeks of a polarized training-intensity distribution leads to greater physiological and performance adaptations than a threshold model in trained cyclists. Journal of Applied Physiology.
2. 02
   Stöggl & Sperlich 2014. Polarized training has greater impact on key endurance variables than threshold, high intensity, or high volume training. Frontiers in Physiology.
3. 03
   Esteve-Lanao et al. 2007. Impact of training intensity distribution on performance in endurance athletes. Journal of Strength and Conditioning Research.
4. 04
   Hickson & Rosenkoetter 1981. Reduced training frequencies and maintenance of increased aerobic power. Medicine & Science in Sports & Exercise.
5. 05
   Hickson et al. 1985. Reduced training intensities and loss of aerobic power, endurance, and cardiac growth. Journal of Applied Physiology.
6. 06
   San-Millán & Brooks 2018. Assessment of metabolic flexibility by means of measuring blood lactate, fat, and carbohydrate oxidation responses to exercise in professional endurance athletes and less-fit individuals. Sports Medicine.
7. 07
   Olsson et al. 2024. Perceived exertion can be lower when exercising in field versus indoors. PLoS One.
8. 08
   Mieras et al. 2014. Physiological and psychological responses to outdoor vs. laboratory cycling. Journal of Strength and Conditioning Research.

More inside Indoor cycling training

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1. 01

   Winter cycling training plan: build an indoor season that holds

   How to structure November-to-February training — base vs intensity indoors, how much is enough, and why winter is when plans break.

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2. 02

   Why your heart rate is higher on the indoor trainer

   Heat, no airflow and cardiovascular drift push HR up at the same watts indoors. What the studies measured, and what to do about it.

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3. 03

   How long should indoor trainer rides be?

   Why a trainer hour is not a road hour, the session lengths that earn their place indoors, and when a longer ride stops paying.

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4. 04

   An indoor cycling training plan for 4 hours a week

   Four hours on the trainer, spent right: how many sessions, which one is hard, and what the evidence says holds fitness on that dose.

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5. 05

   The fan is equipment: airflow for indoor cycling

   How much airflow indoor cycling needs, why a fan switched on late does not fix drift, and where to point it. No brand picks.

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6. 06

   How many trainer sessions a week maintain fitness?

   Hickson's maintenance studies say two sessions hold VO2max if intensity holds. What that means for a winter with less time.

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