Low Heart Rate Training: The Two-Month Mark

So, a quick recap:

• Once upon a time I used to base train properly, ie, running at a very slow, easy pace for hours and hours each week in order to develop my aerobic system, before jumping into race training with speed & tempo & race pace work & what have you.
• In the last few years I've gotten away from that practice, partly out of impatience ("But I want to run this race! And this one! And that one! And wheeeeee!") and partly due to letting a couple of years of setting PR after PR mostly on the strength of my anaerobic system lull me into a false sense of security. (Pro Tip: You can get away with this for a while but it is not really a viable strategy long-term, particularly if you want to run marathons.)
• Since I decided in September to devote some serious time to not racing & instead trying to fix my freakish right hip, it seemed like a good time to also get back to base training.
• Because I'm really good at lying to myself about what a truly easy effort feels like, I've gone back to using a heart rate monitor to keep me honest.

I started this business officially on Sept. 21, mainly just aiming to slow myself down to a completely easy, comfortable pace. After a few days it seemed like most of the time that correlated to heart rates in the 140s, so for the most part I just tried to stick to that. After each run, I would record my average pace and heart rate, following basically the same route every time to try to minimize any variation because of terrain. (I just turn around sooner for shorter runs, and the serious hills are all close to the beginning.)

With aerobic base training, there are two ways to tell you're making progress. Either you can run at the same pace and watch what your heart rate does (hopefully decrease with time), or you can maintain the same average heart rate and watch what your pace does (hopefully get faster). Basically, you want to see that you're accomplishing more with less work.

Now, if I could manage to pull off EXACTLY the same average pace or EXACTLY the same average heart rate on very run, then it would be easy to see whether or not I was improving. Since neither of those things are very easy, though, you end up with data that looks like this:

AVERAGE PACE & HEART RATE, BASE TRAINING MONTH 1

Because both variables jump around so much from day to day, it's hard to draw conclusions just by looking at the numbers. Clearly running at a 10:32 pace with a heart rate of 142 is better than running at a 10:48 pace with a heart rate of 152, but what about 10:32 & 142 vs 10:21 & 150? To make sense of it, I needed some mathematical way of comparing runs with different heart rates AND different paces and determining which was "better."

Luckily, this is exactly the type of thing I am paid to do in my day job. :)

First, here is a quick-and-dirty recap of how base training is supposed to work.

(***Remember, I am a mathematician, not a kinesiologist, so I won't embarrass myself by trying to explain the finer details. There are plenty of good explanations on the internet written by people who are actually experts at this stuff. On a related note, experts, please don't be shy if I have completely misconstrued something.***)

You can google all the biochemical details of how & why it works, but basically, when you spend hours and hours and hours each week bathing your cells in oxygen at a nice, easy effort level, a couple of things happen:

1) Your heart becomes more efficient at pumping blood. That is, you improve your "stroke volume"--how much blood your heart pumps out with each beat or "stroke." Stroke volume is important because the aerobic system relies on oxygen, so greater stroke volume = more oxygen delivered to cells more quickly with less work by your heart.

2) Your body becomes more efficient at using the oxygen you breathe in. That is, you improve your "running economy"--converting the same amount of oxygen into more forward motion. Part of this has to do with delivering more oxygen more quickly (more red blood cells, more myoglobin, etc.) and part of it has to do with cells using the oxygen they get more efficiently (more and bigger mitochondria, more enzymes for metabolizing fat efficiently, etc.)

When you improve both of these things, the result is generating more forward motion with fewer heartbeats. So it seemed to me that the question I should be asking here is, How much forward motion am I generating per heartbeat?

Stick with me; here be equations. I promise they are not too painful.

The variables I have are pace (minutes per mile) and average heart rate (beats per minute). Since pace tells me speed in terms of time rather than in terms of distance, I first converted minutes per mile to miles per minute.

Through a little dimensional analysis, it's easy to show that dividing "miles per minute" by "beats per minute" is the same as "miles per beat" -- ie, how much forward motion is generated with each heart beat, on average.

Now, I had a way of objectively comparing two different runs where neither the heart rate nor the pace were the same and determining on which run my body had been more efficient.

Here's a table listing each run since Sept. 21 for which I have data, along with its pace, heart rate, and "miles per beat":

Then I graphed the table:

Pretty hard to argue with.

A (mathematically calculated) line of best fit makes the exact nature of the trend a little more obvious:

This graph shows that, over time, I was covering more and more ground per heartbeat, on average.

For all that the trend here is pretty clear, it's important to note that if I'd only shown you the data for the first month, it wouldn't have looked nearly as convincing:

You can kinda-sorta see which way the wind is blowing, but
it's not nearly as convincing because there's so much variation day-to-day.

The reason for this is that average heart rate can be affected by a lot of different factors from day to day, like temperature, sleep, blood sugar, stress/mood, medication, etc. For that reason, you really need a longer period of time (Pete Pfitzinger says eight weeks is kind of the minimum) in order to see the trend. You just aren't going to see a nice, neat gain from day to day or even week to week because all of the noise from the factors above. In the long term, though, they'll wash out enough to see the trend.

I'll be honest with you, it is kind of a relief to run those numbers and see real, actual, tangible progress. Yes, I believe in science and (mostly) understand the biology, but there was definitely a small part of me that was all, "OH GOD WHAT IF IT DOESN'T WORK ANYMORE/THIS TIME???" Like I've said before it takes patience, discipline, and consistency to slow yourself down, waaaay down, & go week after week without necessarily seeing obvious improvement in the numbers. But it does work!

Eight weeks is really as long as I have ever formally base trained before, so I'm now in uncharted waters. Though I am starting to add some GMP miles, I'm still doing 85-90% of my miles at a super easy pace, so it will be interesting to see for how much longer I continue to make progress aerobically before the numbers plateau.