This is the second post in a new series where Chad answers training questions.  This question came in through

I have a question on this topic:  How much do you think a persons goal would affect whether they use power, virtual power, or HR?

If ones goal was to maximize results in a time trial, I could see being more interested in building power.  However, if the goal is more endurance related (such as in a mid-distance or longer triathlon), I’d think that HR would be the measurement of choice to ensure staying in an aerobic zone (except for those cases where you might intentionally want to go above that).

So when taking HR drift into account, my concern would be that a longer interval designed to push back lactate threshold would end up getting well into the anaerobic zone for the sake of maintaining power, which would start to defeat the purpose of the workout.

Just curious if there is something about power training that I may be missing, and that perhaps my concern is unfounded.


Chad’s Response:

Heart rate is a useful metric mostly in terms of recovery. A responsive heart rate, one that quickly reaches LTHR during LT intervals, for instance, and then promptly returns to a pre-interval HR indicates an adequate level of recovery while one that is slight retarded – maybe 5-10 bpm – during the work interval and then recedes slowly during the rest interval  is a good indicator of fatigue. A consistently lower-than-LTHR heart rate during said LT intervals can lend insight into a rider’s ability to recover from previous days’ workouts in that further recovery – perhaps an additional day – will bring the HR back within its usual bounds. But, pushing forward at this lower HR can potentially set a rider up for eventual overtraining if carried too far. In a case like this, a rider can use HR to recognize continued fatigue and adjust his training schedule (or more specifically, recovery schedule) in order to reap a better return on the workouts by performing them in a more recovered state.

On the other side of this coin, a rider might interpret this lower-than-usual HR as an immediate need for further recovery and skip what could prove to be a breakthrough, or at the very least, valuable workout and reduce the workload (or skip the workout altogether) when recovery wasn’t necessary and the further training stimulus could have brought with it a new level of post-recovery performance. Remember, we achieve new heights by breaking old boundaries, and pushing through fatigue is often one of those boundaries. Just because you’re tired and your HR is a little low doesn’t always mean you won’t have a great workout in terms of meeting your workout requirements, i.e. power output/duration/repetition.

In the case of this low HR, power would have shown a rider that his capabilities were being pushed but not completely overwhelmed assuming he could hit his targeted power numbers. Had he been shooting for a targe HR, this same workout would have been incredibly hard because his level of fatigue would have required suprathreshold efforts to elevate his HR to only threshold levels, assuming he could reach them at all – the ultimate in HR-training frustration. So HR would have expressed fatigue, but power would have shown that this level of fatigue wasn’t so high that it would prevent a productive workout.

HR alone shows how hard our bodies are working but power shows how much work we’re doing. Couple them and we can see how much work even somewhat fatigued bodies are capable of and allow us to achieve a very high level of training stimulus, one that is very likely higher than what we’re accustomed to when using HR as the defining metric. This greater stimulus will lead to higher performance capacity following adequate recovery.

With this in mind, your goal should be to maximize training stiumlus in order to maximize performance capability, all this regardless of the type of event you’re training for. Being faster for a 40k TT means that you’ll be faster for 112-mile bike leg of an IM, plain and simple. So why not make the most out of your training sessions by using all available metrics, i.e. HR, power, and RPE?

Finally, I think you might have the idea of cardiac drift backwards. As long intervals and workouts progress, HR climbs for the same level of output due to fatigue and/or dehydration. So in your example you would actually start to work at subthreshold output levels because your HR would be climbing due to fatigue/dehydration, you’d consequently reduce your effort to bring your HR back down to LTHR, and you’d then be training at a lower than targeted work level and potentially miss some/much of your workout’s intended stress.

So in response to your question, power would tell you that you’re too tired to hold LT power during that day’s workout and steer you toward modifying (steady state, sweet spot) or skipping your workout whereas HR alone could force you into overdoing the intervals by putting out suprathreshold wattage in an effort to achieve threshold HR with a fatigued body.

-Chad T

“Just because I say it doesn’t make it so – be a skeptic.”