Threshold Training Explained: Lactate, FTP & the Sweet Spot

Cycling coach

For most amateur cyclists, functional threshold power (FTP) is the number. It sets the training zones. It dictates the colour-coded blocks on TrainingPeaks. 

It’s the figure quoted in club WhatsApp groups, usually within 10W of being honest. And yet, what it actually represents physiologically – and what raising it really takes – is the area of cycling training where amateur understanding lags hardest behind the science.

Read on to discover what threshold actually is, what FTP estimates, why ‘sweet spot’ became the most prescribed intensity of the last decade, and the session we still build threshold blocks around.

The muscle-level definition

Inside your working muscles, two processes run constantly: lactate production (a byproduct of breaking down carbohydrate for energy) and lactate clearance (other tissues, especially the heart, the brain, and your slow-twitch fibres, absorbing and using that lactate as fuel via the lactate shuttle).

At low intensities, clearance easily keeps pace with production. Lactate stays near resting levels. As intensity rises, production rises faster than clearance can absorb. Lactate begins to accumulate. 

The first inflection point – the first lactate threshold (LT1) – sits at the top of Zone 2, around 60–70% of FTP. This is where lactate first leaves baseline.

Push further and you hit the second lactate threshold (LT2), also known as Maximal Lactate Steady State (MLSS). This is the highest intensity at which lactate production and clearance still balance, even if at an elevated level. 

Ride at MLSS and you can hold a steady blood lactate of around 4 mmol/L for 40–60 minutes. Push past it by even 10W and lactate accumulation becomes exponential and the clock starts.

FTP is a field approximation of MLSS. The original Hunter Allen/Andy Coggan definition – ‘the highest power you can sustain in a quasi-steady state for approximately one hour’ – is functionally the same thing.

Why a 20-minute test is shorter than 60

The classic 20-minute FTP test multiplies your 20-min power by 0.95. The 5% subtraction isn’t arbitrary. 

It accounts for two facts: most riders can sustain a slightly higher power for 20 minutes than for 60, because anaerobic capacity contributes meaningfully to the 20-minute number; and the longer you ride at any given intensity, the more lactate accumulates from small drift effects.

For most trained riders, the 95% multiplier is roughly right. For riders with very high anaerobic capacity (sprinters, track riders) it overestimates. 

For very aerobic riders with low anaerobic capacity, it underestimates. If your 20-minute test FTP feels suspiciously high and your 1-hour TT power doesn’t match, your anaerobic contribution is doing the work, and 92% is probably closer.

How do you increase your FTP?

Two pathways. Either you raise the ceiling above it (lift VO2max, which we covered last week) and threshold drifts up as a knock-on, or you train the threshold itself directly – which means improving the muscle’s ability to clear lactate at higher production rates.

The adaptations that drive lactate clearance:

  • Mitochondrial enzyme content. More citrate synthase, more β-HAD – the enzymes that handle oxidative metabolism. These come from volume at Zone 2 and from work at threshold and sweet spot.
  • MCT1 transporter density. The molecule that shuttles lactate into the mitochondria where it gets oxidised. Highly trainable.
  • Type IIa fibre conversion. Resistance to fatigue improves in fast-twitch fibres with sustained sub-threshold training.

This is why threshold training works. You’re not just willing yourself to hold a number for longer – you’re remodelling the muscle to handle a higher steady-state lactate flux.

The sweet spot and why it became popular

‘Sweet spot’ – usually defined as 88–94% of FTP – became the dominant prescription of the 2010s because it sits in a productivity-to-fatigue sweet zone. 

High enough to drive threshold adaptations. Low enough that you can do meaningful volume of it (20–30 minutes per interval, hours per week) without the brutal recovery cost of true threshold work.

The risk: it’s also the band most amateurs already drift into on group rides and sportives. 

Prescribe more of it on top of that and you’re stacking moderate-intensity fatigue without the variety the body needs. We use sweet spot blocks – but only for riders who genuinely train polarised the rest of the week.

The 2×20

The 2×20 at threshold has been the bread-and-butter session for forty years for a reason. It’s long enough to drive real lactate-clearance adaptation. 

It’s short enough that most riders can recover from it within 48 hours. And it teaches pacing – the most underrated skill in time-trialling and breakaway riding.

The session:

  • Warm-up: 15 min progressive build, finishing with 2×1min at 105% FTP, 2min easy between
  • Main: 2×20 minutes at 95–100% FTP (depending how you’re feeling on any given day), with 10min easy between
  • Cool-down: 10min easy

Pacing rule: 

The second interval’s average power should equal the first within 5W. If interval 1 was 280W average, interval 2 should land 275–285W. Any wider and you paced badly.

The progression we use across a 4-week threshold block:

  • Week 1: 2 × 20 @ 95% FTP
  • Week 2: 2 × 20 @ 97% FTP
  • Week 3: 2 × 25 @ 95% FTP – extend the duration, keep the intensity
  • Week 4: 2 × 20 @ 100% FTP – by now your old FTP should be holdable for 40 total minutes. Test at the end of the block

How often should you test your FTP?

Less often than most people do. FTP doesn’t move week to week – it moves block to block. Testing more than every 6–8 weeks adds fatigue without adding information. 

A clean test needs a fresh body, a properly built warm-up, and a venue that lets you pace evenly (flat road, indoor trainer, or a long steady climb).

If you don’t want to do a formal test, two field indicators work nearly as well: your average power on the last hour of a 3-hour Zone 2 ride (should be near or above your stated FTP×0.75), and your normalised power on the 2×20 itself (should sit at or above the FTP you’ve prescribed).

The bottom line

Threshold is the floor that determines what hour-long efforts feel like – the climb you can hold, the breakaway you can sit in, the TT you can pace. 

Lifting it requires patience, structured intervals, and a willingness to test less and train more. 

The 2×20 still does most of the heavy lifting forty years after it was first written down, because the underlying physiology hasn’t changed.

This is the third article in our Build a Better Engine series. Next week: Recovery – sleep, HRV, and the supercompensation principle that turns all this work into actual fitness.

Article written by Rowe & King Coach, Matt Rowe

Coached by Rowe & King – built for you.