There are many, many (MANY) possible answers to this question. It all boils down to knowing where you are in your training, what the goal of the particular session is, and which zones you are targeting. Before we dive into some of the more common scenarios, let’s first assume that you have your thresholds and zones set accurately and recently, through a run field test, an FTP test, or lactate threshold or VO2 max lab testing. If you are using the default zones on your watch, they are likely not accurate for you.
Your training will include sessions based on different types of zones at different times, and for different reasons. This is why communication with your coach is important, so you understand what the goal of the session is and can react accordingly if things aren’t going according to plan. For example, if you’re early in your training, you are likely spending some time developing your aerobic base. That involves a good deal of time in “Zone 2,” or “Aerobic” zones. In that case, if your heart rate starts to drift up, you’ll want to slow down to try to bring it back down into zone. Same for recovery sessions. If you go too hard on your easy days, you run the risk of not being able to go hard enough on the hard days to elicit the physiological adaptation that you’re aiming for. So a “too high” heart rate on a recovery session would warrant slowing down. (Of course there are exceptions. Keep reading.)
Let’s look at another situation. If you are in a speed building training block, you may have interval sessions that target pace zones, or perhaps power zones on the bike. In those cases, you don’t want to be concerned with your heart rate, and instead focus on the pace/power prescribed. If you can’t quite hit those zones, or perhaps you are overachieving them, that’s feedback that you and your coach can use to draw conclusions about your fitness and decide how to adapt. Let’s say for example you are doing mile repeats on the track at a threshold pace, but you feel like the RPE was a 3. That might suggest that your speed training is paying off, and that you are ready for a faster pace for that effort.
Keep in mind the limitations of different kinds of zones. Here’s where we get to the exceptions. Pace and power are straightforward measurements of speed or force, but heart rate is actually the physiological response to the work that you are doing. And it can be affected by many other factors: heat, humidity, hydration, nutrition, sleep, menstrual cycles, immune system responses, stress, etc. So on a day when it’s 98 degrees, for example, you might not be able to keep your recovery run in your recovery heart rate zone no matter how much you slow down.
That’s when RPE (rate of perceived exertion) comes in handy. In that same example, on your 98 degree recovery run, if your heart rate is too high but you feel like the effort level is a solid RPE 1, then you might speculate that it’s the heat elevating your heart rate. In other instances when there is a disconnect between the data and the RPE, it can help you and your coach ask the right questions to try to get to the bottom of what might be going on. (Which is why Training Peaks includes the RPE meter for you to fill out after every session!) Also, sometimes technology fails. Batteries die. Power meters go on the fritz. Having an internal barometer of your effort level can be helpful in all these cases as well.
As you can see, knowing what to do when you can’t stay in your zones does not have a simple explanation. But having a good understanding of the different types of thresholds and zones, and paying attention to the specific goals of each training block and training session, can help you determine whether and how to react when your session is not going as planned.