I’ve not been satisfied with Phil Maffetone’s 180 – age rule. He’s derived his formula from decades of observing athletes, but there is no true scientific basis behind the 180 number as far as I can tell. Everyone is a little different and I have a hard time believing everyone falls neatly into his formula. I’m not denying the value of simplifying things down to convenient formulas, not all of us have the money to get tested in a lab. Also, it’s still a good starting point for low intensity aerobic base building.
In the search for a more accurate way to measure MAF, I came across this thread on runningahead that outlines a treadmill test for finding the right zone. A poor man’s substitute for an RQ/v02max test, I’m all for that. So I went ahead and tried it at the local gym.
The basic idea is to measure your heart rate, set your watch to 10 second intervals, and increase the speed on the treadmill in those same increments. This way you have a reading of heart rate over the course of increased intensity. The resulting curve will show plateaus where your MAF and 50% fat/sugar burning zones are.
My results were actually surprising. Based on the formula my max aerobic function is 137, so I was expecting something in the 135-140 range. The results show the main inflection point at around 130-131, a significant amount lower. The smaller plateau at 146 may be my 50% fat/sugar burning zone, but the reading is very shaky at that point.
Stress on my body. I just came off a high mileage week with a trail marathon. This may have taxed my system and effected my bodies ability to run well. Maffetone recommends knocking off 5 beats for individuals who have health issues. I’m pretty healthy, but perhaps a stressful week could have a temporary effect.
Faulty data reading. My heart rate monitor tends to fluctuate and give me inaccurate readings. Also, doing the test in Kilometers per hour is recommended due to its tighter data distribution. The treadmill at the local gym isn’t easily customizable, so I went with mph. This is the first time I’ve done the test, I’ll be doing it again from time to time in order to verify this zone. (even in mph)
Genetics. Probably the most plausible answer is that this may be my natural MAF. I have lower than average resting and max heart rates, so its not unreasonable to assume that my MAF would be lower as well.
Does any of this matter? Not very much really. If this new reading is correct then I’ve been working out my base at slightly higher heart rate levels the last couple of months. I’ll continue for now working out the way I have. I actually run mostly by feel, checking my heart rate every mile or so. As long as I’m relaxed and doing an even easy effort, 6 beats per minute won’t make much of a difference. I may at most pull back just a little to make sure I’m not hitting the high 130’s.
I love doing these tests and I love data, but I can get a little obsessive at times, so its time to toss the watch and monitors and data gathering for a little while and just have fun running. I’m looking forward to my long run today. The sun is out and spring is here.