Thursday, December 9, 2010

Heart Rate Drift and Rate of Perceived Exertion

I'm what you might call a bit of a data junkie, but curiously enough I normally don't use a heart rate (HR) monitor. Let me explain.

On my road bike (or Computrainer stationary trainer), I have a powermeter that measures my output wattage (or workload). On the treadmill, I simply run at a fixed speed. For a given workout, I set the target wattage or treadmill speed manually and doggedly stick to it. Based on the rate of perceived exertion (RPE), I adjust the workout if necessary for next time. The HR falls wherever it may.

But that was before I acquired an iPod Nano 6G back in September (see blog post here). The 6th generation Nano not only records foot pod data but also can simultaneously record HR. Since then, I've been wearing my HR strap on treadmill workouts.

I ran twice earlier this week (Monday and Wednesday) on a treadmill in a very large, climate-controlled gym. There is a completely predictable and repeatable 20 beats-per-minute (bpm) rise in HR over the 50 min or so endurance run. See how remarkably similar the two profiles look below.

[Conditions: The treadmill speed was fixed to a lowly 7.0 mph (11.25 km/h) throughout each entire workout. (There is also a 5 min warm-up before the start that's not shown here.) The climate conditions in the gym were unchanging and perfect throughout. Also, I took a sip of water every 5 minutes like clockwork.]

So the HR drifts. Observable phenomenon confirmed. But what about perceived exertion?

Well, perceived exertion is actually also another pretty dodgy fish. Perhaps it's generally correlated with breathing rate in running but I'm not sure. But initially, I've noticed my RPE is relatively high during the first 10 minutes (despite the 5 min warm-up). I don't feel efficient. But I'm not breathing hard. After that, the RPE actually drops despite the HR steadily drifting upwards. I feel I am in the groove, so to speak. Nearer the end of the workout, I've noticed when my HR hits 168 bpm or so, I start to feel the RPE go up again. However, in neither workout did I exercise to exhaustion, or have to reduce the treadmill speed to accommodate fatigue.

So what can I conclude? If I go by RPE, I'd be adjusting the treadmill speed down, then back up and back down again. If I go by HR, I'd be steadily reducing treadmill speed as the workout proceeds. In the end, I think all of this is really just data for data's sake. There's a lot to say for just ignoring these indicators, beloved of coaches and exercise physiologists, i.e. leave those two up and down buttons alone, and just doing the damn workout and burn your way through those 700 kcal.


  1. I've observed the same phenomenon on the bike using HR. Really, though, its not about running/biking at a constant HR but in a constant HR zone. For me an HR of 159 or 161 like you show is just a hair below my LT threshold. I might hold my HR in a bike session that targets that zone for 20 min, then recover in a lower zone and repeat the 20 min interval with 15 min recovery. Once in the zone RPE feels lower, and as I stretch to maintain the interval near the end (especially of the second interval) it feels harder. In truth as a result, I use my HR more to keep me from going too hard (exceeding the upper limit) than to entice a harder effort. If I go out for an aerobic ride, I don't want to hit 159bpm except maybe on the hills. This is especially important on recovery rides. - Alan

  2. Thanks for the comments, Alan! I agree a HR limit can be a useful tool to put a check (or ceiling) on aerobic effort.