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Heart Rate Monitoring


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Monitoring our heart rate while exercising can be an important tool. For athletes it can help to ensure you are getting the most out of your training sessions to improve performance and fitness level. For those who have an underlying cardiovascular condition, monitoring heart rate can ensure that they are staying within a safe intensity zone that won’t cause additional damage to their heart. In short, measuring heart rate can help us determine if we are pushing our body too hard or not hard enough.

When we are working out or exerting ourselves physically our muscles need more oxygen, the harder they are worked the more oxygen they need. The way oxygen gets to our working muscles is through the blood which relies on our heart for delivery. As a result, during a workout our heart pumps faster in order to supply the increased demand from our muscles.


Types of heart rate:

Resting heart rate = number of beats per minute while relaxed. The average resting heart rate for adults is 60-100bpm. Individuals who are very fit may have a resting heart rate that is much lower than 60bpm, which can be totally normal too! This is because their stroke volume has increased so there is more blood being pumped from the heart with each beat. This means that more oxygen is delivered to the muscles with each beat, so the heart must pump less often to supply the same amount of oxygen for the muscles.

Max heart rate = the highest number of beats per minute that your heart could reach. This is extremely hard to measure without very sophisticated testing so typically we estimate the maximum heart rate. To determine your max heart rate, subtract your age from 220. For example, a 40 year old would have a max heart rate of 180bpm. It is never a good idea to test this through exercise as you can seriously hurt yourself by pushing your body too hard.

Training heart rate = the rate you target during aerobic exercise for improving your fitness. Changing the percentage of your maximal heart rate that you are exercising at will alter the amounts of protein, fat or carbs you are burning for energy. Determining what “zone” you should be exercising in depends on your fitness goals as well as your current fitness level, for some people it may also be a good idea to change this goal frequently. It is important to speak with a healthcare practitioner to decide what percent of your maximal heart rate is an ideal target for you, before beginning any exercise program.

Below is a chart showing the different heart rate zones:


https://commons.wikimedia.org/wiki/File:Exercise_zones.png


Recovery heart rate- This is important for after your workout, the target is within 20 beats of your pre-workout

heart rate.


Heart rate monitors:

These days it is more common to see people with some sort of fitness tracker on their wrist than without. But how do they work and how effective can they be?

Wrist sensors use light to measure your heart rate in process called photoplethysmography (PPG). The LED lights that you often see flashing from the underside of someone’s watch illuminates the capillaries in your wrist and measures the reflection of light from flowing blood. Factors such as contact with the skin due to increased sweat or incorrect fit will alter the efficacy of the reading. This can be combated through wearing a correctly sized band tightly on your wrist, but not too tight! Wearing the tracker too tight can interact with the flow of blood and cause inaccurate measurements as well.

Fun fact: 1 study found that fingertip tracking, like you see on some new smartphones, was actually more accurate than a wrist monitor! This is likely because our fingertips are quite translucent and there is a superficial artery in the tip of our finger that makes the fingertip easier to read through PPG.

Chest straps measure electrical activity through a strap directly around your chest. The activity is then displayed on a connected wrist watch so that the wearer can easily monitor their heart rate. Unlike the wrist monitors, the chest strap needs moisture for the electrodes to pick up a signal. So, sweating while wearing it is necessary for an accurate reading.

But how accurate are they?

In one study by Stahl et al, participants wore 6 different wrist-based heart rate monitors simultaneously as well as a chest strap. They then had their resting heart rate measured, followed by walking and then running on a treadmill. The researchers found that the biggest margin of error occurred during the walking phase, with the resting and running phases being the most accurate (1). It was not clear why this was the case, but it appears that the most accurate heart rate measurement occurred at rest, while the participants created very little movement (1).

A study done through the Cleveland Clinic found that the chest strap monitors were the most accurate, especially for monitoring heart rate throughout activity(2). Regardless of the type and intensity of the exercise, the chest strap proved to be the most accurate(2). The researchers in this study found that the wrist worn devices had a margin of error ranging from 15 to 34 bpm above or below the actual heart rate, which is quite a wide range (2)! These devices were tested against an ECG that the patients were connected to at the same time. The Polar chest strap was tested to be within 1 bpm of EKG readings, making it very accurate and reliable. (4)

Based on consumer test reports it appears that the PPG science is getting better and better. With newer watches and fitness trackers like the Apple watch, Fitbit versa, Garmin and Striiv watches being more accurate than older versions (5, 6).



References:

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5117066/

2. https://www.acc.org/about-acc/press-releases/2017/03/08/14/02/wrist-worn-heart-rate-monitors-less-accurate-than-standard-chest-strap

3. https://arstechnica.com/gadgets/2017/04/how-wearable-heart-rate-monitors-work-and-which-is-best-for-you/

4. https://www.cnet.com/news/how-accurate-are-wristband-heart-rate-monitors/

5. https://www.tomsguide.com/us/heart-rate-monitor,review-2885.html

6. https://www.choice.com.au/health-and-body/diet-and-fitness/sportswear-and-shoes/articles/fitness-trackers-with-heart-rate-monitors-what-we-found