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Training on a treadmill: Accuracy versus precision

— Scott Semple

In early 2018, I bought a treadmill. Training on a treadmill sounds horrible to most–and for long, steady sessions it is–but for high-intensity training, it’s perfect.

Yesterday, I wrote about the big impact that gradient has on climb rates. While writing, I looked online for an angle-to-gradient calculator. I also came across a description of the difference between accuracy and precision:

Accuracy refers to how close measurements are to the “true” value… precision refers to how close measurements are to each other.
~ The Minitab Blog

For example, in shooting, accuracy would be how close each shot is to the bullseye. Precision would be how tight the grouping is of a series of shots:

A diagram of targets with samples of accurate versus precise results

When training by heart rate, accuracy would be how close the average session heart rate is to the target heart rate. Precision would be how wide the variation is between the minimum and maximum.

With a session target of 150 bpm, two athletes could have the same average–let’s say, 145–but very different precision if one athlete’s min/max was 140/150 and the second athlete’s was 125/165.

What does this have to do with training on a treadmill?

I bought a treadmill because I wanted my high-intensity training to be both accurate and precise.

Below aerobic threshold1, training by heart rate is useful.

It’s not perfect, but it’s pretty accurate and pretty precise. In that range, the results of workouts will be repeatable.

Between aerobic threshold and anaerobic threshold2, training by heart rate is less useful.

It’s both less accurate and less precise. The work periods are shorter, and heart rate takes time to adjust. The lag in heart rate makes workouts between AeT and AnT a tricky business.

Training by heart rate over anaerobic threshold is useless.

In well-trained athletes, an anaerobic threshold heart rate can be over 90% of maximum. Metabolic loads during a workout could be 1-8x what they are at anaerobic threshold. Muscular loads could be 1-4x, maybe more. Heart rate doesn’t have the range to reflect those intensities.

Even if it did, heart rate lag takes too long to adjust. Above anaerobic threshold, work periods can be short enough that the work is over before heart rate has even changed at all.

For example:

  • If AnT HR is 180 and maximum is 200; and
  • A short 10-second load is 4x AnT pace;
  • Is heart rate going to jump to 4x 180 (720 bpm)?

No, of course not. And in reality, heart rate won’t even change during the work period. It’ll increase during the recovery. So training by heart rate above anaerobic threshold is neither accurate nor precise. We hates that.

That’s why I bought a treadmill.

But why a treadmill? Why not just use pace outside?

Pace works well for runners on a flat course. But the angle of terrain for mountain athletes is constantly changing. As it does, so does the level of output.

With a long enough interval, the average effort could be accurate enough. But even small changes in terrain create big changes in climb rates, so the precision of an outdoor session will never be high. I wanted both accuracy and precision, thus the treadmill.

Most of the time, the treadmill sits idle in my garage. I’m sure my family wonders why the hell I spent the money. Then on occasion, I open the garage door and my neighbours wonder what the hell that guy is doing hiking inside on a sunny day.

  1. Aerobic threshold is best defined by the pace, power, or heart rate that produces a steady state blood lactate value between 1.5 and 2.5 millimoles. For most people, ~2mM is a good benchmark. ↩
  2. Anaerobic threshold has too many definitions to count… I define it as the pace, power, or heart rate that produces a steady state blood lactate value of ~4mM. Yes, MLSS may have >50% more lactate circulating, but the intensity to produce it is often only marginally greater. Most of the time, the pace and power at ~4mM is “close enough”. ↩