Treadmill Calibration Run

Chose a light regimen today, figuring on about 40 minutes of easy paced running and a lap or two around the weights circuit. I did a little experimenting with the run portion, comparing distance measurements on 3 of the 4 treadmill brands available at Pure Fitness Carlsbad.

I broke the the run up into 3 segments of 1.5 miles each, run at a speed of 6.5 (or 9:13/mile pace). I tried hard to remember details from each treadmill and I'll try to recount them now.

First, was the Precor model. After settling in, I counted my stride rate for 6.5 mph and I was consistently at 172 strides per minute. My heart rate at that intensity was steady at 130. Nike+ was measuring my pace at around 8:20/mile, though it fluctuated. It would cycle between 8:00 and 8:40. It never read out 9:00 or greater though, which is what the treadmill said I was doing. I tried to lengthen my stride to see if I could get Nike+ to "slow down" but it felt unnatural and I just kept my gait at what felt right. I was able to count strides per 1/4 mile, which turned out to be between 396 and 400. Nike+ reached 1.5 miles a little ahead of the treadmill. I had to go an additional 1/10 of a mile on the treadmill, which equates to an error of 6.7% which isn't bad.

A 9:13 pace is 1 mile in 553 seconds. Nike+ pace readout was 8:20, or 500 seconds/mile, or 53 seconds/mile difference. That's about 9% error. If the distance measurement error was really 6.7%, then Nike+ should have been calculating my pace at 8:35. Maybe it was on average and I was just noting the faster pace readout.

I quickly moved to the Star Trac treadmill and got started on my 2nd 1.5 mile segment, at the same 6.5 speed. My stride rate was still 172-173 SPM and my heartrate was steady at 130, just as before. Nike+ had me finishing up the 1.5 mile distance when the Star Trac said I still had .15 miles to go. Nike+ was also indicating I was running a little quicker than Precor, despite the speed setting being the same. On Star Trac, Nike+ was reading out 8:10 or so per mile. I also counted my strides per quarter mile, and on the Star Trac machine, I was taking a few more strides to complete the quarter: 400-404.

Finally, I moved quickly over to the new Free Motion treadmills. Same speed setting of 6.5. I noticed immediately that the incline felt different. I had the Precor and Star Trac inclines set to 0. The Free Motion was set to 0 also, but felt like it was uphill slightly. I could see in a wall mirror that it did look like a little default incline, which must be Free Motion's way of automatically compensating for the nature of the treadmill. I'll have to look that up later and see if I'm right.

But to compare apples with apples, I slewed the incline into a negative setting of -3%, which seemed to me to be level and commensurate with the 0 settings on the other two. Still, at 6.5, I felt like I was having to run slightly faster and my HR suggested the same, edging up to 135 and slightly more toward the end. This could be just cardiac drift since I was going over 30 minutes total by now.

My stride rate was quicker, at 175+ strides per minute. Nike+ was also detecting a quicker speed, measuring me out at about 8:00/mile and even a few seconds under. I counted 400-410 strides per quarter mile based on the treadmill's measurement of distance, but Nike+ had me finishing up 1.5 miles with over 2/10 of a mile left to go on the treadmill.

So...what does all this mean?

Nike+ isn't my measuring stick since I know from experience it can be off by 10%, depending on my pace, hills, fatigue level, etc. It's just measuring foot fall and foot acceleration, estimating distance based on how I've calibrated my stride rate and stride length, both of which vary. But I don't know which of the treadmills is more true. I can line them up from slow to fast:

1) Precor
2) Star Trac
3) Free Motion

If you ask me, the Precor delta from the Nike+ was closest in line with what I experience on a track or level course. Free Motion definitely felt like it was running faster than indicated. If I were to do this again, perhaps I should reverse the order, because maybe my perception and stride rate were affected by fatigue. I doubt that though because 6.5 on all the machines was a pretty leisurely pace.

The 3 Nautilus treadmills were unavailable for comparison: one was broken and the other 2 were being used.

Fun test, and nice way to occupy my mind during a short, easy run. I'm doing 7 miles tomorrow, and incorporated the 5K neighborhood race into it, so I want to be a fresh and recovered from yesterday's hill work as possible. I'll take a day off from running on Friday and then do 16.5 miles on Saturday morning.

I managed to work in a single set of upper body circuit training, but I did go up 10% in resistance, able to produce 12 reps on all exercises except the delt fly and the inclined press.

Physically, I'm feeling good. Ankles are much better. Not perfect, but much, much improved.