Base, build, peak: what each phase changes in your body

Training plans are usually presented as a schedule: base, then build, then peak. That is the order, but it is not the reason. The reason is that each phase asks the body for a different set of changes, some of those changes take months and some take days, and some of them make the next phase possible. Once you can see what each block is actually doing to your muscles, blood and heart, the sequence stops being a rule you follow and becomes something you can adjust intelligently. This post goes through the three phases and, for each, what is changing, how fast, and how you would know.

Base: the slow changes

The base phase is built on volume at low intensity, and its purpose is to change the muscle itself. The foundational review here is Holloszy and Coyle's from 1984, which described what months of endurance training do to skeletal muscle: the number and size of mitochondria increase substantially, the enzymes of oxidative metabolism rise with them, and as a consequence the muscle burns more fat and less carbohydrate at any given intensity, produces less lactate, and spares its glycogen. That last part is what a rider experiences as "I can go longer before I fade." The changes are inside the fibres, they take weeks to months to accumulate, and they are lost slowly, which is why a well-built base survives an off-season.

Alongside the mitochondria, the muscle grows new capillaries. Andersen and Henriksson trained previously untrained people for eight weeks and measured a roughly 20 per cent increase in the number of capillaries around each fibre, which shortens the distance oxygen has to diffuse and increases the time blood spends in the muscle. Capillaries, like mitochondria, respond to duration and frequency rather than to peak intensity, and once built they are among the most durable adaptations; the detraining studies we go through in the off-season post found capillary density unchanged after twelve weeks without training.

The intensity question is where the modern evidence sharpens the old picture. Granata and colleagues' review of exercise prescription and mitochondrial adaptation separates two outcomes: mitochondrial content (how much of the machinery there is) tracks training volume, while mitochondrial respiratory function per unit (how well each piece works) responds more to intensity. Base is about content. That is the mechanistic reason why long easy rides cannot be replaced by short hard ones: the hard ones improve the quality of what is there, and the easy volume is what builds more of it. We wrote about the sessions in base miles.

What you would notice, over eight to twelve weeks of base: heart rate at a given power drifts down; the same long ride leaves you less wrecked; you can eat less on rides without bonking; your first lactate threshold, if you measure it, moves right. Your FTP may barely change. That is normal. Base is not the phase that moves FTP much, and riders who judge it by FTP conclude it does not work.

Build: the medium changes

The build phase raises intensity on top of the base, and its target is different: the systems that limit how hard you can go, rather than how long. Three things are changing here.

• Maximal oxygen uptake and the heart. Intervals near VO₂max push cardiac output and stroke volume. Laursen and Jenkins' review of interval training in already-trained athletes found that this is the training that still moves them: once an athlete is well trained, further low-intensity volume produces little change in VO₂max, but intervals at or near maximal aerobic power do. Our VO₂max guide covers how those sessions are built.
• Threshold. Work at and around the second lactate threshold raises the power at which lactate production and clearance balance. Physiologically this is a mix of more oxidative capacity (already built in base) and better lactate transport and clearance in the muscle, plus the mitochondrial quality improvements that respond to intensity. This is where FTP moves, and where riders who skipped base find the ceiling low: the intensity improves the quality of the engine, but there is less engine to improve.
• Fatigue resistance. The event-specific part. Sessions that put hard efforts late in a long ride train the ability to produce power when glycogen is low and the muscle is tired. There is no single adaptation behind this; it is the combination of glycogen sparing from base and threshold work from build, tested together.

The build is also where the dose matters most and where individual variation shows. Montero and Lundby's study is a useful corrective to the idea of "non-responders": people who showed no VO₂max improvement to a given weekly dose of training did respond when the dose was raised. Some riders need more stimulus in build than the standard plan gives them, and some need less. It is also the phase where you can most easily do too much; recovery weeks are not optional. How fast to raise the load is in ramp rate: how fast to build.

What you would notice, over eight to twelve weeks of build: the power you can hold for 5, 20 and 60 minutes all rise; the hard sessions get harder to recover from; your fitness score climbs and your form score stays negative most of the time. Efficiency also improves through a season: Hopker's group measured gross efficiency in competitive cyclists across the year and found it rose from the winter into the competitive period, which means a little more of the power you produce is reaching the pedals for the same oxygen cost.

Peak and taper: the fast changes

The peak phase is short and works on a different timescale. The last hard sessions are done in the first part of it; the taper is the second. Nothing in the taper builds fitness. What it does is remove fatigue faster than fitness decays, so that the fitness built over the previous months is finally expressed. Mujika and Padilla's review of the scientific bases for tapering describes what changes: blood volume and red cell mass recover, muscle glycogen stores fill, muscle damage markers fall, hormonal markers of stress normalise, and muscular strength and power improve, all in the space of one to three weeks. The performance gain is modest, a few per cent in the tapering literature (the figures are in the cycling year), and comes from being fresh, not from being fitter.

There is a specific reason to keep intensity through the taper. The adaptations that respond to intensity, including the mitochondrial quality improvements and the neuromuscular sharpness, decay quickly if the stimulus is removed entirely, while the volume-driven adaptations from base are durable. A taper that cuts volume but keeps a little intensity holds the fast-decaying pieces in place while the fatigue clears. A taper that stops everything loses them. See the taper: last two weeks.

What you would notice: legs feel oddly bad in the first few days of the taper, then suddenly good; resting heart rate drops a little; your form score turns positive. If it turns positive too early, the taper was too long or the last build block too light.

One adaptation that runs across all three: blood

Plasma volume expands within days of starting or resuming training, by roughly 10 per cent according to Convertino's review, and this early change raises stroke volume before the heart itself has adapted. It is why the first fortnight of any block feels better than it should, and why the first fortnight off feels worse than it should. Red cell mass follows more slowly. Neither belongs to a single phase, but knowing about them explains a good deal of the week-to-week noise in your numbers, especially in the first two weeks after a break.

The phases at a glance

The time courses are approximate and drawn from studies mostly on trained or previously untrained men; individual responses vary widely, and your own year of data will tell you more than the averages.

Why the order matters

Put the three together and the logic of the sequence is clear. Base builds capacity that is slow to make and slow to lose. Build sharpens that capacity in ways that are faster to make and faster to lose, and its effect is proportional to the base it works on. Peak clears fatigue so the combination is expressed on one day. Reverse the order and you sharpen an engine you have not yet built, then try to build it while carrying the fatigue from sharpening. It can be done, and there are variants such as reverse periodisation that shift the emphasis for good reasons, but the underlying biology does not change: content first, quality second, freshness last.

If you build a plan in Moveee, the wizard lays these phases out from your target date and weekly hours, and the Power Profile will show the 5-, 20- and 60-minute power moving through the build, which is the fastest way to see whether the phase is doing its job. The full calendar view of how this fits into a year is in the cycling year.