Because average power counts every coasting second as zero watts while your heart is still working hard. On technical trail you surge and coast constantly, so the zeros pile up and the average collapses, even on a ride that wrecked you. Low watts with a maxed heart rate doesn't mean you're weak. It means the crank only saw half the ride.
Road power is mostly continuous. Trail power is a strobe light. You punch 400 watts for ten seconds to clean a rock garden, then coast the next thirty through the chunder, then punch again. The average blends those coasting zeros right in. A ride that felt like an hour of intervals can come back with an average power that looks like a recovery spin.
Meanwhile your heart rate never got the memo to come down. It stays pinned through the coasting because your body is still paying for the surge you just did and bracing for the next one.
A power meter measures one thing: force through the drivetrain. Trail riding is full of work that never touches it.
It gets worse. Research from the University of Central Lancashire, reported by MBR, found power meters under-reading by roughly 8 percent off-road compared to the lab. Vibration and impacts mess with the strain measurements. So even the watts you do produce on trail get shortchanged.
Normalized power, a weighted average designed to give surges more credit than a plain average does, helps some. But it was built for road racing, and it still can't account for coasting-heavy technical riding or any of the off-the-crank work above.
Heart rate measures the total cost to your body: legs, arms, core, the carrying, the braking, the recovery debt between surges. It doesn't care whether the work went through the drivetrain. It's a blunter instrument than a power meter, and on a steady road climb the power meter wins. But on technical trail, heart rate is telling the truth and the power number is leaving most of the ride out.
Score your trail rides from heart rate and let your road bike keep its power meter. This is exactly how RideTool works. Each bike in your Bike Profile has an "Effort from" setting: heart rate by default, power meter if that bike's watts are trustworthy. Your rocky trail ride and your steady road ride both land in one honest effort currency, and your fitness is the long-term trend of that. The ride that wrecked you gets counted like it wrecked you.
And stop comparing your trail average watts to road numbers. They're measuring different rides. If the climb felt hard and your heart rate agrees, it was hard.
Connect Strava, Garmin, or Wahoo and RideTool turns every ride into a plain-English read on your fitness. Three short emails a week tell you if the riding is working. $29 a year, no ads, your data stays yours.
Start with your next ride