TL;DR: When I tell you to do 5 minutes of “Zone 5” I do NOT mean that your goal is to get your heart rate into Zone 5 for 5 minutes. Your goal is to stay set your power/pace/output to zone 5, and let your heart rate do what it will.
Zones
In order to allow us to survive in a large array of situations, our body has evolved multiple systems to supply energy to our muscles, allowing us to perform many different activities at many different speeds and for very different amounts of time. There are three different energy systems we use, and they’re all working at least a little all the time, but the ratio of each changes as we change the intensity (easy vs moderate vs hard). The external manifestation of the interaction between energy systems is described by the Power/Pace Duration Curve (PDC).
This graphic shows what we intuitively know, which is that the faster/hard you go, the shorter the time that you can maintain that speed. This makes sense, right? If you’re on a treadmill at a fast speed, you can only stay on so long. If you’re going at a slower speed, you can stay on the treadmill (ignoring boredom) for longer.
In order to get faster or more durable in the exact ways that we want to, it’s important to target specific ratios of energy systems, and since the PDC is a representation of the energy systems, you could say that for training it’s important to target different areas of the PDC. For this reason sports scientists, coaches, and athletes commonly use a framework called either Zones or Levels. This allows athletes and coaches to simply and easily speak the same language around energy system ratios or areas of the PDC. The simplest way to make zones is to simply use the “Lactate Threshold” or LT2, or OBLA, or whatever you want to call it (I’m just going to call it “threshold”) and split the PDC into two zones. One zone is easy, one zone is hard.
If we want to be more precise in talking about our training (which we do), we want to slice our zones up finer, so that we can speak with more nuance about how long we can hold on to the treadmill, and by association what’s happening inside our bodies vis a vis our energy systems.
In this five-zone model, you can see that things haven’t fundamentally changed, you still have zones above threshold, and you still have zones below threshold, but you’ve sliced them up finer so that we can be more specific about describing or prescribing your intensity.
Note that the two-zone system above uses “Zone 2” to talk about one ratio of systems, and the five-zone system uses “Zone 2” to talk about a completely different ratio of energy systems! The important thing is simply to make sure that you and your plan or coach (or that video you’re watching on youtube) are speaking about the same area of the PDC (and thus the same energy zones) when speaking about zones!
Input vs Output
Up until this point, we’ve talked about intensity as a single thing. The human body, however, is anything but simple. There are many ways to measure intensity, and largely we talk about them in two main categories. First, we can talk about “Output” metrics of intensity. This is something like pace or power. It’s how fast you’re moving, it’s the RESULT of your effort. Additionally, we can talk about “Input” metrics. These are things like heart rate, lactate, or VO2. These aren’t measures of the result, they’re measuring how hard your body is working to GET that result. In a perfect world, all the input metrics correlate perfectly with the output metrics: If you’re in Zone 5 in heart rate, you’re in Zone 5 by power. Unfortunately this isn’t always the case!
As you can see from this workout (5 times five minutes of Zone 5, with four minutes of Zone 1 between), the heart rate (red) and power (purple) curves ARE NOT THE SAME SHAPE. The power (output) changes instantly, but the heart rate (input) takes time. In the first couple reps, the heart rate doesn’t get to Zone 5 at all, and in the last couple reps, it’s only in Zone 5 for the last 1-2 minutes of the rep.
This is called “Heart Rate Lag.” Your heart rate will ALWAYS lag behind high intensity efforts. It’s slow to rise, and slow to lower.
When training, most of the gains we want to make correlate with OUTPUT, not with INPUT. Sure sometimes we want to hit specific heart rates, but 95+% of the time, we want to hit the outputs! This means that the workout above IS A NEARLY PERFECT WORKOUT. This is what a zone 5 workout should look like. Nail the output, let the inputs do what they will. This is the way to get the MAXIMUM amount of time spent at Zone 5 power, which is what we want.
In contrast, here’s an athlete trying to “hit zone 5 heart rate.” See the HUGE spikes in power trying to get his heart rate up, and then being unable to sustain it, and just DYING for the whole rest of the workout. This athlete, in this workout, NEVER HIT ZONE 5 HEART RATE, and indeed spent fairly little time in Zone 5 power. He wore out his legs before his heart could even get pumping.
SO, when I give you a zone to work in, I want you in the OUTPUT zone, and we’ll watch and see what the input zone is. Especially during intervals, I mostly don’t want you looking at HR at all, and certainly not for the first three minutes of each effort!
Now, if you’re in a sport that doesn’t have measurable outputs (most sports), this is a little tricky! It’s going to take time for us to calibrate your “internal power meter,” so that you can feel the output. If you’re a cyclist, we might spend a little time on the trainer first, if you’re a runner/skier, we might spend some time on the treadmill first.
In Summary: When I tell you to do 5 minutes of “Zone 5” I do NOT mean that your goal is to get your heart rate into Zone 5. If it happens, that’s great, and that gives us a lot of good information, but that shouldn’t be the metric that you act upon in the workout. Instead, I want you to keep steady output for 5 minutes, and if you’re at the right output, the HR will end up in zone 5!
