They are the same as they were, as the HR-based rec’s match up with the guidelines.
Would you be able to point me to evidence that the MET-based and HR-based recommendations line up? They don’t line up at all for me, and my cursory search found some research that suggests the same is true for others.
In a 2016 study by Sarah E Stahl, Hyun-Sung An, Danae M Dinkel, John M Noble, Jung-Min Lee, entitled “How accurate are the wrist-based heart rate monitors during walking and running activities? Are they accurate enough?”, the researchers recorded the heart rates of recreational runners (3x/week training, aged 19-45) at various paces. At 3 mph, the average heart rate was 90-95 bpm (47-50% of max HR). At 4 mph, the average heart rate was 105-110 (55-58% of max HR). Even at 5 mph, the average heart rate was only 135-140 (71-74% of max HR).
Those walking paces (3, 4, and 5 mph) are listed at 3.5, 5, and 8.3 METs in the compendium. The 5 mph pace would be at the high end of Z3 based on METs, but still in Z1 based on heart rate. The paper also reports standard deviations for the heart rate measurements, so I can estimate that approximately one person out of the 50 participants had a heart rate high enough to hit Z2 at the 4 mph pace. I know that these participants are fitter than the average American, but they seem like they’re probably typical of the people who are actually trying to follow the physical activity guidelines.
I also found a study from 1983 (“Cardiovascular and metabolic responses of trained and untrained middle-aged men to a graded treadmill walking test”) with untrained participants walking at 3.5 mph on a treadmill at 5% grade, with an average HR around 65-70% of max, while that activity would be at least 5.3 METs. The listing is for “walking, 2.9 to 3.5 mph, uphill, 1 to 5% grade”, so the 3.5 mph walk @ 5% grade is at the extreme end of that and presumably worth more than 5.3 METs. Again, based on heart rate we’re in the middle of Z1, but based on METs we’re at the high end of Z2.
I understand that there is individual variation in heart rate responses to exercise, as well as some possibility of measurement error, but this discrepancy seems to go way beyond that. I’m using a Polar watch, but I’ve also tested my heart rate with the monitors on various pieces of exercise equipment, as well as just by manually counting my pulse, and the measurements are usually pretty close. I’m open to correction if there’s better evidence out there, but so far I’m not finding anything that suggests that 3-6 METs and 75-83% of max HR are roughly equivalent targets.
The other thing to note here is that I would predict your HR to go up with a longer effort than 15-minutes.
This is a good point, but it creates some practical difficulties. If I need to do an hour of conditioning to accumulate 20 minutes of time in Z2, reaching the activity targets gets pretty difficult.
In any case, I would encourage you to look at this as an opportunity for additional gainzZz.
That’s true. From my point of view as a fitness enthusiast, I’m excited to see how I can fit higher intensity conditioning into my training and have another avenue to improve. However, from a health perspective, it’s discouraging to think that I’m so far away from the minimum activity targets. Even beyond my own situation, it presents a much bleaker picture for the general population. When I think about how much effort it’s taken me to accumulate about an hour of Z2 cardio in the last week, I can’t imagine any of my loved ones even getting close to the minimum activity targets. When you said that only 20% of adults met the physical activity guidelines, I was imagining a scenario where people are getting 60 minutes of 3-6 MET activity a week instead of 150. Based on the heart rate targets, I now think that the large majority of my friends and family get zero activity in Z2 or higher, and I sadly don’t see much prospect of that changing.