What does your resting heart rate, SpO2, REM sleep, etc. really show?

An important note in advance: Most wearables do not have medical certification - and we are not doctors either. If you are seriously worried about your health, you should always consult a doctor and not rely on articles online or the readings of a fitness tracker or smartwatch.

Contents:

1. Heart rate
   1. Why take your pulse?
      1. Resting Pulse
      2. Maximum heart rate (and VO2max)
      3. Training pulse
   2. Optical pulse measurement
   3. Electrical pulse measurement
   4. Electrocardiogram
2. Running and jogging
   1. Pedometer
   2. Cadence
   3. Ground contact time
   4. Vertical movement
3. Oxygen saturation SpO2
4. Sleep

1.1 Heart rate: why measure your pulse?

In the animal kingdom, heart rates vary between four and over one thousand beats (!) per minute. Healthy adults, on the other hand, fluctuate between 50 and 80-minute contractions - physical exertion left out, of course. But why are such values important at all?

The heart rate is an important indicator of the current load on the cardiovascular system. Resting heart rate and maximum heart rate or VO2max, on the other hand, allow you to draw conclusions about your general training status and to better plan your next endurance training session.

Resting Pulse

Resting pulse is the heart rate that the body needs to maintain its function without any physical or mental stress. Since the resting heart rate is measured while the body is virtually idle, it allows a good conclusion to be drawn about the training condition of the heart. The lower the resting heart rate, the better trained it is.

For example, endurance athletes can achieve resting pulses below 30 beats per minute. And even for amateur athletes, resting can be exciting to track training progress. There are also long-term studies where a lower resting heart rate indicates a lower risk of cardiovascular disease.

At the same time, your resting pulse provides valuable information on a daily basis. If the body is struggling with stress or an infection (possibly still symptom-free), your resting heart rate is often increased. So if you notice an increased resting heart rate in your fitness app, you should perhaps go a little easier during training in order not to put additional strain on your body.

At this point, again a hint: the above consideration of the resting pulse rate is only valid for healthy people. There is also the clinical picture of bradycardia, where the heartbeat is abnormally low, which then manifests itself in the form of dizziness or shortness of breath. Consult a doctor if in doubt.

Maximum heart rate (and VO2max)

For the maximum heart rate, there is the much-cited and much-discussed formula according to Fox: 220 minus age gives the maximum possible heart rate. But outside of this are body size, individual genetics, the altitude of the training location, and so on.

As an alternative to this and other rules of thumb, the maximum heart rate can also be determined by means of performance diagnostics or self-testing. Since the self-tests are logically maximally strenuous, you should exercise caution here, especially if you are overweight, older than 40 years, or have previous illnesses.

The exciting thing about the maximum heart rate is that it can be used to determine different training zones that correspond to different load levels. While I don't believe in the much-discussed "fat-burning pulse", training in a staggered per pulse range according to "endurance" and "load" is quite useful.

An alternative to the maximum heart rate is VO2max, the body's maximum oxygen uptake capacity. In hobby sports, this is estimated by the so-called Cooper test. Here the test person tries to cover the maximum possible distance within twelve minutes.

Several Garmin heart rate monitors, for example, interpolate the VO2max value based on heart rate and distance traveled without the subject having to reach his or her load limit. Depending on the training system, the VO2max value can be used as an indicator of fitness or to determine load ranges for training.

Training pulse

Why should you measure your heart rate during training? There are several answers to this simple question.

For endurance sports such as running or cycling, the pulse rate can be used to control the intensity of the training. With training in the higher pulse ranges, for example, you train the maximum load - ergo your speed. Therefore you will often find an interval training in the HIIT training plans, in which phases with high and low load respective to heart rate alternate, whereas in lower heart rate ranges you rather train the metabolism and the long-term endurance.

The training heart rate also provides an indication of your energy requirements during training - and thus your daily calorie requirements - which is interesting for those who want to lose or gain weight or have problems maintaining their weight.

If you want to build up a calorie balance in order to gain or lose weight in a targeted way, you must also record the calories you have consumed. For this purpose we have also a further article for you:

• The best calorie counter apps: why keeping track is so important!

1.2 Optical pulse measurement: what is to be considered?

Whether it costs $25 or $500, it flashes green everywhere. Almost all fitness trackers and most smartwatches nowadays have a pulse sensor on the bottom. Most sensors work with a photoplethysmogram. Since red blood cells absorb light of the complementary color green best, green light is usually used. A photocell then measures the wave-like contractions of the blood vessels and deduces the heart rate of the wearer.

However, the pulse measurement on the wrist has two major problems. First, it works best on light, hairless skin. Those with dark skin and very hairy arms have to live with limitations in accuracy.

Second, precise heart rate measurement requires free wrists. While optical pulse sensors provide precise results when cycling or jogging, deviations of 30 to 40 percent are not uncommon in yoga, HIIT with Burpees, or strength training. This applies to all activities with bent or very tense hands or wrists. The alternative to optical pulse measurement is then:

1.3 Electrical pulse measurement: chest straps measure better

They may be uncomfortable - or simply not as incredibly practical as a pulse sensor that you wear on your wrist all day anyway - but chest straps are much more accurate. They measure the electrical impulses from the heart directly on the chest with the help of two electrodes.

This type of measurement is not only very precise, but also works independently of all kinds of twisting of the various extremities, and recently electric pulse belts for the upper arm have also become available, for example from Polar or Wahoo. If you find chest straps uncomfortable, you might be happy with these.

If you are interested in a chest strap, you are facing a compatibility problem. Not every wearable ecosystem supports its own or other pulse belts. For example, there is currently no way to use a chest strap together with the Fitbit app. Other manufacturers are more open: the Huawei Health App, for example, supports the in-house fitness trackers as well as Bluetooth chest straps from Polar or Suunto.

Another compatibility problem concerns the wireless standard. In addition to Bluetooth, ANT+ is also widely used for heart rate belts (and other fitness accessories), which is supported by various heart rate monitors but not all smartphones. So be sure to inform yourself before you buy the corresponding accessories. Many mobile phones from Samsung, for example, can support ANT+, but for iPhones, you will not find the standard in the datasheet.

What now: optical or electrical pulse measurement?

In summary, both methods of pulse measurement offer their advantages and disadvantages. The optical pulse measurement is convenient and excellent for capturing the pulse throughout the day. The results are sufficiently precise for endurance sports such as jogging or cycling. If you want to be more precise or if you regularly do sports that put a lot of strain on your arms, electrical pulse measurement is the instrument of choice. Plus point: many chest straps also record other key data, such as the vertical movement during running and the time spent on the ground. But more about that later.

1.4 ECG via a smartwatch: does that work?

Apart from the two pulse measurement methods mentioned above, another type of electrical heart rate measurement has recently been discovered: the electrocardiogram. Here, too, the heart rate is recorded with two electrodes. This method is currently supported by the Apple Watch Series 4 and the Withings Move ECG.

Whether Apple or Withings, the number one electrode is always on the smartwatch. Electrode number two provides the other hand with which the user must touch the watch case during the measurement. There are also special ECG devices that connect to the smartphone via Bluetooth.

The goal is always the same: to detect potentially dangerous conditions like atrial fibrillation or cardiac arrhythmia. In the course of this, I would like to share with you an interesting test that colleagues from ComputerBILD (German) carried out together with doctors.

2.1 Counting steps: 10,000 as a measure of things?

Admittedly, February 21, 2015, was a few years ago, and the algorithms are certainly a few generations ahead of their time. But on that day I was wearing two trackers on the same wrist - one from Fitbit and one from Withings. As a result, the Fitbit tracker had almost 70 percent more steps on the watch that night.

In reality, however, both numbers are probably equally far off the mark, but that does not matter. After all, fitness trackers continue to count even when washing up or chopping wood - and thus measure activity rather than the steps actually taken. And that is exactly what it all comes down to!

Google Fit, for example, has been counting not only the steps but also the activity minutes since 2018, which offers a differentiated picture. Polar sports watches, on the other hand, record (in addition to the steps) different activity intensities. After a day on the couch, for example, the daily goal can be achieved with a demanding HIIT workout - or with many small activities spread out over the day.

Whether as activity minutes or steps, it is important that you find a comparable parameter for yourself to find your activity level. The goal is then to improve on your "past self".

If you are curious and want to measure your daily activity, here you will find our selection of the best pedometer apps:

• The best pedometer apps for Android

2.2 Cadence when running

From steps to cadence: most of the better running watches record the cadence while running, and increasingly this value can also be found in cheaper fitness trackers, such as the €35 Honor Band 5. A lower cadence means a relatively longer stride length - and thus more strain on joints and tendons.

So try to take smaller steps rather than fewer larger steps when running. If you run with music, here's a trick: build a playlist with songs whose BPM is at your target cadence and run in time. Don't overstrain yourself right at the start, though. If you are currently running at 140 steps per minute, the frequent recommendation of 180 steps per minute is certainly too high. Better to increase slowly and listen to your body!

2.3 Ground contact time when running

I already mentioned it at the beginning of the topic of electrical heart rate measurement: apart from the heart rate, modern chest straps also record further data, which is especially exciting when running. The HRM Run from Garmin, for example, also measures the ground contact time while running.

The ground contact time is a good tool to increase your running efficiency. The longer your feet are on the ground, the more energy you lose. A good way to start reducing the contact time is to place your feet directly under your body when running - and not in front of it.

2.4 Vertical movement while running

What counts when jogging? Right, you want to get ahead. Accordingly, running watches such as fitness trackers increasingly include vertical movement in their program. The less you bounce up when running, the more energy flows into your propulsion.

The vertical movement while running is difficult to capture with trackers on your wrist. The movement of the forearms when running is virtually impossible to calculate out of the total movement. Therefore the total vertical movement of the body when running is also part of the portfolio of more advanced chest straps. Like the time of contact with the ground, it is one of the parameters of effective running training.

3. oxygen saturation: What is the SpO2 value?

Especially in connection with the coronavirus, oxygen saturation in the blood was repeatedly mentioned. This so-called SpO2 value is also recorded by various fitness trackers, including even inexpensive models such as the Honor Band 5. In healthy people, oxygen saturation is in the range of 90 to 100 percent, lower values indicate acute lung disease or altitude sickness.

But now the question arises: how reliable are the values of the fitness trackers, especially at a value that indicates a potentially life-threatening situation? In the course of a corona test, my oxygen saturation was measured with a professional pulse oximeter, and a short time later I determined a similar value with the Honor Band 5 at almost 100 percent. But what does that say?

Until there is a scientific test, I would not rely on the SpO2 values - especially since manufacturers such as Fitbit always talk about estimates and thus withdraw from their responsibility. Or have you already come across a meaningful test for this?

4. How do fitness trackers measure sleep?

Whether you want to take the fitness tracker with you to bed is certainly a matter of taste. Those who can sleep with the big block on their arm will be rewarded with sleep data. Usually, the fitness trackers and smartwatches distinguish between light and deep sleep and indicate waking phases in which you have been disturbed, for example. Many trackers also record REM sleep during which the dream phases take place, and the fitness trackers now measure the transitions between the different sleep phases using movement data and heart rate.

What do I do with the sleep data?

Especially if you do not sleep well and want to work on your sleep quality, long-term sleep data are very interesting. For example, stop using your smartphone in bed at night for a month. Or don't eat for two weeks after 20:00 and watch how your sleep changes.

Good sleep is worth a lot, and the sleep data gives you great feedback, for example, to work on your sleep hygiene. Some apps even give you concrete suggestions on how to improve certain aspects of your sleep.

In my personal experience - after parallel tests with fitness trackers worn at the same time - the recorded sleep data is mostly congruent.

That's all?

I hope we've been able to shed some light on the quantified self chaos. Have we forgotten an important measurement? The various body fat scales and their measurements will be dealt with in detail elsewhere. Which of your body values do you regularly record or which would you like to record? We are looking forward to your comments!