Getting Nerdy With Fitness Trackers
by Mark Braithwaite, for Shockoe Bottom Performance

Technology has come a long way in fitness tracking. The new Apple watch series 6 can measure blood oxygen levels and perform an electrocardiogram (ECG). In addition to what has been measured in previous models and other fitness trackers (heart rate, steps, standing, distance, sleep), that’s pretty impressive. 

There are tons of wearable reviews out there. So, instead of weighing in on which one is better, I decided to geek out on several of the metrics they provide and share some interesting information. 

Steps

We’ve come a long way from the old school pedometers that were worn on the hip to count steps. Virtually every device these days comes with an accelerometer. Counting your steps may seem archaic to some, but don’t negate this metric just yet. According to an article in the Journal of the American Medical Association, a greater number of daily steps was significantly associated with lower all-cause mortality [1]. So the next time your work/family/friends offer up a step tracking challenge, take it on in stride. 

Heart Rate

Heart rate is commonly used in the fitness world as a means to maximize a workout. There are some fitness businesses out there in which their entire model is built around it. Getting into and staying in certain HR zones is said to not only burn more calories during the bout of physical activity, but burn more afterwards. Now, what energy your body is burning at what intensity or time can be complicated, but it looks like maximal fat oxidation happens between 60-80% of one’s max HR [2]. 

On a different note, HR monitoring is extremely helpful information when used in training and racing/competing. Using running as an example; your average mile pace might be a 6:45, but when there are variables like the condition your body is in, weather, altitude, etc. they can have a significant impact on your ability to hold that pace. If you are going to use HR as a tool for training, racing, or competing, I would highly recommend a chest strap monitor. My experience with those was that they are much more consistent and accurate no matter the activity (running, cycling, swimming,etc.). I will note that there was a 2017 study in which several wrist-worn trackers were tested for HR accuracy, where most of them had a median error of less than 5% for measuring HR [3]. They noted that BMI, skin tone, and sex impacted the accuracy of the reading, as did the activity. One factor to consider is the different technology used between a chest strap and a wrist tracker. Chest straps pick up the electrical signals of the heart and wrist trackers use a light based technology to pick up blood volume in skin tissue. 

Sleep

We spend about 1/3 of our life sleeping, or at least trying to sleep. In my opinion, this is one of the most important metrics that can be tracked. With 1 in 3 Americans not getting enough sleep (according to the CDC), it’s one that we need to work on and be conscious of, as a whole. There is a slew of acute and chronic health issues that can arise out of lack of sleep. 

Speaking to athletes and fitness enthusiasts, I’m sure you are well aware of the importance of sleep for recovery to achieve your goals. I noted a study in my last post on the importance of adequate sleep for building and maintaining muscle strength [4]. In addition, when spring rolls back around and you start preparing that beach body, lack of sleep can compromise your efforts in leaning up [5]. If you’re putting in the hard work, make the effort to hit that sweet spot of 7-8 hours. 

Another factor to note is sleep consistency. Studies have shown that inconsistent sleep patterns can be just as detrimental as not getting enough sleep [6]. I know it can be tough, but try and hit the sack the same time each night and get up the same time each morning. Yes, even on the weekends. 

That nifty device on your wrist tracks your sleep, so take some time and review that data. If you notice bouts of sleep disturbances or less time in those deep sleep zones, it might give you some insight on underlying health issues like sleep apnea or help you realize and deal with a stressful situation you’re going through. 

Side note: I found that using a wearable as an alarm clock is rather nice. No sound, just vibration, is not as startling of a way to wake up. 

Heart Rate Variability (HRV)

Measuring HRV is one of the more recent additions to wearables. Where HR monitors measure beats per minute, HRV monitors measures in between beats. HRV variation is impacted by the autonomic nervous system (ANS), which regulates our body’s reaction to different stimuli. In addition to providing information on the impact of stress vs rest, this information is said to be a viable way to measure impact of physical training and recovery [7]. Generally, a higher HRV is a sign of the body being recovered and primed to go and an increasing HRV is a positive adaptation to physical training. 

I found an article in the Behavioral Pharmacology journal in which HRV has shown to be a viable biomarker for recognizing a slew of health disorders [8]. It states that a lower HRV is associated with the development of diabetes, cardiovascular disease, inflammation, obesity and psychiatric disorders. Before you freak out about a low HRV, remember that it is a measurement controlled by your autonomic nervous system (ANS), which is impacted by different stimuli to the body. Stress (mental and on the body), lack of sleep, poor eating habits, and sedentary lifestyle impacts the ANS, which are all factors of those disorders, which you probably already know. It’s similar to high blood pressure being a viable biomarker for recognizing the risk of a heart attack or stroke. 

Now comes the next question, what’s a normal HRV? That depends, and this is where we dive in   deep. There are several ways in which HRV is calculated and, depending on what device you have, what is normal is different. From what I can find, Apple calculates by using the standard deviation of the inter-beat (RR) intervals between normal heartbeats (SSDN). WHOOP calculates by using the root mean square of successive differences between heartbeats (RMSSD). In addition, it’s not as simple of a metric as something like blood pressure, where if you’re under 120/80, it’s generally thought of as normal. I found an article in the Frontiers of Physiology journal and plugged their numbers into a spreadsheet to spit out a graph [9]. 

Original data for this chart – Table 3 – Front Physiol. 2018; 9: 424

Original data for this chart – Table 4 – Front Physiol. 2018; 9: 424

As noted, HRV is not something easily measured. The variation of your HRV from day to day could be dramatic. I know with my own experience, I had measurements as high as 97 and as low as 19. What you’re looking for is your average HRV, which is usually provided in the app of whichever wearable you are using. 

Final Thoughts

You have a mini research lab strapped to your wrist that’s collecting tons of data day in and day out. We get so tied up and busy shuffling through the weeks that we can often overlook what’s going on internally. Hopefully this article helped add a little more insight and appreciation for all that data your device is catching and use it to optimize your everyday life! 

References:

1. Saint-Maurice PF, Troiano RP, Bassett DR Jr, Graubard BI, Carlson SA, Shiroma EJ, Fulton JE, Matthews CE. Association of Daily Step Count and Step Intensity With Mortality Among US Adults. JAMA. 2020 Mar 24;323(12):1151-1160. doi: 10.1001/jama.2020.1382. PMID: 32207799; PMCID: PMC7093766.
2. Carey DG. Quantifying differences in the “fat burning” zone and the aerobic zone: implications for training. J Strength Cond Res. 2009 Oct;23(7):2090-5. doi: 10.1519/JSC.0b013e3181bac5c5. PMID: 19855335.
3. Shcherbina, A.; Mattsson, C.M.; Waggott, D.; Salisbury, H.; Christle, J.W.; Hastie, T.; Wheeler, M.T.; Ashley, E.A. Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort. J. Pers. Med. 2017, 7, 3.
4. Chen Y, Cui Y, Chen S, Wu Z. Relationship between sleep and muscle strength among Chinese university students: a cross-sectional study. J Musculoskelet Neuronal Interact. 2017 Dec 1;17(4):327-333. PMID: 29199194; PMCID: PMC5749041.
5. Nedeltcheva AV, Kilkus JM, Imperial J, Schoeller DA, Penev PD. Insufficient sleep undermines dietary efforts to reduce adiposity. Ann Intern Med. 2010 Oct 5;153(7):435-41. doi: 10.7326/0003-4819-153-7-201010050-00006. PMID: 20921542; PMCID: PMC2951287.
6. Huang T, Redline S. Cross-sectional and Prospective Associations of Actigraphy-Assessed Sleep Regularity With Metabolic Abnormalities: The Multi-Ethnic Study of Atherosclerosis. Diabetes Care. 2019 Aug;42(8):1422-1429. doi: 10.2337/dc19-0596. Epub 2019 Jun 5. PMID: 31167888; PMCID: PMC6647049.
7. Dong JG. The role of heart rate variability in sports physiology. Exp Ther Med. 2016 May;11(5):1531-1536. doi: 10.3892/etm.2016.3104. Epub 2016 Feb 23. PMID: 27168768; PMCID: PMC4840584.
8. Thamm A, Freitag N, Figueiredo P, Doma K, Rottensteiner C, Bloch W, Schumann M. Can Heart Rate Variability Determine Recovery Following Distinct Strength Loadings? A Randomized Cross-Over Trial. Int J Environ Res Public Health. 2019 Nov 7;16(22):4353. doi: 10.3390/ijerph16224353. PMID: 31703468; PMCID: PMC6888606.
9. Young HA, Benton D. Heart-rate variability: a biomarker to study the influence of nutrition on physiological and psychological health? Behav Pharmacol. 2018 Apr;29(2 and 3-Spec Issue):140-151. doi: 10.1097/FBP.0000000000000383. PMID: 29543648; PMCID: PMC5882295.
10. van den Berg ME, Rijnbeek PR, Niemeijer MN, Hofman A, van Herpen G, Bots ML, Hillege H, Swenne CA, Eijgelsheim M, Stricker BH, Kors JA. Normal Values of Corrected Heart-Rate Variability in 10-Second Electrocardiograms for All Ages. Front Physiol. 2018 Apr 27;9:424. doi: 10.3389/fphys.2018.00424. Erratum in: Front Physiol. 2019 Nov 01;10:1373. PMID: 29755366; PMCID: PMC5934689.

Mark Braithwaite

Mark has competed in Bodybuilding, Triathlon, CrossFit, Weightlifting, and Strongman. He is dedicated to maintaining health and overall wellness while navigating the day to day grind and enjoys helping others along their journey.