The science behind the workouts
Every kwod workout links to one of these lessons. Read them once and the plan stops being a list of instructions: you will know what each session is for and what it will — and will not — change.
Two thresholds, three zones
LT1 and LT2 are the two points where your body changes how it produces energy. Every kwod zone is defined by them.
As effort rises, blood lactate stays near resting levels for a long time. The first point where it starts to climb is lactate threshold 1 (LT1), sometimes called the aerobic threshold. Below it you can go for hours, burning a large share of fat.
Keep pushing and you reach lactate threshold 2 (LT2): the highest effort at which lactate production and clearance still balance. Above it lactate accumulates and the clock starts ticking on how long you can hold the effort.
kwod uses three zones built on those two points. Zone 1 is below LT1, Zone 2 sits between LT1 and LT2, and Zone 3 is above LT2. Percentages of max heart rate are only a rough stand-in for these points, which is why the plan includes a field test every fourth week to keep them current.
Takeaway: Zones only work while they match your current thresholds — re-test them as you get fitter.
Why intervals: VO2 max, stroke volume and LT2
High-intensity work is what raises your aerobic ceiling. Easy volume cannot stand in for it.
Intervals at or above LT2 — around 107% of LT2 power and up — drive oxygen uptake close to its maximum and keep it there for minutes at a time. That repeated time near VO2 max is the signal that makes the heart pump more blood per beat (maximal stroke volume) and raises the most oxygen your body can use.
Short, all-out efforts such as a 15-second sprint every minute add a different stimulus: they recruit your largest, most powerful motor units, improving maximal power output.
Both also lift LT2 itself, so the effort you can sustain before lactate runs away from you goes up. None of this is triggered by easy work, however much of it you do: the body only adapts to demands it actually meets.
Takeaway: If you want a higher ceiling, you have to visit the ceiling — briefly, and with recovery around it.
Why easy volume: LT1, fat oxidation and the aerobic base
Long Zone 1 sessions move your first threshold to the right and make you burn more fat at the same pace.
Training below LT1 — around 63% of max heart rate for many people — is easy enough to repeat often and to do for a long time. That volume builds mitochondria and capillaries in the slow-twitch muscle fibres.
The result is that you produce less lactate at a given power and burn more fat doing it. LT1 shifts to the right: you go faster before the first rise in lactate, and you can use a larger percentage of your VO2 max before reaching it.
This is long-duration endurance: the ability to hold a good pace for hours. It is real and valuable — and it is a different adaptation from the one intervals produce.
Takeaway: Easy should be genuinely easy: the benefit comes from duration and frequency, not from pushing.
Specificity: you adapt to what you do
The body adapts specifically to the stimulus it is given. That is the whole reason a plan mixes zones.
Low-intensity training improves LT1, fat oxidation and endurance. High-intensity training improves VO2 max, stroke volume, maximal power and LT2. The overlap is smaller than people expect.
So if you replace every interval session with easy steady-state work, your fat-burning efficiency can keep improving while your VO2 max, LT2 power and sprint output stop moving. kwod's forecast shows exactly that: no Zone 3 sessions means a flat projection for the high-intensity markers.
The reverse holds too. A diet of only hard sessions leaves the aerobic base underbuilt and makes recovery the limiting factor.
Takeaway: Pick sessions for the adaptation you want, and track the marker that adaptation moves.
Polarized or pyramidal: distributing intensity
Structured plans decide how much time goes in each zone before deciding what each workout is.
A polarized week puts most sessions in Zone 1, adds one or two targeted Zone 3 sessions, and keeps Zone 2 to a minimum. A pyramidal week also has Zone 1 as its base, but with more Zone 2 than Zone 3 — Zone 1 > Zone 2 > Zone 3.
Both work. What tends not to work is spending most of your time in the middle, the "yellow zone" between LT1 and LT2. It feels productive, but it is too hard to recover from quickly and not hard enough to drive the high-intensity adaptations.
kwod builds either model from how many days you can train, keeps hard days apart, and compares what you actually did with what the model intended.
Takeaway: Most of your training should be easy, a little should be hard, and the middle should be a deliberate choice.
Recovery is where adaptation happens
Intervals give a lot of stimulus per minute, and ask for real recovery in return.
A hard session is a stress; the adaptation is the body's response while it recovers. Stack hard sessions without easy days between them and the second one is rarely as hard as it should be, so it delivers less of the stimulus it was meant to.
That is why kwod never programs two hard days in a row and puts an easy day after every Zone 3 session. Very easy movement on those days keeps blood flowing without adding to the load.
Resting heart-rate variability (HRV), taken the same way each morning, is a useful trend to track: a run of unusually low readings is a reason to keep the day easy.
Takeaway: Make easy days easy so hard days can be hard.