Watts Held, Body Broken
Coach Peter — Training Philosophy · Part 2
Watts Held,
Body Broken.
"The power file will show you a perfectly executed session. Your body will show you an athlete who is now three days into recovery from a workout that was supposed to be a training stimulus."
The first article in this series made a philosophical argument: heart rate has to be the governor of training intensity. Power is external load. Heart rate is internal cost. Under a fixed wattage, in almost every real athlete on almost every real training day, the internal cost drifts upward while the wattage stays flat — and the athlete who chases the power number is quietly overtraining while calling it discipline.
That article generated more messages than I expected. The most common one, in three different languages, was some version of: show me.
Fair enough. Coaches like me argue this all day. Numbers argue better. So this piece is the physiology behind the philosophy — the actual curves that heart rate, power, and blood lactate trace across a long ride and a threshold interval, in the one condition where power alone works, versus the far more common condition of a real athlete on a real training day.
The graphs are illustrative, drawn to realistic ranges rather than from any single athlete's file. But the shapes are the shapes. Every coach who has done lactate testing during rides has seen exactly these curves. I have run this test on my own athletes many times, in cool European conditions and in Bangkok's heat, in fresh legs and in fatigued ones. The result does not depend on the climate. It depends on the athlete's condition on the day — and on whether the wattage prescribed is genuinely the one their body can hold internally, right now.
Exhibit One — Four Hours at 200 Watts
Start with the simplest scenario. A rider goes out for a four-hour Zone 2 endurance ride. The plan says hold 200 watts. The rider does exactly that. In both cases the power file is essentially identical: a flat line at 200 watts for four hours, gold star, session complete.
The physiology tells a very different story.
Left: heart rate holds within a few bpm across four hours for a rider who is fully fresh at exactly the right target. For any typical athlete on a normal training day, HR drifts around 30 bpm even though the power is unchanged. Right: lactate stays at the aerobic threshold for the peak-shape rider, but for the typical rider it leaves Zone 2 around minute 90 and keeps climbing. Illustrative curves, realistic ranges.
The peak-shape ride finishes as intended — a Zone 2 endurance session, an accumulation of easy aerobic load, low glycogen cost, minimal recovery demand. The typical-rider ride, by hour four, is no longer a Zone 2 ride. Heart rate has drifted past the Z2 ceiling. Lactate has climbed out of the aerobic zone. The rider is now doing sustained tempo-into-threshold work while their head-unit tells them they are doing an easy ride.
The next day, the training log looks identical for both sessions. The bodies feel wildly different. One was a deposit into the aerobic bank. The other was a withdrawal. Repeat that mistake across a training block and one athlete adapts, the other flatlines — and neither of them can point to a bad session on paper.
This is why coaches who understand physiology do not let athletes hold a fixed wattage on long rides — unless the rider is genuinely in peak shape and the target is genuinely correct. Everyone else gets a target power with a heart-rate ceiling. When HR hits the ceiling, watts come down. Every time. No exceptions. That instruction protects the workout across the 95% of training days when the athlete is not perfectly fresh, the target is not perfectly right, and the day is not perfectly cooperative.
Exhibit Two — The Threshold Interval Trap
Now the more interesting case. A rider is prescribed a 20-minute threshold interval at 350 watts. This is a classic session — sustained just at or slightly under FTP, designed to shove the aerobic ceiling up. Done correctly, it produces a controlled climb in heart rate to around threshold HR (~170 bpm here) and a steady-state lactate around 4 mmol/L. Repeated over a block, it works beautifully.
What "done correctly" actually requires: an athlete who is fully recovered, whose current FTP genuinely is around 350 watts, on a day where nothing is compromising the aerobic engine. In that specific case, the session executes as intended. Same 350 watts, same lactate steady-state, same HR settled at threshold.
Now the same 350 watts, prescribed to the same athlete when any one of those conditions is not met — recent hard session, disturbed sleep, hydration debt, warm day, the target itself is slightly optimistic. The power file will look identical. The physiology will not.
Watts pinned at 350 for the full 20 minutes in both cases. Peak-shape rider at the perfect target: HR settles just above threshold, lactate finds a steady state near 4 mmol/L — a clean threshold interval. Typical rider on any real day: HR runs 15+ bpm above threshold and lactate escapes to 7+ mmol/L. Same file. Very different session.
Look at the bottom right. In the peak-shape case, lactate rises to threshold and plateaus. That plateau is the whole point of a threshold interval — you sit at the lactate ceiling, your body works flat out to clear what it produces, and the aerobic system adapts. For the typical rider on a typical day, there is no plateau. Lactate climbs, and keeps climbing, because you have crossed the athlete's actual threshold on this day — even though the wattage says you are still at it.
Every extra millimole is not free. It is deeper glycogen depletion, more hormonal stress, more fatigue signalling, more damage to recover from. The athlete finishes the session having executed the prescribed power to the watt, and having done a completely different workout than the one they thought they did. It was not a threshold interval any more — it was a VO2max effort in a threshold interval's clothing.
Do this session weekly, holding the wattage no matter what the body is doing underneath, and you get exactly the profile I see in unfortunately many amateur athletes: FTP that will not move, chronic fatigue, and a strong sense that they are working harder than everyone else and going backwards.
Exhibit Three — The Same Interval, Governed by Heart Rate
Now flip the prescription. Same athletes, same 20 minutes, but the instruction is different. Instead of "hold 350 watts," the instruction is "hold 170 bpm." Power becomes the reference. Heart rate becomes the boss.
HR pinned at 170 for the full 20 minutes. Peak-shape rider: holds ~350 W the whole way. Typical rider on a real day: power sags from 350 to around 300 W as the body sheds output to keep HR at threshold. The critical result: lactate finds a steady state near 4 mmol/L in both cases. The stimulus is delivered. The body is not overcooked.
The two rides now look completely different on the power file — but from a training-adaptation perspective, they are the same session. Both delivered ~20 minutes at threshold heart rate. Both produced a controlled lactate plateau at the target intensity. Both stimulated the aerobic adaptations the session was designed to produce.
The typical rider who executed Strategy B did the correct workout, on a day when Strategy A would have destroyed them. The wattage was lower on paper. The physiological stimulus was identical. This is not a compromise. It is the correct execution.
The Two Strategies, Side by Side
To make the trade-off unavoidable, here is the same 20-minute interval, in typical training conditions, scored across five things that actually matter for training adaptation and recovery.
Strategy A finishes 32 W higher in the file. The body pays for those 32 watts with 16 bpm more strain, 3 mmol/L more lactate, 14 minutes above threshold instead of 1, and 2.5× the recovery cost. Only a fully-fresh rider at exactly the right target avoids this trade. For every other real training day, the trade is not worth it.
Note carefully what is not being said here. This is not an argument that Strategy B is always better than Strategy A. It is an argument that Strategy A only works in one specific case — the peak-shape rider at the perfect target. For every other case, Strategy A is not the workout the file makes it look like. It is a much harder session, at a much larger recovery cost, delivering a completely different stimulus. Strategy B delivers the workout as prescribed regardless of what condition the athlete is in on the day.
Why the Body Does This
The physiology is not mysterious. It is worth understanding what is actually happening under the skin, because once you see the mechanism, the training decision becomes obvious.
Cardiac drift is the default, not the exception
Any sustained aerobic effort will produce a rising heart rate as central blood volume decreases and stroke volume drops. The cardiovascular system compensates by beating faster. In an ideal case — perfectly-fresh rider, ideal target, cool conditions, hydrated — the drift can be as small as 3–5 bpm per hour. In real training weeks, in most riders, on most days, it is meaningfully more. Add residual fatigue, heat, hydration debt, altitude, illness recovery — the drift accelerates. The direction, however, is always the same. Cardiac drift is not a warning that something is wrong. It is what a working aerobic system does.
Lactate accumulation depends on today's engine
Your lactate threshold is not a fixed number. It is a state your engine can be in today, which depends on how recovered you are, how fuelled you are, how hydrated you are, and how warm the day is. A wattage that produced 3.5 mmol/L in fresh legs at the start of a block can easily produce 4.5 or 5.0 mmol/L in the same rider two weeks later, on a tired day. The wattage said threshold. The lactate said above it. Environmental heat is one accelerant here; laboratory studies show lactate accumulating around 30% faster at the same power in hot conditions. But heat is only one variable. Every real training day has several.
Every real variable pushes internal load up at fixed external load
Fatigue. Poor sleep. Hydration debt. Menstrual cycle phase. Post-illness recovery. Altitude. Heat. Nutrition status. Cumulative training load from the previous block. Every single one of these variables raises the physiological cost of a given wattage. Power alone sees none of them. It cannot. It is measuring the bike, not the rider. Heart rate integrates all of them automatically, in real time, without any modelling required from you.
Lactate clearance is the point, not lactate production
A well-executed threshold interval sits at the balance point between lactate production and lactate clearance. The plateau is the training stimulus. Cross the point where production exceeds clearance and you are no longer training the aerobic engine — you are training the anaerobic system and paying a much larger recovery bill. Strategy A on a real day blows past the balance point in the first five minutes and keeps piling on lactate the athlete cannot clear. Strategy B stays on the balance point for the full 20 — the wattage adjusts down so that the body can still oxidise what it produces. That is the difference between a session that builds you and a session that hollows you out.
What This Means in Practice
These four graphs are not a call to throw away your power meter. Nobody serious does that. The professionals on my team ride with power visible every second of every session. Power tells the truth about the bike. It is essential for short intervals where heart rate is too slow to react, and it is the cleanest way to prescribe a target for any hard effort.
But the target and the governor are not the same thing. The target is what you are aiming to produce. The governor is what tells you to back off. Only in the specific case of a peak-shape rider at exactly the right target on a good day can power be both. In every other real case, they have to be different metrics — target in watts, governor in heart rate.
In peak shape · perfect target
Power can lead
Target and governor can both be power. Heart rate confirms the internal load is where it should be. Sessions execute cleanly against the prescribed watts. This is the exception, not the rule.
Every other real training day
Heart rate has to lead
Power is the reference — where you would like to be. Heart rate is the ceiling — where you must not exceed. Watts adjust down to protect the physiological cost. The session lands.
For long endurance rides, the rule is simple: set a target power, then a heart-rate cap 5–10 bpm above your Z2 top end. When HR touches the cap, watts come off. No matter what the plan said an hour ago.
For threshold intervals, set the target in watts and the ceiling in HR. If the interval requires the target watts and pushes HR above threshold within five minutes, the workout was written for a different day. Reduce the target, or shorten the intervals, or defer the session — but do not simply grit through it. Grinding through Strategy A does not build you. It just delays adaptation and costs you tomorrow.
The Physiologist's Rule
The best workout is not the one with the biggest numbers on the file. It is the one that delivers the intended physiological stress at the intended cost. Only the internal signals — heart rate, RPE, breathing, lactate if you have it — can tell you whether the workout you did matches the workout you prescribed. The external numbers cannot. They can only tell you what the bike experienced.
The Coaching Point
Everything in this article is downstream of one idea: training adaptation is a response to internal load, not external load. Your body does not know it did 350 watts. It only knows what those 350 watts cost — and on any real training day, they cost far more than the wattage suggests.
The athletes I see progress fastest are not the ones with the highest peak-power files. They are the ones whose files show a consistent, disciplined pattern: hard workouts that hit the intended internal load and stop; easy workouts that stay easy; long rides where power sags when the body needs it to and nobody panics about it because the heart rate stayed where it was meant to.
That pattern is not luck. It is coaching. Someone is watching the internal signals, adjusting the prescription in real time, and holding the line on the boring principle that the file is not the workout — the physiology is.
If your training has stalled, if your FTP has not moved despite honest work, if you are finishing rides more broken than you should be — the problem is very rarely fitness. It is almost always measurement. 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.