Train the Cost, Not the Number
Coach Peter — Training Philosophy
Train the Cost,
Not the Number.
"Watts tell me what you did on the pedals. Heart rate tells me what it cost you. Only one of those numbers reflects what your body actually experienced."
I want to make an argument that many riders find uncomfortable, because it contradicts a decade of power-meter marketing. It goes like this:
Under a fixed wattage, in almost every athlete on almost every real training day, heart rate and lactate drift upward across the session. Not because the athlete is unfit. Not because they are undisciplined. Because that is what the human body does when you pin an external output and force it to hold. The physiology is not obedient. It responds.
There is exactly one situation where an athlete pinned to a wattage sees heart rate and lactate hold steady alongside the power: a rider in peak shape, executing at exactly the correct target, on a day when everything — sleep, hydration, fuelling, recent training load — is genuinely dialled in. That combination is the exception, not the rule. For every other real training day, the underlying story looks the same: watts flat on the file, HR climbing, lactate climbing, body absorbing more cost than the number suggests.
I have run this test on my own athletes many times, with lactate strips and heart-rate straps. The pattern does not vary by climate, only in speed. In cool European conditions it happens more slowly. In Bangkok's heat it happens faster. But the direction is the same. And that is the point of this article.
External Load vs. Internal Cost
The single most useful concept in modern endurance science is the split between external load and internal load. External load is what you did — kilometres, watts, elevation, time. Internal load is what it cost — heart-rate response, perceived effort, lactate accumulation, hormonal stress, recovery demand. Two athletes can complete the exact same external load and pay dramatically different internal costs. The same rider on two different days can complete the same wattage and pay dramatically different internal costs.
Stephen Seiler, whose research underpins most of the modern polarised training model, is explicit on this: for genuinely easy aerobic work, internal load is the correct target. It is not close.
"Power in Zone 2 that sits slightly too high produces the exact grey-zone drift that is the most common amateur training problem."
Power is a magnificent tool. It is precise, immediate, and honest about what the bike experienced. For a five-minute VO2 interval, it is unbeatable — heart rate is 30 to 60 seconds too slow to be useful. But for the 80% of your training that should be easy, and for almost any interval held longer than ten or fifteen minutes, power alone gets you into trouble. It tells you what you produced. It cannot tell you what it took out of you.
Two Ways to Read the Same Ride
Riding to Heart Rate
Trains the Body
Adjusts to today's condition automatically. Lets the body burn lactate rather than accumulate it. Detects fatigue, poor sleep, hydration debt, heat, illness in real time. Delivers the workout you actually prescribed, day after day. Slower to respond to short surges — but for anything sustained, the more honest number.
Riding to Power Alone
Trains the Screen
Perfectly precise about what the bike did. Cannot see fatigue, sleep debt, dehydration, or accumulated load. Rewards the athlete for holding a wattage while the physiology drifts underneath. Only works exactly right when the rider is fresh, the target is exact, and the day cooperates. Which is rare.
Both metrics are useful. The mistake is treating them as interchangeable — or worse, treating power as always right and heart rate as legacy technology. Heart rate is not legacy. It is the only signal on your handlebars that measures you.
What Actually Happens Under a Held Wattage
Take a straightforward endurance ride. Target 200 watts, four hours, Zone 2. Do it in cool conditions in April, do it again in the same climate in July when the rider is a bit tired from the previous block, do it in tropical heat on any day of the year. The power file for all three rides looks the same — a clean flat line at 200 watts.
The heart rate and lactate do not look the same at all.
Cardiac drift is not a heat phenomenon. It is a fatigue phenomenon that heat accelerates. Any sustained aerobic effort, held long enough, will produce a rising heart rate as stroke volume decreases and the cardiovascular system compensates. In an ideal cool session with an ideal rider on an ideal day, that drift can be as small as 3–5 bpm per hour. In a typical training week, in most riders, on most days, it is meaningfully more. In tropical heat with an average rider, it can be 15–20 bpm per hour or worse. But the direction is always up. Never down. Never flat except in that idealised case.
Lactate behaves the same way. A wattage that produced 1.8 mmol/L in fresh legs at the start of a season can produce 2.5 or 3.0 mmol/L in the same rider after four weeks of accumulated load — even in cool conditions. Add heat, add sleep debt, add a hard session yesterday, and the same wattage climbs into 3.5, 4.0, higher. The wattage said Zone 2. The lactate said Zone 3.
Fatigue, hydration, fuelling, illness, altitude, heat — every one of these variables pushes internal load up at the same external load. Power alone sees none of them. It cannot. It is measuring the bike, not the rider.
If you train to power alone across a block, none of that shows up on your screen. You just watch the watts and press on. The body, meanwhile, is running a metabolic tab that will come due later — in a stalled adaptation, a lingering illness, or a season that never quite got going.
Five Reasons Heart Rate Has to Lead
It Keeps Zone 2 Genuinely Zone 2
The single most common amateur mistake is riding Z2 slightly too hard. Ten watts over the ceiling turns a mitochondrial session into a moderate-intensity grinder that builds almost nothing. Only a perfectly-fresh rider at exactly the right target can hold the correct watts and stay in Z2 physiologically the whole time. For everyone else, the wattage that sat in Z2 last week sits above it this week. Capping by heart rate keeps the aerobic stimulus clean regardless of what the day brings.
It Catches the Days You Should Not Push
Bad sleep. A stressful week. A meal that did not sit right. Early illness. Cumulative load from the last block. None of that appears on a power file. All of it appears in heart rate. If your HR is 8–10 bpm higher than usual for the same easy warm-up, the day is telling you something. An athlete who listens trains for years without breaking. An athlete who overrides it with a wattage target eventually gets ill, injured, or stuck.
It Lets the Body Burn Lactate, Not Accumulate It
This is the point that changes everything. A ride paced by heart rate stays at whatever internal intensity you targeted — and at that intensity, the body oxidises the lactate it produces. It never accumulates. A ride paced by watts, when the athlete is not perfectly fresh, quietly crosses the balance point. Production exceeds clearance. Lactate starts to stack. Every extra millimole is deeper glycogen depletion, more hormonal stress, more damage to recover from. Same session, on paper. Different physiological outcome, in the body.
It Adapts to Every Real Variable, Automatically
Fatigue, hydration, temperature, altitude, sleep quality, nutrition status, menstrual cycle, illness recovery, jet lag — every one of these shifts your internal cost at the same wattage. You do not need to model any of them. Heart rate does the maths for you. You obey the ceiling and the workout self-adjusts to what the body actually has today. This is not less rigorous than power. It is more rigorous. It respects a reality the power meter cannot see.
It Turns Every Long Ride Into a Free Fitness Test
Pw:HR decoupling — the gap between your power and heart rate over the course of a ride — is one of the cleanest markers of aerobic fitness there is. If your HR stays flat while your power holds, your base is doing its job. If HR drifts 10 or more beats over the last hour at the same power, either you started dehydrated, the day was harder than the file shows, or your aerobic engine needs more easy volume. No lab required. Just look at the file.
How I Actually Use Both
This is not an argument for throwing your power meter away. Nobody serious does that. The professionals on my team ride with power visible every second of every session. The point is to know which number is telling the truth right now — and to have the discipline to obey the one that matters.
For short, hard intervals — anything under about eight minutes — use power to prescribe and execute. Heart rate is too slow to catch a 30-second VO2 rep or a two-minute threshold pull. Set the target in watts, hit it, move on.
For anything easy or endurance — Z1, Z2, long tempo — use heart rate as the ceiling. Power sets a rough floor so you are not soft-pedalling; HR sets the ceiling that keeps you honest. If HR drifts above the cap, ease off. Do not fight the body to hit a wattage that was set on a fresher day.
For long sustained efforts — any interval longer than about ten to fifteen minutes, and any ride over two hours — heart rate wins conflicts. Full stop. Your first-minute watts are not defensible in the twentieth minute unless you are the rare fully-fresh rider at exactly the right target. That is not most rides, and it is not most riders.
Every day, before the session — glance at resting HR and, if you track it, HRV. A resting HR 5–7 beats above your baseline is not a coincidence. It is your body telling you what today's session should actually look like — regardless of what the plan says.
The Ceiling Rule
On any sustained aerobic effort, set a heart rate cap before you start. Whatever number comes up on your power meter, if the HR exceeds the cap, back off. Every time. Not sometimes. The rider who obeys the ceiling for a full block builds an engine. The rider who breaks it for the watts builds fatigue.
The Uncomfortable Truth
The athletes who get faster fastest are not the ones producing the biggest numbers on any given file. They are the ones whose internal cost stays where it was prescribed — workout after workout, block after block, season after season. That is only possible when the metric that governs the ride is the one that measures the rider, not the bike.
Only a rider in peak shape, at exactly the right target, on a genuinely good day, can hold a wattage and see HR and lactate stay stable underneath. If that is you every day, congratulations — you may not need heart rate at all. For every other athlete, on every other real training day, heart rate is the metric that keeps the workout honest. It is the metric that catches the days you should not push. It is the metric that lets the body burn lactate rather than accumulate it. It is the metric that adapts to the reality your power meter cannot see.
You bought a power meter because you wanted the truth. Good instinct. But the truth about how fast you are getting is not on the power meter alone. It is in the relationship between what you produced and what it cost you. Train the cost, not the number. The watts follow. They follow only when the cost is right first.
If your training has flatlined, or you are riding to numbers that stopped making sense somewhere in the last block, it is almost never a fitness problem. It is a measurement problem. And a measurement problem is exactly what coaching is built to solve.
P
Coach Peter
Endurance Coach · The Coaching Philosophy · Bangkok, Thailand
I build training plans for UCI-licensed riders competing across the Asian racing circuit — and for amateur cyclists who want to know what that level of preparation actually feels like. Based in Bangkok. Head of Performance, Roojai Insurance Winspace UCI Continental Team. If your numbers have stopped agreeing with each other, or your fitness has stalled despite honest work, that is a measurement problem — and a measurement problem is exactly what coaching is built to solve. Start here.