Power/Heart Rate Analysis

Heart rate is not usually a very good indicator of how you are working on the bike because it is so variable.  The variation comes from the fact that your heart rate depends on many factors of your whole body, including sleep, stress, nutrition, time of day, how hard the effort is, etc.  In other words, there is a lot of variation and error to the heart rate numbers.  My guess is heart rate can be as much as +/- 5%.   This can make HR changes irrelevant in a lot of cases. There is no such variation in power, because it directly measures the effort you are putting on the pedals.

But outside of this 5% errror, I think that HR is relevant.  I figure heart rate can tell you something if there is a big enough change.   Some coaches will tell you that HR is worthless no matter what, but I say it is one more piece to the puzzle! I’m not a coach, as that sentence might imply, btw.  Below is a chart with a semi-relevant example.  The green is a 10 min interval I did at the end of my 2011 season going into my last race.  I averaged 316 normalized power, with a HR average of 161 bpm.  Compare this the yellow, which is an interval I did after 3 weeks off the bike, averaging 253 watts at a HR of 165 bpm.  Ideally, the power averages would be the same so we could compare one variable, but I couldn’t find a better example!  Just keep in mind that Green was higher watts at lower HR.

HR vs Power before and after Break

This change is pretty drastic and I think over 5%.  It’s hard to say for sure because of the change in wattage.  If it was just a few bpm change at the same wattage, it would probably be statistically insignificant and irrelevant.  But this is a BIG change, especially when you take into account that my wattage on the yellow interval was much lower.  Pretty much, it shows I lost some fitness, and my heart was having to work harder even though I was at a lower wattage.  I was expecting this after 3 weeks of no riding.

It could also be that variation in HR is more like 25% depending on things like diet, sleep, and fatigue, and this example is worthless.  If this is the case, I would argue that all HR data is too inaccurate to really use well.  But I will carry on assuming this is not the case.

A lesson for people without a power meter: don’t freak out over a few bpm change.  If you are doing a loop at a given speed, and one  workout your HR is 165, and the next time your heart rate is 170 for the same speed, it’s nothing.  You are within your body’s error for the same effort (assuming course conditions are similar).  If the heart rate change is much more drastic, then it could be something more.  Just remember to use a BIG range when dealing with HR, because so many outside influences can affect it.  For instance using a big range for threshold HR (155-170 bpm) is much better than saying, “my threshold HR is 165 exactly.”

Feel free to disagree completely.  And thanks for reading as I reason myself in circles.