Experts, Explain it to me like I'm 5

bwogilvie wrote: »

Your maximum heart rate is the fastest that the old ticker can go. It appears to be limited by the flexibility of the heart walls and your nervous system's ability to stimulate the heart. There may be some neurological limits there too, because if the heart stops working, so do you.

The 220-age formula has no scientific basis. There are more accurate formulae that have been developed since then, but they still have a lot of variance in the study populations. The only accurate way to determine your personal MHR is a stress test. Believe me, it's not a lot of fun. When the formulas predicted mine should be somewhere in the 175-185 range, my test showed that mine was 192.

If you are exercising for weight loss, you need to strike a balance between the effort that burns the most calories per minute and the effort that burns the most calories over the course of the whole workout.

Sure, at MHR you can burn a lot more calories per minute (maybe 20 calories/minute for a trained 150 lb. male), but you'll be doing that for a couple minutes at most. That's not a lot of calories. You simply cannot exercise at your maximum heart rate for more than a very brief period.

At the other extreme: Walking at 3 mph you can walk all day, but if you weigh 150 lb., you'll only burn 4 calories per minute. So walk for 4 hours, burn 960 calories. Not too shabby, but who has 4 hours a day to exercise?

The same 150-lb. person running at an 8:00/minute mile (7.5 mph) might burn 13 calories per minute. To burn the same number of calories you'd burn in 4 hours of walking would require "only" an hour and 15 minutes of running. I put "only" in quotation marks because, if you've run 9.25 miles in an hour and a quarter, you know what it requires.

The 60% of maximum figure that is often recommended is kind of conservative, depending on your fitness and your tolerance for punishment. On my last one-hour run, I averaged 85% of my MHR. That was not exactly a walk in the park, but it wasn't too hard either.

The 60% figure is sometimes justified by the fact that, around that intensity, the greatest proportion of calories burned in exercise comes from free fatty acids. That might be true, but if you're exercising harder, the total calories from fat will be higher even if the proportion is lower.

And to answer the final question: the calories that don't come from fat come from carbohydrates: glycogen stored in muscles (which can usually, by itself, fuel up to 2 hours of moderately intense exercise in a fit athlete), liver glycogen (which fuels the brain), and any ingested carbohydrates. If you're exercising beyond your aerobic (lactate) threshold, accumulated lactate that can no longer be metabolized aerobically by being reduced to pyruvate will circulate in the blood, where it can serve as fuel for those parts of the body that aren't working crazy fast. In extremis, your body will start breaking down proteins, but that's really only in long-distance endurance events when you've exhausted other sources.

Basically, though, it doesn't matter how many calories burned during exercise come from fat. As long as you're in a calorie deficit, your fat stores will provide the energy that ingested food doesn't provide. In my last one-hour run, I burned around 820 calories. Probably 40% of those calories came from my fat stores. But I was at a 500-calorie deficit that day, so all 500 calories that I didn't eat ultimately came from fat stores—if not during the run, then over the next several hours.