We’re often told to walk 10,000 steps per day, but what is the science behind that? Why not 20,000? 5,000? Why step count, and not the type of exercise, intensity, or total minutes?
With the advent of Apple Watch and Android Wear, millions of people now have heart rate sensors strapped to their wrist.To answer the questions above, we analyzed 458,000,000distinct data pointsfrom Cardiogram for Apple Watch.
Manpo-Kei, or “10,000 step meter”
The idea of 10,000 steps originated as a marketing sloganduring the 1964 Tokyo Olympics, when an athletic company released a pedometer named manpo-kei, which means “10,000 step meter”. Unfortunately, the clinical evidence is scant: while there is some work correlating step count with outcomes like BMI and blood pressure, and a set ofofficial activity guidelines, “no study to date has systematically evaluated dose-response effects of different steps/day goals.“
Your heart rate while at restnot moving, but not necessarily sleepinghas been shown to be a strong measure of overall health. A Chinese study showed the risk of dying of any illness went down by 9% for every 10 bpm decrease in resting heart rate. If your resting heart rate is below <75bpm, your chance of sudden cardiac death is halved. Even beyond cardiovascular health, a resting heart rate of <70bpm reduces your chance of cancer.
The good news is resting heart rate is highly modifiable. Exercise strengthens the heart muscle, so that it can pump the same volume of blood with fewer strokes. Marathon runners sometimes achieve resting heart rates in the 40’s.
According to the Mayo clinic, a normal resting heart rate is between 60 and 100 beats per minute. Among Apple Watch users with Cardiogram installed, the median is 76 bpm. About eighty percent of us are between 67 and 88 bpm.
So what happens if we plot average step count per day from iPhone vs your resting heart rate from Apple Watch? It looks like this, with size of bubble proportional to number of distinct people in each group:
In cardiovascular terms, the drop in heart rate from 1000 steps/day to 2000 steps/day is significant: a full 3 bpm decrease. And as step count increases, resting heart rate steadily dropsuntil you reach about 5000 steps per day. After that6000, 7000, even up to 10,000 stepsthe curve flattens.
What if it’s not step count that matters, but the intensity of exercise? To quantify this, we broke the chart above into three groups of people: high intensity gym rats who get their heart rate above 150 bpm for at least an hour each week, medium intensity people who get their heart rate above 130 bpm for an hour a week (say, power walkers), and low intensity (couch potatoes, the rest of us).
When broken down by exercise intensity, step count becomes much less relevant:
Even if you get 10,000 steps per day, if your heart rate doesn’t go over 130 bpm, there’s not much impact on your resting heart rate. In contrast, even 4000 steps / day of high intensity exercise delivers a benefit: about a 4bpm absolute drop in resting bpm, which doubles to8 bpm at 10,000 steps / day.
The chart above was for an hour of exercise per week. But how much exercise do you need to get the best “bang for the buck”? Surprisingly little:
Even 45 minutes per week of high intensity activity (heart rate >= 150bpm) placed participants in the lowest tier of resting heart rate.
More research funding is needed. Just kiddingisn’t that how all of these are supposed to end?