Gradient: Small changes in angle, big differences in climb rate

September 11, 2018  ●  3-minute read

When using climb rates as benchmarks, make sure that the grade of the terrain during tests is consistent. Small variations in gradient will create big differences in climb rates and misleading results between tests.

I often use a treadmill as part of my warm-up for strength sessions in the gym. At my usual gradient and speeds, my heart rates in the gym are 5-8% lower than on my treadmill at home.

For a warm-up, that hardly matters. But I’ve been using my treadmill at home for lactate tests. And I’ve been using my previous best lactate test as a benchmark, but I did that test on the treadmill at the gym.

The difference heart rates was a good clue. I wondered if I was comparing apples to apples between lactate tests. What if the gradients and speeds between the two treadmills were actually different?

I’m not sure how to compare the speeds, but measuring the angles seemed simple enough. I downloaded an app on my phone and measured the slope of the running surface on each treadmill. It turned out that there was a 1.4° difference: 12.5° at the gym; 13.9° at home.

A 1.4° difference doesn’t sound like much. But convert the degrees into a percentage gradient. Then calculate the difference in climb rates at a constant speed. It’s a difference of 11.6%. That is significant.

Here are the details. When set to a 25% grade (14.0°):

  • The gym treadmill is actually 22.17% (12.5°);
  • My home treadmill is actually 24.75% (13.9°); and
  • The difference between the two is 2.58% (24.75 – 22.17).
  • 2.58 / 22.17 = 11.6%

To make it easier, let’s compare climb rates at a constant speed. Let’s say that the two treadmills each appeared to be set at a 25% grade and 4.0 kmh. As described above, the actual grades are less than 25%. Let’s assume that the speed is accurate. In this case, the climb rates would be:

  • 4 kmh = 4,000 meters per hour (mhr); so
  • 4,000 x 22.17% = ~887 meters per hour on the gym treadmill; but
  • 4,000 x 24.75% = ~990 meters per hour on my home treadmill;
  • ((990 / 887) – 1) x 100 = 11.6%; so
  • Despite appearing to be set to the same grade and speed, my home treadmill is demanding a pace 11.6% faster.

So no wonder I thought my performance was sucking. Despite more training volume and better fitness, my lactate tests were worse. I assumed I was just getting (more) old.

Now I know that my February 2016 test was overstated. By assuming that the gym treadmill was actually set at 25%, I felt like I was climbing faster than I actually was. My actual pace was much slower.

The Good News: Correcting my previous test

Next, I corrected the climb rates on my February 2016 test. The comparative results are more in line with what I’m feeling. I’m climbing better now than in 2016.

Graph of lactate test results before calibration
Lactate test results before treadmill calibration: My February 2016 lactate test (the blue line) is lower and further right than all of my 2018 tests (the cluster of red lines). If it were true, then my metabolic performance in February 2016 would have been better than it is currently, despite a higher training volume and better overall fitness.
Graph of lactate test results after calibration
Lactate test results after treadmill calibration: Once corrected for the difference in slope, my February 2016 lactate test (the blue line) is higher and further left than most of my 2018 tests (the cluster of red lines). As such, my metabolic performance now is better than it was in February 2016, which is in line with how I feel and other performance measures.

The Bad News: My previous pace was really exaggerated

The disappointing part is that my ~2mM climb rate was never as good as I thought. Reality sucks, I guess…

In February 2016, I loved thinking that my ~2mM climb rate at 25% was 1,200 mhr. That felt pretty respectable.

But now I see that I’ve never been that fit. My best ~2mM climb rate is where I’m at now: ~1,100 mhr. It’s still decent, but… Doh!

The takeaway: Measure to make sure

I realize now to never trust the speeds and grades that a piece of equipment displays. I’ll measure to make sure.

From now on, I’ll always calibrate equipment that I use for testing. This is especially true when using more than one device for the same purpose.

And it’s very important with climb rates. Small variations in gradient will create big differences. To compare apples to apples, it’s necessary to adjust for the slope of the test surface.

Posted in: endurance training