The following is a question from a coach. He and the athlete he coaches are somewhat frustrated by the difference inpower numbers they produce relative to heart rate when riding indoors as opposed to out on the road. This is a common complaint this time of year, one that I hear a lot.
QUESTION FROM THE COACH: I'm hoping to find an answer or an explanation to an athlete's FTP [functional threshold power] testing results.
Here's the 411…. the cyclist is a a masters racer… 44 years old, has been cycling for the last 6 years. Whenever he conducts a FTP test outside he is posting better 20 minute numbers vs a test on a trainer. I looked at HR and that piece of the data is the same. We have tried a multitude of things to improve his "trainer" FTP (to match his outside FTP), such as fans, using the trainer outside, music…. but there still seems to be a difference. His prior coach actually had two FTP values for him depending on if he was doing a trainer workout or an outside workout.
Here are the numbers from a past test. I had him do a test on a trainer resulting in 258W and an average HR of 174. He was frustrated at his low number so 10 minutes later he went outside and put down a 278W with 175 HR.
MY ANSWER: This is a common question every winter. A lot of riders experience this. But I’ve also heard from some who have exactly the opposite problem—better numbers indoors than out. So it remains a mystery. There are a couple of possibilities that may explain it, but they are rather lame I’ll have to admit. (I'm assuming the same power meter and bike were used on both tests.
One is that when riding on the road there are brief episodes of “recovery” due to slight hills, cornering and changes in wind direction. These little changes don’t occur indoors on a trainer where power is usually much steadier. Some research shows that these little “mini-recoveries” allow the rider to go harder overall. In fact, I’m told that when going for an hour record on the track they sometimes train to be “unsteady” in power output order to take advantage of this little oddity.
The next is that when riding on the road the bike actually rocks from side to side slightly while the body remains relatively steady over it. On the trainer it’s the opposite – the bike is locked in place so the rider’s body moves slightly side to side while pedaling. This requires a somewhat different technique and muscle recruitment and so may elevate HR and effort “artificially.”
It sounds like you have already eliminated the third one – that heat build-up elevates HR when riding indoors more so than when on the road. Lots of fans help here.
I’d also be interested in seeing if the rider paced the two TTs the same way. In other words, did he negative or positive split the two trials in the same manner? Someone may be more adapted to riding on the road than the trainer and be much more unsure of how to pace themselves indoors. Or the other way around.
Could one of these explain a 7% difference? Probably not. Could some combination of them account for that? Possibly.
Bottom line is that I really don’t know.