Every damn day I see you people walking into the gym with your fancy watches meant to measure heart rate, which supposedly correlates to how hard you’re working. And every damn day I tell you its crap but I had yet to sit down find enough data to support my argument – UNTIL NOW. To understand the problems with using your heart rate to calculate your “work zone” you need to understand Age-Predicted Maximum Heart Rate (APMHR), where the formula comes from, post-exercise oxygen consumption (EPOC), and the issues with calculating your max heart rate (HRMax),
Post-Exercise Oxygen Consumption (EPOC)
Let’s start with EPOC. The idea of EPOC is that if a person can keep a certain maximum heart rate during a workout, they will continue to burn calories long afterward through an oxygen-burning process – sometimes called the “afterburn” by GymBros. It’s the idea that your metabolism stays raised after you commit to all-out exercise. Exactly how long it lasts is a contentious issue and some scientists say that it lasts more like two to three hours, post-training.
A good example of how this theory is applied in exercise is Orangetheory (OTF). Now, let me get this out of the way. I am not bashing OTF (I am) – any exercise is better than sitting on your butt. However, the science behind their approach is not founded in any current scientific research. A typical OTF workout follows the high-intensity interval training (HIIT) format, which we’ve already determined is superior to continuous moderate exercise. If you remember, HIIT has moments of intense exercise and moments of rest or less physically demanding movement. This back-and-forth format is more effective in weight loss and building strength. A key component of OTF workouts is the heart rate monitor you wear on your arm, which measures your heart rate to determine how hard you’re working. Despite the wearable heart rate monitor having huge variation in the measurement, the define five heart rate zones to determine your work. The first zone is the easiest, while the fifth is the most intense and will probably feel uncomfortable. The infamous fourth zone is where they claim that if you spend 25 to 30 minutes “in the zone” and 12 to 20 minutes “in other zones” four or five you’ll see incredible results. The goal of the class is “to get into the orange zone’ — which is above 84 percent of your maximum heart rate — for at least 12 to 20 minutes of the hour-long workout. OTF outrageously claims that you can burn calories for up to 36 hours after you finish the class. Essentially, if you work out on a Monday morning, and hit your target for the HR zone, you could still reap the elevated metabolism benefits on Tuesday evening. As you will be wearing a heart rate monitor during your session, your workout summary — including your calories burned, your splat points, and a breakdown of how long you spent in each of the five zones — will be conveniently emailed to you in a report after every class. How nice.
Now enter the mythical “splat points” …. Splat points are points awarded for each minute you spent in the red or orange zones. Anything over 12 splat points is seen as a job well done and as further proof that your body will indeed continue to reap the rewards of EPOC and the afterburn. But it becomes harder to earn splat points as you exercise more regularly, as your body adapts, and it gets more difficult to maintain your time in the orange zone. READ THAT AGAIN! As your body gets stronger and in better shape its harder to achieve the heart rate that says you’re “in the zone”. Enter the measuring HRMax and the Age-Predicted Maximum Heart Rate (APMHR) Calculation.
How to Measure HRMax
The first issue is HOW TO MEASURE HEART RATE. Given the rise of the wearable smart watches that measure heart rate, it seems important to discuss the accuracy of these devices before moving on. In athletes, the preferred measurement method has always been a chest strap, which are generally more accurate than smartwatches. But watches are far more convenient (and trendy) than an ugly chest strap. Unfortunately, a recent extensive systematic review of the literature found that a lack of common validated methods for qualifying wearable devices (all the studies are using different metrics to evaluate the devices), therefore the literature and scientific data are incomplete and not comparable. The only conclusion that can be drawn from my review of the data is that ONLY chest monitors and other measurements techniques found in physiology labs can be considered accurate enough for even using these crappy calculations. Sorry but the HR showing up on your watch is essentially meaningless.
The most accurate way to determine HRMax is by maximum exercise testing and measurements from sophisticated equipment. Because this is not always practical or feasible for the general population, a heavy reliance on age-predictive formulas has ensued. However, this practice is not rooted in science and the dissemination of this misinformation needs to end!
The Crap Numbers Behind Age-Predicted Maximum Heart Rate Calculations
APMHR is considered an essential measure for healthcare professionals in determining cardiovascular response to exercise testing, exertion, and prescription. Although multiple APMHR prediction equations (Fox, Tanaka, Nikolaidis, among others) have been used for specific populations, the accuracy of each within a general population requires testing. The most common equations for calculating APMHR are given elow:
|Fox||HRMax = 220-age|
|Tanaka||HRmax = 208 – (0.7 x age)|
|Nikolaidis||HRmax = 223 – (1.44 x age)|
The Fox equation from 1971 (HRMax = 220-age), is unbelievably the most used common metric for measuring HRMax – despite it being based on invalid test results.
So how did the authors determine these equations? They performed large studies on various populations of people and collected their HRMax then analyzed the data to create these predicative equations. Most importantly, how did the authors determine the constants (220 in Fox) in the above equations? You would be shocked to know that this formula was never derived from original scientific evidence, but rather, from observation and compilation from other invalidated research sources. Subsequent validated studies showed Fox’s equation to be significantly inaccurate in some populations, hence the different models used to predict HRMax. There are numerous variables unaccounted for in all of these formulas. These include training experience, stress level, efficiency, adaptation level, hemoglobin content of the blood (oxygen carrying capacity), training phase, recovery status, health status, pre-existing health conditions, medications, and more. All of these can drastically affect heart rate and perceived exertion at a given exercise intensity each day. HRMax generally decreases with age and diminishing fitness levels and can also be influenced by gender – although that data is still inconclusive.
Numerous large studies have proven these equations do not correlate to real life. In one study of 4000 individuals on a treadmill, HRmax from 99 graded treadmill exercise tests (GXT) were measured in a laboratory using sophisticated equipment. Significant differences between measured and predicted HRmax were found. Statistical analysis revealed wide limits of agreement for all APMHR equations, suggesting poor agreement between measured and predicted HRmax. Another study of 3500 adults showed that the average standard error using any of the equations was up to 12 bpm. That is a huge error on top of the huge error measured by your smartwatch. Finally, a very recent study of 10,000 people concluded “from the current study in conjunction with the overall body of related literature, there appears to be clear consensus that one can expect the prediction error associated with APMHR using any of the currently available prediction equations with 95% CIs, to be approximately ±24bpm. These findings, both current and previous, all point toward significant limitations in the validity of APMHR in predicting the actual maximal HR response to exercise”
So what does this mean? It means that the prescribed heart rate intensities using APMHR models is inaccurate at best and damaging to your exercise progression at worst. Look at this example using John Doe, Age 45 and the most commonly used HRMax equation (Fox, HRMax = 220-age):
|Intensity||% HRMax||John Doe’s Predicted HRMax using FOX||John Doe’s ACTUAL HRMax|
|175 bpm||186 bpm|
|Easy Effort||65-79% HRMax||114 to 138 bpm||120 to 147 bpm|
|Moderate Effort||80-85% HRMax||140 to 149 bpm||149 to 158 bpm|
|Mod-High Effort||82-88% HRMax||144 to 154 bpm||153 to 164 bpm|
|High Effort||90-100% HRMax||158 to 175 bpm||167 to 186 bpm|
|All out Effort||97+% HRMax||170+ bpm||180+ bpm|
As we can see from comparing the actual versus the predicted numbers, the actual appropriate intensities for John Doe are staggered by nearly an entire pace level up, which represents a 6 bpm to 10 bpm difference from the predicted values. Using the reduced HRMax values in John Doe’s training technique is selling himself short. Since there is not an even distribution across all intensities, it isn’t safe to assume that all adaptations to training are relative to the initial measurements. Clearly the values are not directly proportional to one another. Furthermore, anything with a 6% error rate (or ±24bpm from a larger study) is hardly considered a scientific fact… yet, every fitness professional and even some medical professionals will tell you your HRMax is 220-age!
The whole idea of working within a targeted “zone” is not based in any scientific literature. It is not possible to compare one 40-year-old lazy person to a 40-year-old athlete. As you exercise your body ADAPTS to the exercise, which makes it more difficult to raise your heart rate during exercise. Therefore, we vary the workouts at PLF – so your body never has the chance to adapt. Since the only way to accurately measure HRMax is using sophisticated equipment in a laboratory setting, a convenient method to employ is the Age-Predicted Maximum Heart Rate (APMHR) Calculation. However, the most common HRMax equation used, Fox’s model from 1971 (HRMax=220-age), is fraught with issues. Numerous studies show that all 9 APMHR equations used to predict HRMax are significantly inaccurate, with a standard deviation of ~±24pm – that is a HUGE difference. New studies are being done to determine a better way of predicting HRMax in different populations, but the general population is hard to model given the extensive number of variables that different from one person to the next. So until there’s enough solid data, don’t bother asking me about “target heart rates” and “fat burning zones” because they don’t exist for you yet.
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