I have been training indoors on my trainer since January first, all of that time with my trainer at a consistent setting, and recording speed, heart rate, and cadence. Because of this consistency, I figured there must be some way to assess how I’m improving. I have done FTP tests (estimating the “P”, since I’m not training with power), but these are only once per month or so. The common thing to measure if how your FTP increases, but with only two measurements this seems like a poor metric to measure.

So what I decided to analyze was my power output at low intensity. The reasons for this are two-fold. First, I felt that last season I had good anaerobic power, but my aerobic power was lacking. Thus this is an area that I’m keen to improve. Second is because I’ve been doing polarized training this winter, so I’ve spent a lot of time in zone 2, i.e. low-intensity aerobic. Therefore I have a lot of data to work with.

I’m sure that with more time and commitment I could analyze this data in a more precise, nuanced manner. However I just wanted to get a quick feeling for how things are trending. I chose to analyze my speed @ 120bpm. For reference my max hr is about 185bpm, and my current FTHR is around 165bpm. Now admittedly 120bpm seems a little low, and is not something I plan to spend a lot of time racing at. However I chose it because in both of my FTHR tests, 120bpm lied in my zone 2, where I spent a lot of time riding. Thus I have a lot of data to work with. For reference, 120bpm was near the top of Z2 at my first FTHR test, and at the bottom of Z2 in my second FTHR test.

So what I did was open up all my Z2 rides since January 1. Then I looked for a 10+ minute span where I was riding close to 120bpm, and then recorded my average speed. I made sure to avoid places where my hr/speed spiked, or where I was just warming up, or where the time interval was very short. This isn’t a perfect analysis, but it at least gives a “big picture” idea. So without further ado, here is my chart:

 What can be seen is a roughly positive linear correlation (the line is the least squares fit). I find this encouraging, because it means it’s “easier” for me to ride at higher speeds, or conversely, I get more speed for the same heart rate. In fact, this graph underestimates the improvement a bit. Ideally what I would do is not to fix my heart rate at 120bpm, but rather fix it at some fixed percentage of my FTHR. After my first FTHR test, 120bpm = 77% of FTHR. After my second FTHR test, 120bpm = 73% of FTHR. Thus the better thing to do would have been to perhaps keep the percentage at 77%, which would mean measuring speed at .77*SecondFTHR =  126bpm. With this change we would expect a steeper increase. The problem with that is that I only have measured my FTHR twice so far this winter, so I’m not so confident with the stability of those numbers.

In all I find this encouraging, and think it’s a simple way to check your fitness. Coaches and bloggers love to talk about their power curves and how they change with winter training. But for those of us that only have trainers, we have something very close to a power meter already.

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