Balancing the information acquired from Virtual Power or a power meter with that which you obtain from the myriad heart rate recording methods available nowadays can be a bit confusing.
In this article, we'll try to help you understand some of the more frequent power/heart rate mismatches by explaining some power and HR feedback fundamentals and then explaining some ways in which the two interact.
It's important to keep in mind that heart rate readings can fluctuate on a daily basis. General fatigue, incipient illness, day-to-day stress, dehydration, quality and quantity of sleep, caffeine intake, training stress and other factors can influence your heart rate.
In the case of impending illness an elevated heart rate can even be used as a warning sign to back things off a bit, maybe rest a bit more, but again is an illustration of how sensitive HR is to numerous internal and external factors.
These demonstrations of why HR-based training is problematic are not to imply that it's without merit.
Athletes who know their bodies and have years of training history can often associate their perceived levels of exertion (RPE) with HR abnormalities and circumvent training errors, but these athletes are in the minority.
For most riders, HR becomes more usable when associated with power output. This too involves a learning process but one that's more straightforward than the RPE/HR relation.
You've probably heard the phrase, "Watts is watts", and while statements like these are often maddeningly obvious and meaningless (not to mention grammatically incorrect), this one effectively pushes the point that you can't cheat power numbers.
One day you can hit 300 watts and the next day you can't. You're seldom left mystified by the host of potential bodily and environmental influences that might be affecting your power, rather you just know you're tired.
But what happens when you're heart rate tells you one thing but your power indicates another?
If your workouts, especially workouts near and above FTP, just don't seem very demanding then it's likely that you have an underestimated FTP. Further evidence of this too-low FTP value are evident when your HR doesn't rise to the expected level.
Underperforming on an FTP test is not uncommon; in fact, I've yet to meet a rider who nailed his/her first FTP assessment. Due to this assessment learning curve, many first-time or early testers won't perform to the height of their abilities.
As a result, their FTP is underestimated and consequently, their HR is lower than expected at nearly all work levels.
Indications You Underperformed
If the workout's intensity classification indicates a gut-wrenching set of intervals, e.g. Intense or Insane, yet you're sailing through them wondering when things will get challenging, you've probably underestimated your FTP.
If your next reassessment grows by leaps and bounds, you probably underestimated your FTP the first time around and things are about to get a lot more demanding with your new, more accurate FTP estimate.
Often, riders finish an initial assessment and feel like they still have a lot of gas left in the tank, or they'll start the assessment interval at a much higher power output than they finish with.
In both cases, the FTP estimate is likely to be low, probably too low to raise the subsequent workouts to the appropriate effort level and this can have a minor to major effect on the training response you derive from a later set of workouts or even an entire training plan.
What If I Held Back Too Much?
Riders who have underestimated their capabilities during their FTP test have two options.
Option 1 is to retest and work harder.
Option 2 is to incrementally increase your FTP by 5-10 watts over the course of your next couple/few rides until your HR (assuming you're rested and fresh) is more in line with your LTHR and associated percentages.
A third option for riders new to training with power is quite a bit more work but lends itself really well to making your training as effective as possible. By using the first ride of each week during the first two to three weeks to do an FTP test, you'll better learn to evenly pace your effort over the course of 8 or 20 minutes.
Discrepancies between your actual FTP and the FTP your tests yield can be mitigated by learning how to properly pace and push yourself during the extent of a test interval. This is something that comes with experience, and doing an FTP test at the top of each week for two to three weeks would help bring about such skills.
Other Common HR/Power Discrepancies
If you notice a heart rate that's below what it should be based on the power you're putting out, this is a good indication you're overreached. But overreaching is a necessary component of improvement and it only becomes a problem when it doesn't relent.
You can only overreach for so long before, worst case, you start dangerously venturing into overtraining territory or, best case, you continue to train a tired body and fail to see any noticeable improvements in fitness. Time to rest.
If workouts that once seemed impossible or at least very demanding are becoming more tolerable and maybe even a little too manageable, you're right on track but it's time to consider your next assessment.
For example, your power may be right on track but your HR is a little lower than you'd expect it to be, you're breathing a little more easily and another minute isn't quite as intimidating as it once was. This is the difference between improvement and overreaching too far.
In both cases, your HR is lower than expected but in the case of fitness improvement, your power is on track, i.e. steady graphs and on-target numbers. But with with overreaching, your power output is erratic or falling and you can't seem to hit your numbers as consistently as you once did.
Learning these subtle differences can be the difference between increasing and decreasing your training load, and as a result, can be the difference between building and losing fitness.