Useful quotes from the one and only legendary triathlete Mark Allen himself, regarding aerobic training:
"Max HR is a number that is partially a reflection of your age (the older you are the lower it will get), partly a reflection of your overall fitness, and very much influenced by your aerobic/anaerobic development. If a person has done tons of speedwork but little true aerobic base building their max HR will be very high for their age. The average as you most likely know is supposed to be 220-age=maxHR. However, if someone always trains fairly fast they will never develop a good aerobic base and their max HR will be well above that predicted number. Not only that but their heart comfort levels will be skewed way up also. So if that person has say a max aerobic HR in our training of 140 they may still feel comfortable running at 160. However, just because a person is comfortable training at a high heart rate that does not mean that they are necessarily going to be going extra fast at that high heart rate. But back to our formula and max heart rate. Once you start to develop your aerobic/anaerobic fitness balance by doing some pure aerobic work (which can be painfully slow for someone who has never done this) they will find that their max HR begins to drop, but also that they are going to be going faster at that lower max than they did at the higher max because they are now balancing their body's fitness development. If you base training zones off of an out-of-balance person's high max HR, they will alway be training at too high of a HR and will never ever maximize either their fitness or their racing potential. As a personal example, I came from a swimming background where all my training was anaerobic and almost no aerobic. My max heart rate at age 22 was about 215, which was almost 20 beats above what it should have been. Then in triathlons once I started to work with heart rate training and slowed my workouts down my max heart rate began to drop. Eventually at age 28 it was about 178, almost 15 beats below what would have been expected from the formula. But I was going so much faster at that lower max than I did at the high max simply because I had a good aerobic/anaerobic fitness balance. The process was not an easy one, however. Initially I had to slow my run pace down by over three minutes per mile from my normal training pace just to keep my HR from skyrocketing. I had to walk every hill and slow to a crawl at the end of every moderately long run. But over time I was able to run aerobically at under a 5:30 pace! I hope that helps,"
"Above that max aerobic heart rate is the point where you cause the adrenal system to rev up and kick in its hormones, some of which will turn off fat burning. This is what happens in speed work. Some stimulation of this system increases fitness. You want to do thing periodically at the right times in your training to get really fast and strong. You don't want to do this every week of the year because it will inhibit your potential. These zones do not relate to max heart rate. Your max HR (heart rate) is linked to the amount of aerobic and anaerobic fitness you have. If someone goes hard all the time, they will stimulate the anaerobic system all the time (a high stress situation) and end up with a very underdeveloped aerobic fat burning system as well as find that their max HR is very high. However, just because their max HR is high DOES NOT MEAN THAT THEY ARE GOING FAST! In fact, most people who have a higher than predicted max HR are going relatively slow at that heart rate. This is because they do not have aerobic fitness, which you gain by training at or below your aerobic max heart rate."
"What you are describing is what all athletes should be hoping for in their fitness gains. You are going faster at a lower heart rate. That is the ideal situation for an endurance athlete. And this happens when one does the appropriate amount of base aerobic work along with just the right amount of high heart rate speed work. One of the reasons that the max HR drops has to do with how the body accesses fuel. If someone does not have a developed aerobic system, they cannot access fat for fuel, but can access carbs. Carbs come into play in as a majority source of fuel above your aerobic max HR. If you do not have the capacity to burn fat for fuel, the body will actually kick the heart rate up quickly so that it can access carbs. Keep in mind that this fat/carb fuel mixture is exactly that...a mixture. You are never burning only fat, and almost never burning pure carb except at the extremely high output level. But without the ability to burn fat, the heart has to work really hard to keep delivering carb to the working muscle. There are additional hormonal reasons why the max will change with a better balance in the workouts, but that is the short version."
