Playing with Heart Rate and Speed

After a grueling day of driving through 90 degree weather, my wife and baby were finally asleep. We had been driving from Bend to Portland all afternoon with a crying two year old who kept complaining that she was "stuck." It was hard to explain to her that she was supposed to be "stuck," the seatbelts keep her "stuck" in her car seat for her safety. She was having none of it.

I had hoped to do my long run earlier in the day, but temperature and family and the drive home made that impossible. So last night at 11:30 pm I strapped on my heart rate monitor, turned on the GPS and took my dog out for two hours and 10 minutes of "running."

I'm testing out a marathon training program and right now, at week three of a fourteen week program, I am supposed to keep my heart rate between 60% and 70% of my maximum heart rate. I walked for two hours last night with my heart rate bobbing between 113 and 132 beats per minute (bpm). For most runners, this is torture because it means that you're intentionally going "too slow" to keep your heart rate in the right range.

During the run I had a sticky thought that I couldn't get out of my mind. For the first two weeks of training, I kept my heart rate under 132 bpm and spent most of the time walking at about 4.1 miles per hour. (Real runners use pace to measure their speed, but no matter how hard I try, can't get my brain to think in minutes per mile). Last Thursday, I went out for my workout and during my ten minute warm-up I noticed something. I wasn't strolling. In fact, each time I walked at my normal warm-up speed of 3.0 miles per hour, my heart rate monitor started beeping to tell me that I needed to increase my heart rate. I did my ten minute warm-up at 3.8 miles per hour and the workout at 4.7 miles per hour, all while keeping my heart rate below 132 pbm.

"Of course," any experienced runner will say, "you're getting in better shape." From an Exercise Physiology standpoint, I've experienced Adaptation. For the same amount of exertion, my body doesn't have to work as hard because it is operating more efficiently. Adaptation occurs when your body is stressed at a high intensity, then given the proper amount of time to recover. But for me, that is not enough. I'm a "quant" as my friend Joe would call me. I like numbers. In fact, I love numbers. I want numbers and data even if it’s for something that I am not currently tracking because I might want to go back and look at the data later.

I kept trying to find a really easy to use number that would quickly show this increase in efficiency. After thinking about all the ratios I could think of, (calculating total heart beats for an exercise and then figuring out your beats per mile, calculating beats per hour and dividing by miles to find beats per hour per mile, and a bunch of other options that really wouldn't give me anything useful or easy to wrap my brain around) I arrived at what I think is a pretty good measure of efficiency based on the numbers that I have, Heart Rate and Speed.

Its' this:

beats per minute per mile per hour.

What? Yeah, it's ugly, but it gives you a really simple relative number to compare workouts and see if your efficiency is increasing. I think of it like fuel economy for a car, compared to its speed or range on a single gas tank. Sure, a Ford Aspire can drive at 85 miles per hour, but its fuel economy at that speed is crap and it is practically redlined the whole time. So if you take my workouts, (excluding the ten minute warm up and cool down from each workout) here are the numbers you get:

Date: HR Avg: Speed Avg: Efficiency Index:

7/10/2011 123 3.8 32.6

7/12/2011 126 2.5 49.8

7/14/2011 123 4.0 30.4

7/16/2011 123 3.8 32.5

7/17/2011 125 4.0 31.5

7/19/2011 127 3.7 34.1

7/21/2011 129 4.4 29.4

7/23/2011 127 4.1 31.1

7/24/2011 128 4.3 29.5

Over time, the efficiency index keeps dropping. I look forward to seeing how this progresses. Sure it's not efficiency as a measure of Vo2 max, lactate threshold or work done. But it does provide a crude but simple index to see efficiency with the technology that most athletes have available, namely HR monitor and a GPS (either paired with the HR monitor or in their smart phone).

I'm going to keep an eye on this index and see where it takes me.