Wrist-based heart-rate monitors are all the rage the days; many of my clients wear them to the gym each day. Yet, aside from the fact that these wrist-based heart-rate monitors are less accurate than the chest-strap versions, especially as you work harder (some models can be off by several beats per minute according to new research), they don’t offer users any perspective.
What does it truly mean if your heart rate is 135, 145 or 155 beats per minute? Unless you know your maximum heart rate (MHR) and what percentage of it you’re working at, that info is useless.
The well-known calculation of 220 minus your age to calculate MHR has long been known to be quite inaccurate. Yet, unfortunately, it remains a common standard for maximum heart rate calculations. For example, a 40-year-old person would subtract 40 from 220, indicating a maximum heart rate of 180 beats per minute.
Known as the Fox formula, this classic maximum heart rate calculation was developed for untrained healthy adult males, though it was also considered valid for untrained healthy adult females and children.
The Fox formula originated in the late 1960s, when William Haskell, an exercise physiologist who is now at Stanford University, and Sam Fox, a cardiologist, came up with a formula to determine the maximum heart rates of people who were having treadmill stress tests for heart disease.
However, it was not really a scientific effort. Rather, it was a suggestion, based on a decidedly non-random and small sample of people. When Haskell and Fox proposed the formula, they never intended to give an absolute number for athletes or people who are used to exercising vigorously.
Yet, it’s now recognized that that an individual’s maximum heart rate can vary widely from what that formula predicts — by as many as 30 beats per minute higher or lower.
In essence, the Fox formula is as likely to be wrong as it is to be right. In fact, this formula’s reliability is 0.51. In other words, it’s reliable about half the time.
The Tanaka formula for maximum heart rate is more accurate. It was developed based on studies of thousands of test subjects, can be used for all age and gender groups, and is considered to be more reliable than the Fox formula.
• Fox: Maximum Heart Rate (beats/minute) = 220 minus Age
• Tanaka: Maximum Heart Rate (beats/minute) = 208 minus (0.7 x Age)
Using the Tanaka formula, a 40-year-old person would multiply 40 by 0.7, which equals 28. Then they would subtract 28 from 208, indicting a maximum heart rate of 180 beats per minute. You may remember that this is the same figure determined by the Fox formula. However, this is a coincidence, not the norm.
For example:
• Fox formula: maximum heart rate = 170 (220 minus 50)
• Tanaka formula: maximum heart rate = 173 (208 minus 35)
• Fox formula: maximum heart rate = 190 (220 minus 30)
• Tanaka formula: maximum heart rate = 187 (208 minus 21)
Ok, so now you can calculate your maximum heart rate. What does that mean? How does that help you?
Reaching your MHR doesn’t mean your heart will explode! And it doesn’t guarantee a heart attack either. People train at this level for brief periods during their workouts every day, and they do it again and again and again.
Most well-trained, fit people can maintain their maximum heart rate for just 1-2 minutes. Yet, world-class athletes may sustain their MHR for 3-4 minutes. On the other hand, a person who is quite out of shape may only be able to sustain their maximum heart rate for about 10 seconds or so.
Yet, the ability to sustain MHR is just one measure of your fitness. How quickly your heart is able to recover from high-intensity exercise, during periods of lower heart-rate work or rest, matters a lot. The ability to recover (lower) your heart rate quickly is also an important measure of fitness.
Familiarity and experience with exercise matters. Someone who works out regularly may be very accustomed to briefly pushing their heart rate to 85, 90 or even 100 percent. Yet, someone who is sedentary may find that pushing their heart rate to 70 percent is excruciating and scary. It’s all about perspective.
It’s important to recognize that you can’t improve or increase your maximum heart rate with exercise. MHR is determined by genetics and age; as we get older, our MHR declines. Furthermore, not every 30-year-old will have the exact same maximum heart rate, nor will all 40-year-olds or 50-year-olds, etc. The calculation is an approximation, not a scientific assurance.
What’s important is not what your maximum heart rate is, but what heart rate you can sustain for an extended period. Someone with a MHR of 190 isn’t necessarily more fit than someone with a MHR of 180. Person A, with the 190 MHR, may only be able to sustain 70 percent of that. Meanwhile, Person B, with the 180 MHR, may be able to sustain 85 percent of that. Person B is more fit, which is what matters — not an age-determined MHR.
It’s important to remember that your heart rate has variability when you’re exerting yourself, such as when you’re working out. Most exercisers can’t simply pin their heart rate at 70, 80 or 90 percent of MHR. That’s because heart rate fluctuates throughout exercise. Your heart rate may rise quickly at the onset of a workout, then level out at a lower number. As you continue to workout, it may, over time, begin to climb again. And the harder you work, the more your heart rate may continue to rise throughout the workout.
This is why you should pick a target heart-rate zone for your workouts, so that it doesn’t go to low or too high.
The target heart rate during moderate intensity activities is about 50-70% of maximum heart rate, while during vigorous physical activity it’s about 70-85% of maximum, according to the American Heart Association.
Of course, you can push your heart rate higher for brief periods and then allow it to recover. This is the essence of high-intensity interval training (HIIT), which is used not only by athletes but also by older folks and those with chronic conditions, such as diabetes or heart disease.
Lastly, another important determinant of fitness is resting heart rate. The average resting heart rate is usually between 60 and 80 beats per minute, though it can even be as high as 100 bpm. Yet, some athletes have resting heart rates as low as 30 to 40 bpm. So, exercise can indeed lower resting heart rate, and this is important because every heart has a pre-determined number of beats from birth. The slower we get there, the better!
In 2013, a team of Danish researchers published16 years of work on just under 5,200 men in the journal BMJ Heart. The researchers determined that higher resting heart rates correlated with a greater chance of dying.
Those participants who ranged between 71 to 80 beats per minute had a 51 percent greater chance of dying prematurely than those with a resting rate of under 50 beats. At 81 to 90 beats per minute, that risk was double. And over 90, it tripled.
So, there’s a lot more to the heart-rate story than just the number flashing on your watch.
In conclusion, if you’re using a heart rate monitor while working out, it’s important to calculate your maximum heart rate, and to know your target heart-rate zone, as well as your resting heart rate. Once you know all of these things, then you’ll have some context for that heart rate number popping up on your watch.