A 5K run came in 50 seconds slower after variable-power cycling than after steady cycling. Same distance. Same average power. Only the smoothness of the watts changed.
That’s the finding from a 2007 lab study on triathletes. Your bike file matters more than the average watts number suggests. You can hit your target power for the whole ride. You can still arrive at T2 with less left than a smoother rider. That rider held the exact same average power as you did.
What Bike Pacing Variability Actually Measures
Your bike computer shows two power numbers after a ride: average power and normalized power. Average power is just the mean watts across the whole ride. Normalized power weighs the ride toward your surges. A ride full of hard bursts and soft coasting reads higher than the plain average. That’s true even when the average never moved.
Variability Index, or VI, is normalized power divided by average power.
VI = Normalized Power / Average Power
A VI of 1.00 means dead-flat, perfectly even power. A VI of 1.10 means your ride was harder than the average watts alone suggest.
Your FTP (Functional Threshold Power) is the highest power you can hold on the bike. That’s roughly one hour, all-out. Racing at a percentage of FTP for the bike leg is your Intensity Factor, or IF. IF is normalized power divided by FTP. A higher IF means a harder-feeling bike leg for the same fitness level.
IF = Normalized Power / FTP
Here’s where sources disagree. This deserves naming outright instead of picking one silently. Field-data coaching guidance comes from thousands of real 70.3 age-group files. It puts target IF at 0.73-0.78 for most trained age-groupers. First-timers should aim for 0.70-0.72. That’s a conservative, safe starting point for a first 70.3 bike leg. High-volume veterans can push up to 0.78. The classic Hunter Allen and Andrew Coggan power-training textbook lists 0.83-0.87 as the “optimal” half-Ironman range. That’s a full 10 points higher.
| Source | Suggested 70.3 IF Target |
|---|---|
| Field-data coaching guidance (age-groupers) | 0.73-0.78 |
| Allen & Coggan textbook, “optimal” range | 0.83-0.87 |
Which one is right for you? The textbook range models cycling performance alone. The field-data range comes from athletes who still had to run a half marathon afterward. Start near the lower, field-tested number. Only go higher if you’re a bike specialist with a proven durable run leg.
VI Targets by Course: Flat, Rolling, and Hilly 70.3 Bike Legs
Coaches who review hundreds of 70.3 power files converge on a similar range. The exact numbers shift slightly by source. Treat these as coach-reported heuristics, not a single controlled study result.
A flat course punishes a high VI the most. There’s no terrain excuse for the swings. A hilly or rolling course pushes even a disciplined rider’s VI up. Gravity is now part of the equation.
That last point matters more than most pacing guides admit. Coasting down a fast descent drops your average power. It barely drops your normalized power at all. Normalized power leans hard on your recent effort. It barely credits a stretch of zero watts. The formula raises short power spikes before averaging them back down. A handful of hard climbing surges can outweigh a long, soft coast. Your VI can read high on a hilly course even when you paced well. This is why the same VI number means different things on different courses. Check the elevation profile before you blame your pacing discipline.
The Physiology: Why Surging Costs More Than Coasting Saves
Here’s the mechanism behind the number. A 2020 study had 15 elite cyclists ride at matched average power. Some rode dead steady. Others swung power plus and minus 15 percent every two minutes. At high intensity, the surging riders’ blood lactate came in at 3.9 mmol/L. The steady riders sat at 2.8 mmol/L, about 39 percent lower. Heart rate and oxygen use barely moved between the two groups. Neither lab number shows up on a bike computer in real time.
Translation: two riders can look identical on a watch. Same heart rate. Same oxygen cost. Their internal engines are running very differently. The surging rider’s muscles work anaerobically harder for the exact same average output.
Think of your anaerobic capacity like a phone battery. It only recharges when you’re pedaling easy. A hard surge up a roller drains it fast. Coasting down the other side barely charges it back. Real recovery needs sustained easy effort, not a few seconds of freewheeling. A rider who chases every roller and coasts every descent drains that battery all day.
Same watts on paper. Different bill in your legs.
A separate study found something else worth flagging. After variable-intensity cycling, riders showed more neuromuscular fatigue than after steady cycling at matched average power. Your legs, not just your blood chemistry, come out of a surging bike leg more beat up. That fatigue compounds every mile of the run leg that follows. The same logic favors a steady target over Critical Power’s higher number for Ironman bike pacing.
What the Research Actually Shows: Variable vs Constant Power and the Run That Follows
The clearest test comes from a 2007 study of ten triathletes. Each rode 20 kilometers at either constant or variable intensity. Power swung between 68 and 92 percent of maximal aerobic power. Then each triathlete ran 5K immediately after. The variable-power ride cost 50 seconds on the run that followed, at matched average power.
That’s a small study over a short run. Stretching that 50-second cost across a full 21.1-kilometer 70.3 run leg is a reasonable guess. It is not a proven number. This article treats it as an illustration, not a settled multiplier.
Here’s the twist most pacing guides skip. A different 2007 study found the opposite result. Eight triathletes rode 30 minutes at constant power. Others alternated every 5 minutes between plus and minus 20 percent power. Then they ran to exhaustion on a treadmill. The variable group ran longer, not shorter: 15:09 versus 10:51 on average.
Why the contradiction? Look at the structure of each protocol. The first study used short, sharp surges with barely any recovery between them. That’s the same pattern real 70.3 climbs and rollers create. The second study built a full easy segment into every 5-minute block. That segment was long enough to act as genuine recovery right before the run started. Short surges without recovery hurt you. Longer swings with a real easy block built in do not, and can even help. Both effects point the same direction: smoother pacing preserves more for the run.
It’s not variability itself that costs you. It’s surging without recovering.
Two Riders, Same Average Power, Very Different Run Splits
Take two fictional 70.3 age-groupers, Dana and Marcus. Both average exactly 200 watts on the bike leg. Dana rides smooth, VI of 1.03, which puts her normalized power at 206 watts. Marcus surges every roller and coasts every descent. His VI is 1.12, normalized power of 224 watts, for the identical 200-watt average.
| Rider | VI | Normalized Power | Average Power |
|---|---|---|---|
| Dana (steady) | 1.03 | 206W | 200W |
| Marcus (surging) | 1.12 | 224W | 200W |
Coach-reported race outcomes for this pattern put Marcus 15 to 30 minutes slower overall. Of that, 10 to 15 minutes shows up specifically in his run split. Same average watts. Different bill.
Hunter Allen’s version of the same math makes it sharper. A rider targets 196 normalized watts, 70 percent of a 280-watt FTP. At a VI of 1.05, that rider delivers 187 average watts. At a VI of 1.15, the same rider only delivers 170 average watts. That’s seventeen fewer watts of forward motion for the exact same physiological cost. The lesson holds at any wattage, not just 200.
How to Actually Hold a Lower VI on Race Day
Ride to your target power, not to the terrain in front of you. On rollers, resist matching the hill with a surge. Shift into a smaller gear and keep your cadence up instead.
On descents, pedal a little instead of fully coasting. That keeps your average power closer to your normalized power. It also avoids a compensating surge on the next climb.
If you overcook the first half, coaches recommend a specific fix. Drop your normalized power target by about 5 percent for the following hour. Don’t try to correct it in the next 10 minutes. You cannot un-burn glycogen. You can limit how much more damage you do. Build that discipline in training with regular brick sessions that rehearse the run-off-bike transition.
Steady beats hard-then-soft, race after race.
After a race, most athletes can only guess why the run went badly. Was it fitness? Fueling? Something buried in the bike file? AthleteOS’s session analysis compares your bike variability index against your run-off-bike drift ratio from the same race. A surging pattern on rollers and climbs then shows up as the likely cause of a faded run split. It stops getting blamed on fitness or fueling you didn’t actually get wrong. That distinction changes what you fix in your next training block.
Taper timing matters too. Ironman 70.3 taper science sets the fitness and freshness window for race week. That window is what lets you hold this pacing plan on race morning. Explore AthleteOS’s race-day tools to review your own power file this way.