Here's something that shouldn't be possible: two athletes with identical FTPs, riding the same course at the same average power, finishing the bike within two minutes of each other — then running splits forty minutes apart despite similar standalone run fitness.
It happens more than you'd think. And when it does, the explanation is never in the average numbers. It's in the numbers behind the numbers.
Think of TSS as a currency. You have a budget for each discipline — an amount you can spend on the swim and bike and still have enough left to run. Exceed the budget and you don't stop — you keep going, but every kilometre costs more than it should.
The numbers aren't arbitrary. A well-paced IRONMAN bike typically produces 270-300 TSS. A well-paced 70.3 bike sits around 150-180. These aren't targets to hit — they're ceilings to respect. Cross them and the run doesn't gradually get harder. It gets harder faster than the maths suggests it should.
IF is where most athletes make their first mistake — not because they don't understand the number, but because they don't respect how dramatically it changes with distance.
An IF of 0.87 is aggressive but appropriate for a competitive 70.3. Applied to a full IRONMAN it's a guarantee of a bad run. The physics are unforgiving — the longer the race, the narrower the band you can operate in. A 70.3 gives you roughly 0.75 to 0.87 depending on your experience level. A full IRONMAN gives you 0.65 to 0.78. Stray above the ceiling and TSS accumulates faster than your nutrition can compensate for. The margin between a well-paced and poorly-paced bike is as little as 15 watts of normalised power. Most athletes would never notice 15 watts in training. On race day it's the difference between running and surviving.
Consider two athletes. Same FTP, same race, same IF of 0.80 on the bike — by every headline metric, identical rides. Rider A finishes with a VI of 1.03. Rider B finishes with a VI of 1.11.
Rider A arrives at the run with full capacity. Rider B arrives compromised — and the run shows it. Etxebarria et al. (2013) put the effect on the clock: at matched mean power, cyclists who rode with substantial variability ran their subsequent 9.3 km time trial 42 seconds slower than those who rode steady, with the gap appearing primarily in the first half of the run. The same study found elevated blood lactate and higher perceived exertion after variable-power cycling — physiological signatures of repeated above-threshold efforts the average power doesn't reveal. Friel's coaching standard of VI ≤1.05 sits roughly where this cost stops being negligible for long-course racing. Same IF. Completely different run.
The question a serious athlete should be asking before any race isn't "what power should I hold?" It's "what TSS can I arrive at T2 with and still run to potential?" Everything else — IF, VI, pacing strategy — is in service of that number.
IF determines how aggressively you're drawing from the budget. VI determines how efficiently. A high VI at any IF means you're spending more than the watts suggest — fast-twitch recruitment burns glycogen at a rate normalised power doesn't fully capture. A well-controlled VI at the right IF means the TSS you accumulate is the TSS you planned for. That's the difference between a race that unfolds as expected and one that falls apart in the second half of the run.
Everything discussed so far assumes you arrive at T1 with a clean slate. Most athletes don't.
Exceed your CSS in the swim — easy to do in cold water, with adrenaline, in a mass start — and you arrive at the bike with elevated heart rate and accumulated lactate that affects how the early bike feels and what power you can hold for the opening kilometres. The targets you set for the bike were calibrated for a fresh start. A hard swim means those targets are slightly too aggressive, and TSS accumulates faster than planned from the first kilometre. The recovery you'd take to clear that spike at low intensity isn't available at race pace — you're carrying it for the rest of the day.
None of this is meant to make racing feel like a maths exam. TSS, IF, VI, CSS — these are the inputs that make a real race plan possible. They're also exactly the numbers you shouldn't have to think about when the gun goes off.
Keiro works through all of it before race day. Your swim pace is derived from your CSS and your event. Your power plan is built around the right IF and VI for your course, your fitness, and the forecast conditions. Your run pace is planned to its full potential — not salvaged from whatever the bike left behind. All three disciplines, one connected plan, built for the complete race.
Find your race and start planning.