There's a tier of training metrics above FTP that amateurs hear about, nod along to, and never really use: critical power, W', fractional utilisation, the whole alphabet of performance physiology. Alex Welburn lives in that world — he's a coach and a PhD researcher at Loughborough working specifically on critical power and W', and he's the physiologist I had on to talk about the metrics Pogačar's team actually pays attention to. So I expected the conversation to be deep in the weeds.
It went somewhere more useful. Welburn can explain the metrics as well as anyone, but the thread that mattered most for ordinary riders wasn't a number at all. It was a way of thinking about how much you can take on in a day before you start doing yourself harm.
Critical power and W', without the jargon
Start with the metrics, because they're worth understanding even if you never measure them directly. FTP gives you a single number — the power you can roughly hold for an hour. It's useful, but it's one point on a curve. Critical power and W' describe the whole curve.
Critical power is, in plain terms, the highest power you can sustain in a genuine steady state — the boundary between effort you can hold for a long time and effort that's burning a fuse. It's close to FTP in spirit, but it's derived from several efforts of different durations, which makes it a more honest description of where your sustainable ceiling actually sits. The deeper picture of how these load metrics fit together is in our guide to reading your training data.
W' — said "W-prime" — is the part that changes how you race. It's the finite amount of work you can do above critical power before you're empty. Picture a battery. Every time you surge over your critical power — an attack, a steep ramp, closing a gap — you drain some of that battery. Drop back below critical power and it slowly recharges. W' is why you can only answer so many accelerations before your legs simply won't fire, no matter how willing you are. It quantifies the thing every racer feels but few can name: you have a set number of matches, and once they're gone, they're gone.
You don't need a lab to use this idea. Knowing that you have a finite reserve above threshold — and that it recharges only when you ease properly below it — should change how you spend it. Burn your matches early chasing every move and you'll have nothing for the moment that decides the race.
Two or three hard sessions, and build around them
When the conversation turned to how much hard work an amateur should actually do, Welburn's answer was refreshingly bounded. "I think two to three hard interval sessions per week can fit most people," he said, "and then the volume kind of is moved around that."
That framing — intensity first, volume arranged around it — is the right way up. Most amateurs do it backwards, scattering hard efforts through every ride until the whole week becomes a grey smear of medium-hard work. Welburn's model is cleaner: pick your two or three genuine quality sessions, treat them as the priority, and fill the rest of the week with endurance riding that supports rather than competes with them. Our interval training guide covers how to build those sessions; the discipline is in not adding a fourth and a fifth.
He also explained why the pros look different, and why you shouldn't simply copy them. Full-time athletes riding 20 to 30 hours a week naturally drift toward a polarised approach, because the maths forces it. "If you're sticking six hours or whatever of high intensity, they're going to be pretty cooked," he said. Nobody can absorb that much intensity. So the pros do a lot of easy volume and a small, sharp dose of hard work — not because a coach drew a pie chart, but because anything else breaks them. The amateur lesson isn't to ride 25 hours. It's that even the fittest humans on earth ration their hard days, so you probably should too.
The stress budget that nobody balances
Here's the idea I keep coming back to, because it reframes training as part of your whole life rather than a thing that happens in a vacuum. Welburn described each day as having a budget — a capacity for total stress.
"Each day you have a capacity for stress," he said. "You've got stuff that goes within your life, then you've got your training, and you want to typically have a little bit of space left over, because then that's kind of your safe space." Work stress, family stress, poor sleep, the training session — they all draw from the same account. And the goal isn't to spend every penny. It's to leave a margin, so that when work runs long or life throws something at you, you're not already overdrawn.
This is where so many amateurs come unstuck, and Welburn named the specific failure mode: the hero effort. "If work gets a little bit longer, or you push on and do a couple of hero efforts — which you shouldn't, but you do those," he said, you blow the margin. The easy ride becomes a half-hard ride because you felt good and chased a segment. The recovery spin turns into a tempo grind because a mate came past. Each one feels harmless. Together they overspend the budget, and the cost lands later, when your actual hard sessions fall flat and you can't work out why.
The fix is a kind of restraint that doesn't come naturally to motivated riders: protect the margin on purpose. On an easy day, stay easy even when you feel strong — especially when you feel strong, because that's exactly when the hero effort tempts you. Account for the rest of your life when you plan the week; a brutal stretch at work is a reason to pull training back, not to prove something. The riders who get this right rarely look heroic on any given day, and they're the ones still progressing in three months while the segment-chasers are flat and frustrated. It's the same trap our piece on the signs of overtraining keeps describing from the other end.
Spotting when you've overspent
The stress-budget idea is only useful if you can tell when you've gone overdrawn, so it's worth knowing the signals. The overspend rarely announces itself on the day you do it — that's what makes it dangerous. It shows up a few days later, and usually in the same handful of ways: the hard session that should feel sharp instead feels heavy and flat; your easy pace creeps up because holding the old easy power suddenly takes more out of you; your sleep gets worse rather than better despite the training; your motivation dips and the bike starts to feel like a chore. Resting heart rate drifting up and a sense of being permanently slightly tired are the classic tells, the early end of the overtraining spectrum.
The key insight from Welburn's framing is that the cause is often not the training at all — it's the total. You can run a sensible training week and still dig a hole if work blew up, sleep collapsed, and you bolted a few hero efforts on top. So when those signals appear, don't just look at your training log; look at the whole budget. The fix is usually to spend less somewhere — pull back a session, protect your sleep, or simply stop turning easy rides into races — until the margin returns. Riders who treat fatigue as purely a training problem keep adjusting the wrong variable.
A note on testing critical power
If the metrics interest you, the good news is that critical power is testable without a lab. The simplest field approach is a few all-out efforts of different durations — something short like three minutes and something longer like twelve — on a road or indoors, with proper rest between. The power-duration relationship across those efforts estimates both your critical power and your W'. It's more involved than a single FTP test, and for most amateurs FTP is enough to set training zones. But understanding that the two extra efforts reveal your finite reserve above threshold — and retesting occasionally as you get fitter — gives you a richer picture than one number ever can. Treat it as a useful refinement, not a prerequisite: the training principles above matter far more than which metric you use to describe them.
What to take from it
Welburn operates at the level of critical power and W', but the gift he gives amateurs is simpler than any metric. Understand that you have a finite reserve above threshold and spend it deliberately. Cap yourself at two or three real hard sessions a week and build your easy volume around them. And treat each day as a stress budget with your whole life in it, leaving a little unspent rather than chasing hero efforts that quietly bankrupt you.
You don't need the pros' metrics to train like the pros think. You need their discipline about how much is enough — which, it turns out, is usually a bit less than you want it to be.
Hear the full conversation with Alex Welburn on the Roadman podcast. For more on managing your training load, read our guide to reading your training data, and bring your numbers to the community on Skool.