All this goes back to my original question. And I'm asking this because I'm sincerely curious. I'm not trying to be difficult. We all have read the advantages of running slow. Cutting and pasting:
Why do long slow distance runs?
The LSD run has many benefits. First, it helps to adapt your joints and muscles to give them the endurance for long runs. Second, it improves your cardiovascular system, strengthens the heart and increases the blood supply in the muscles; it therefore enhances the body’s capacity to deliver oxygen to your muscles. Third, it enhances your body’s ability to burn fat as a source of energy. Fourth, it teaches your body to store more energy as glycogen in your muscles. And finally, long slow runs teach the body to run efficiently, minimising the energy expenditure needed to move you along. Even if you are not training for a marathon, the long slow distance run is a key element in your overall fitness programme.
There seems to be differing opinions however about what heart rates you should usually run these long, slow runs at -- usually I've seen about 70 to 75% of max HR but I've even seen as high as 85%. My question is why should we train at 70% if our body feels comfortable at 75%? What are we not getting at 75%? I thought, perhaps (I don't know) it might be better for glycogen storage and fat burning training which, although important, aren't as critical for shorter distances.
Here's an example from an actual metobolic test I took a few years ago.
Max hr 185.
Aerobic base 127 hr (68.6%) - AB is the maximun rate at which you burn fat.. In this test I burned 7.2 KCal/min fat of a total 14.9 KCal/min.
Anaerobic Threshold 167 (90.3%) - I burned 30.2 Kcal/min but only 0.5 KCal/min fat. So while I doubled the calories burned, the fat calories burned were about zero.
At 142 or mid zone 2 (77%) - 20.4 Kcal/min with 5.8 Kcal/min being fat..
So to answer your question, while I felt comfortable training at 77% or 142 hr, I was burning 5.8 cal of fat but when I slowed down to an almost silly pace of 127 hr, I burned 7.2 cal of fat.
For whatever reason, running at 127 teaches my body to use fat as a fuel source. I didn't re-test after doing slow base work, but the theory was that if I had done so, that 0.5 KCal/min fat at 167 hr would have increased. Matter of fact, the %fat burned should have increased across all zones.
My specific example isn't going to apply to everyone due to fitness and base, but it's a real world example of why 70% is better than 75%.