Threshold training: the physiology, and why it is the best return on your time

If you only had one type of hard session to spend your week on, threshold work would be the one. Not because it is fashionable, but because it is the quality that improves most in an amateur runner, and it improves for years.

What threshold actually means

Your muscles produce lactate all the time, even at rest, and your body clears and reuses it as fuel. At easy paces, production and clearance sit comfortably in balance. As you speed up, production climbs. There comes a point where clearance can only just keep pace — that is your lactate threshold. Beyond it, lactate and the acidity that comes with hard glycolysis accumulate faster than you can deal with, and the clock starts running on how long you can continue.

Physiologists usually describe two thresholds. The first, sometimes called the aerobic threshold, is where lactate first rises above resting levels — roughly the top of easy running. The second, the one people mean by “threshold”, is that highest sustainable steady state. It is what a tempo run trains.

Why it matters more than VO2 max

VO2 max — the maximum rate at which you can use oxygen — gets the attention, but it is heavily influenced by genetics — roughly half the variation in how people respond to training is familial — and in already-trained runners it tends to flatten while performance keeps improving. The best-known illustration is a five-year case study of an Olympic runner whose VO₂ max barely moved while her running economy improved by around 15% and her race times kept falling. Threshold is different. Threshold is about what fraction of that ceiling you can sustain, and that fraction keeps improving for years with the right work.

Two runners with an identical VO2 max can finish a half marathon many minutes apart, and the gap is largely threshold. That is why an experienced runner beats a fitter-on-paper newcomer: they are using more of what they have.

How the training week divides up

Think of running intensity in three bands:

Easy — conversational, below the first threshold. Builds the aerobic base, capillaries, mitochondria and, crucially, tendon and bone durability. This should be the large majority of your running.
Threshold — comfortably hard, sustainable, right at that second line. Moves the ceiling on sustainable pace. One session a week for most amateurs.
Interval / VO2 max — hard repetitions of 3–5 minutes with real recovery. Sharpens the top end. Useful in the weeks close to a race, unnecessary year-round.

The classic error is running everything in the middle: too hard to recover from, too easy to drive adaptation. It feels productive and produces very little. That is not just coaching folklore: across 151,813 marathons, recreational runners spent about 35% of their training in that moderate band, and more time there correlated with slower finishing times. Easy days genuinely easy, hard days genuinely hard.

One honest qualification, because our own Zone 2 guide makes the point: across that same dataset, what tracked most closely with performance was total volume, and the proportion of easy running rose in faster runners. Threshold work is the best hard session per minute spent. It is not a substitute for the easy miles around it.

What threshold sessions look like

Continuous tempo — 20 to 40 minutes sustained, depending on your training age.
Cruise intervals — 3 × 8 min, 4 × 6 min or 5 × 5 min with 60–90 sec jog between. More total threshold time at a more accurate pace.
Threshold long run — a long run with two or three threshold blocks inside it. Demanding; save it for a marathon build.

Total time at threshold in a session usually lands between 20 and 40 minutes. Beyond that you are into race territory and the recovery cost climbs steeply.

How to tell it is working

The honest answer is that you will not feel it week to week. What you will notice over two or three months is that the same pace requires a lower heart rate, that the pace itself drifts faster at the same effort, and that a distance which used to feel hard now feels routine. Threshold adaptation is slow, cumulative and unglamorous — which is exactly why the runners who stick with it pull away from the ones chasing a new session every fortnight.

A note on heart-rate zones and watches

Watch-estimated threshold and zone models are useful as a rough guide, but they are estimates built on formulas, not measurements of your body. Heart rate also drifts upward over a long effort even at constant pace, and lags behind changes in speed. Use effort and the talk test as your primary guide and the watch as a sanity check — not the other way round.

References

Faude O, Kindermann W, Meyer T. Lactate threshold concepts: how valid are they? Sports Med, 2009;39(6):469–90. · Bassett DR, Howley ET. Limiting factors for maximum oxygen uptake and determinants of endurance performance. Med Sci Sports Exerc, 2000;32(1):70–84. · Jones AM. A five year physiological case study of an Olympic runner. Br J Sports Med, 1998;32(1):39–43. · Bouchard C, et al. Familial aggregation of VO₂max response to exercise training: results from the HERITAGE Family Study. J Appl Physiol, 1999;87(3):1003–8. · Muniz-Pumares D, et al. The training intensity distribution of marathon runners across performance levels. Sports Med, 2025;55(4):1023–35. · Seiler S. What is best practice for training intensity and duration distribution in endurance athletes? Int J Sports Physiol Perform, 2010;5(3):276–91. · Scherr J, et al. Associations between Borg’s rating of perceived exertion and physiological measures of exercise intensity. Eur J Appl Physiol, 2013;113(1):147–55.