Many runners who take their sport seriously have a heart rate monitor. They know their heart rate zones, and they know where they perform best. This allows them to train and measure their progression in a targeted manner.
Why is heart rate monitoring so popular with runners?
In 1982, Polar was the first company to come up with a watch for a wide audience that could measure heart rate. With smartphones, Fitbits, heart rate monitors, pedometers, and the Apple Watch, we now have endless brands and devices with which we can measure heart rate, steps, speed, sleep, activity, and stress. But in the early eighties, Polar was actually the first to launch a watch with which you could collect data from your own body. And the data you could collect was heart rate. With a strap around your chest and a watch connected to it, you could see what your heart rate was while you were running.
Many researchers and exercise physiologists used these devices to conduct research on topics such as the relationship between heart rate and fuel consumption. We learned that at a low heart rate you use your fats relatively more, and that at higher heart rates you use your glycogen stores more (for an explanation of glycogen, see box). The relationship between heart rate and lactic acid was extensively explored, and the tipping point became a value that, as an avid runner, you simply had to know. The tipping point (or anaerobic threshold, AT point) is the heart rate at which you produce more lactic acid than you break down. Lactic acid is some- times mistakenly seen only as a waste product. What not everyone knows, is that you also produce lactic acid with light exertion, and that in low doses, lactic acid is used by the heart as fuel. It’s only when you produce more lactic acid than you can use, that it starts to become an issue. As lactic acid accumulates, you start to lose performance capability, and you use your glycogen stores to the fullest. You can maintain this intensity for about an hour.
ANAEROBIC THRESHOLD AND FUEL
Is the anaerobic threshold a new concept for you?
We’ll briefly explain what it is and why it’s so valuable for a runner to know about it. If you exercise more intensively, your muscles need more oxygen because energy must be released faster. That’s why your heart rate goes up with physical exertion: your heart pumps more oxygen to your muscles. If you accelerate after a gentle warm up, your heart rate will increase, as will your breathing frequency and tidal volume. In the beginning you will breathe deeper, but not faster. There will automatically come a point (if you keep increasing intensity) at which you can no longer breathe deeper, but your thighs do require more oxygen. That’s when you start to breathe faster. This is the moment when the training stimulus starts. At your aerobic threshold, and above this effort, you will build up fitness. It’s called the aerobic threshold because when you’re taking in a lot of oxygen, you can mainly use your fats as fuel. You can maintain this intensity for between four to six hours. If you keep increasing intensity beyond the aerobic threshold, you will reach your anaerobic threshold. You breathe less deeply and exponentially faster, lactic acid accumulates, and you can maintain this effort for about an hour while mainly burning glycogen.
You have four fuels to draw from: ATP, Creatine Phosphate, Glycogen, and Fat. Glycogen and fat are particularly important for endurance athletes. You can run quite a few marathons with your fats as fuel. The big advantage of fat is that you can take a lot of energy with you, even if you have a relatively low weight. Even if you have a fat percentage of 8% (which is very low) and you weigh 70 kilos (154 lbs), you still have 5.6 kilos (12.3 lbs) of fat, good for more than six marathons.
A disadvantage of fat is that it does provide energy, but the energy is released slowly. A lot of oxygen is needed, and you don’t get the energy directly. So if you run slowly, you can use your fats, but if you increase intensity, you also need another fuel: your glycogen. You have about 500 grams (1.1 lbs) of your glycogen stored in your muscles and in your liver. These 500 grams equal approximately 2000 kCal.
Every runner has enough fat to live on for weeks. We sometimes joke that you can easily find out exactly for how long you can persist with fuel stored in your fat stores: stop eating, and then wait until you die. That’s how long. You can go without food for more than 40 days, which shows that you have fuel that you can really use for a long time. A property of fats is that they are energy efficient and last a long time. When you sit in a chair and you’re relaxed, you mainly use your energy-efficient fats. When you go for a run, you also use your energy-fast sugars.
You should not confuse these sugars with the sugars in sweets or sports drinks. It’s a general term that includes slower carbohydrates. In exercise physiology, these sugars are called glycogen stores. This glycogen is stored in your liver and around your muscles. A well trained runner can run on maximum glycogen use for about one and a half to two hours. However, it’s not true that the combustion of one fuel stops before the other continues. In other words, you’re never running on only fat or only glycogen. And, fat always contributes. As you keep adding intensity, more muscle fibers are involved that “eat” something else.
So the idea that one system is switched on (glycogen) and that your other system (fats) no longer participates, is not the case. The fact that oxygen uptake increases proportionally with increasing load provides evidence that both systems remain active. If the “fat burners in your muscles” (which use the most oxygen) stop, the oxygen uptake would no longer increase linearly. Oxygen uptake does increase linearly, showing that fat burning does indeed continue throughout.
Okay. But how is this knowledge useful?
Because you always have more fat than glycogen, as a runner, you want to achieve two things with training:
1. Run as fast as possible on your energy-efficient fats
2. Store as much glycogen as possible in your liver and muscles.
FUN FACT: a kilo (2.2 lbs) of fat is good for 9000 kcal. So, if you weigh 70 kg (154 lbs) and you have a body fat percentage of 20%, you have 14 kilos of fat x 9000 kcal = 126,000 kcal fat. You can store about 500 grams of glycogen, which is about 2000 kcal. With this knowledge, you immediately know why many training schedules emphasize that variation is important. You want to train your muscles to run efficiently on fats, and you want to stimulate your muscles to store glycogen.
Many heart rate monitors therefore work with zones based on this tipping point.
There are three main zones:
- Very easy, little training effect. It’s about recovery. For your body, fuel consumption is comparable to sitting on the couch: not much happens. In terms of training, however, it’s a part of your schedule that should not be underestimated. In this zone, you run, but you don’t build up fatigue or damage your muscles, tendons, ligaments, and joints.
- If you exercise more intensively, you will reach the aerobic threshold. This is where the training effect begins.
- If you keep increasing intensity, you will arrive at your lactate acid turning point, and you will no longer be able to maintain your power. This is the anaerobic threshold.
Though we describe 3 zones here, you will often see 5 zones in heart rate training because 3 zones are distinguished between the aerobic threshold and the anaerobic threshold (low, medium, high) and there is also a separate zone above your turning point.
You can enter your own heart rate zones when training by heart rate. This way, you’ll have (reliable) values that you already know. Even if you don’t enter zones, you’ll still see different zones from your heart rate monitor.
Note: this is unreliable. If you take your new heart rate monitor out of the box and you turn it on, you’ll have to answer some questions. What language do you speak? What time is it? Do you want a 12h or 24h time format? Are you male or female? Do you wear your watch on the left or right? How much do you weigh? What’s your year of birth? And that’s where it goes wrong. Your watch determines your maximum heart rate based on your age. There’s a standard formula that determines your maximum heart rate as follows: 220 minus your age.
Based on this invented maximum heart rate, the monitor will determine your zones. This is a shame. Because, for many runners, this standard formula does not apply at all. Suppose, you’re 45 years old and you have an actual maximum heart rate of 195 (which is not surprising at all). Your watch says 220 minus your age (45) = a maximum heart rate of 175. If you then train in your zones in a targeted manner, you’ll become quite annoyed. Because, every time you run smoothly, your heart rate monitor starts to beep that you should slow down. In this way, you structurally train too carefully, and at a certain point you’ll no longer make progress.
That’s a shame. You can do better.
Heart rate measurement has taught us a lot in the last 40 years. And with a reliable heart rate monitor and sufficient knowledge, you can train on flat terrain as long as you don’t do short intervals. In some cases, however, a heart rate monitor is not suitable. For example, with your interval training of 200 or 400 meters. By the time your heart rate is high, you’re already at the end. Running uphill is another issue when training by heart rate. Your pace drops, your heart rate shoots up, and your results are hard to compare to your flat training laps. But the biggest “danger” of heart rate training is an inaccurately measured heart rate. Many major brands have switched from measurements via a strap around the chest to a wrist monitor, and those are still far from accurate for everyone. Runners who often suffer from cold hands generally get inaccurately low heart rate values from a wrist monitor. Ron himself has a different experience. His wrist monitor indicates values that are too high.
If you look at the physiology of a person, it actually makes much more sense to measure breaths, instead of heart rate.
The three stages we mentioned earlier can be registered faster and more clearly by looking at your breathing. When you get up from the couch and go outside to run, you start to breathe deeper. So, the first zone is that you take deeper breaths, without breathing faster. At the aerobic threshold, your thighs ask for more oxygen and you breathe faster, as we described in the box about aerobic threshold. You keep increasing intensity, so you’ll breathe faster and faster, with the same tidal volume until you reach your tipping point, the anaerobic threshold. At this point, you can no longer deepen your breathing. Because you still need more oxygen, you’ll breathe faster, but more shallowly.
Physiologically, your breathing responds faster than your heart rate, which makes it ideal for targeted training. It’s just that, back in 1982, it was easier for the company Polar to measure heart rate via a belt than to measure respiratory frequency and tidal volume, so they decided to go with the heart rate monitor.
Well before heart rate monitors emerged, enthusiastic runners trained by pace.
Funnily enough, training by pace in the 1970s and 1980s was very reliable, but nowadays that’s no longer the case. How is that possible? Fifty years ago there were no watches that determined your speed via GPS. So, if you started training focused on pace, you were forced to train with a stopwatch and to know your distance very precisely. On a running track you could calculate exactly how fast you had to run your 200, 400 or 1000 meters to train at a certain pace. That way of tempo training is of course still reliable and still popular with track training. With your fastest time at a certain distance (3, 5 or 10 kilometers,) you can calculate what your possible times are at other distances, and you can also determine what your intensive interval paces or your leisurely endurance runs are.
So, what’s the problem?
Many runners have a Garmin, Polar, or COROS and train by pace using their watch as a compass. Unfortunately, this way of measuring is not always reliable because the watch bases your speed on GPS data. Your watch is connected to satellites and uses the distance between the different position measuring points to know how fast you took to go from point A to point B.
Tall buildings, trees with wet leaves, winding roads, or not enough connected satellites can all contribute to GPS measurement errors. Pace based on GPS varies from moment to moment and is not very useful. Of course, the measurements are more stable over longer distances because deviations average out.
Therefore, if you run a marathon, your distance will always be around 42.195 km (26.2 miles),although you’re not always running the ideal line and your watch is always slightly off. We all know that when training on a track and using Strava it sometimes seems as if you have cut straight across the middle area. That’s simply because your watch combined two “satellite” points and missed the curve in between.
Another disadvantage of running with a heart rate monitor is that your heart rate responds slowly. When you run up a hill, your muscles immediately use more energy, but your heart rate takes time to notice that your muscles need more oxygen and it needs to pump faster. So, if you walk up a hill with a constant heart rate, you have to work hard for the first part and you have to walk very slowly for the second part.
That is one of the great advantages of training with Stryd. You train by power and Stryd does not depend on GPS or heart rate, but measures your power with accelerometers on your foot. And that turns out to be extremely reliable.
The breakthrough in power running came with the use of inertial measurement units (IMUs). We prefer to call them accelerometers. These are small instruments in a chip that can be used to measure accelerations. The measuring principle is based on the fact that the crystals in the chip produce a piezoelectric effect under the influence of an acceleration. This piezoelectric effect results in a voltage that can be measured. The Stryd chip accurately measures this voltage more than 100 times per second, which makes the device ingenious and reliable.
“Ehhhh, Ron and Hans, I really don’t know what you’re saying now. Plain human language please."
* Laughter * “Okay, we have a great every day example.”
Thanks to an accelerometer, your mobile phone knows whether you are holding it horizontally or vertically. If you watch a video on YouTube and you tilt your phone, the image on your screen will tilt. With the same type of technology, your running watch knows your cadence and the number of steps.
Accelerometers today are very cheap, very accurate, and they are found in all kinds of devices, such as smartphones, cars, tablets, pedometers and running watches. A smart power meter uses this technology to determine your speed and stride cadence. And that turns out to be much more reliable than GPS. The Stryd power meter is currently leading the way in converting this technology into reliable speed (and power) measurements for runners.
The sensor includes 6 accelerometers. These measure the acceleration of your body while running in 3 directions: horizontal, vertical, and lateral/sideways. Obviously, in running, it’s important to limit vertical and lateral movements, as this consumes energy that doesn’t contribute to forward displacement. Everyone has a certain optimal economic step frequency and technique. With Stryd you can determine which running technique suits you best. As mentioned, Stryd takes measurements many times per second, which makes the accuracy of the device very precise. And Stryd doesn’t just measure your movement from side to side, top to bottom, and your speed forward, but also air pressure, temperature, and humidity.
These measurements, combined with your weight and height, together with Stryd’s well thought-out algorithms, accurately reflect your power. When you run, you can see your power (in watts) via your smartphone, your Apple Watch or your running watch. Power (P) is calculated from your weight (m) (in kg), the measured acceleration (a) (in m / s2, in 3 directions), the speed (v) (in m / s), and the air resistance with the basic formulas:
F= m * a P = F* v
Stryd’s breakthrough is the software it has developed to calculate the power by using all data from the accelerometers continuously and in real time. As we saw, the basic formulas are simple but a complicated algorithm is needed to accurately calculate the power based on the accelerations in all directions.
The advantage of the Stryd footpod is that it gives you a pure and exact measurement of power in real time. This gives a much better and objective picture of your effort than your feeling, your speed, or your heart rate alone.
And the biggest advantage for runners who prefer simplicity, rather than to read complicated books about training: you only need to train with 1 number in mind. As long as you know which level of power you need for which training, that’s enough.
Why the wattages from Polar and Garmin are wrong
One of Koen’s running friends wanted to know what his expected time was on the half marathon. To calculate, he used the power of a fast 10K.
“I may not have gone all the way because I was running on my own, but I did my best anyway,” said Joost.
His time on the 10 kilometer was 44:15 and his power was 357 watts. So, Koen and Joost used 357 watts as his Critical Power and started calculating.
According to the formulas, after some calculations, Joost would end up with a half marathon of 1:16:34 ”. That can’t be true, said Koen.
“Did you enter your correct weight in Stryd?”
“I don’t have Stryd,” said Joost. “I measured it with my Polar.”
What did we find out? The power measured by Polar (and Garmin) is far too high. Hans and Ron have done several studies that showed that the wattages of Polar and Garmin were 25% to 35% higher than those of Stryd. The differences can be explained by the fact that Polar and Garmin derive their power readings from measurements with force plates in the lab. However, this doesn’t take into account the energy recovery in the muscles during the landing phase. Stryd bases calculations on the actual power required to move around while running, resulting in this difference of 25-35%. In addition, Polar and Garmin use GPS, which is much less accurate. Incidentally, the relatively new brand COROS does make use of the necessary power to move around while running. COROS has fully integrated the information from the Stryd Footpod. This makes the COROS watch a strong duo with Stryd. COROS can also measure power based on GPS, but this is less accurate, especially with changes in speed and course. COROS also doesn’t take into account the resistance from the wind. So for now, Stryd works best.
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