yeah, I think it's more related to LT than HR. It's the muscular-force that determines fat vs. carb. utilization and once you're exerting so much force that you're over LT, very little fat is burned. So it would appear that using lower-gears and spinning would utilize more fat because it requires less muscle-force for the same speed. Which raises HR too.
As for LT vs. RHR.... I'm not sure if there's a correlation since LT can occur over a variable range depending upon gearing. Pushing bigger gears would have LT occur at say... 150bpm while spinning smaller gears would have LT be around 170bpm. As you get more fit, RHR drops while LT increases..
"I've read in a lot of places that fitter people burn a higher percentage of fat at a higher HR percentage but never figured out why. Is it then just a matter of looking at the numbers, the only adaptation responsible being a more efficient cardiovascular system?"
It's partly muscle-efficiency, that is, aerobic-power generated for any given amount of oxygen consumed. With training, LT and VO2-max does increase, but not as much as muscle-efficiency. You may see a 10-20% increase in LT & VO2-max, but power-generated at LT can increase by 100-200%. Let's take some examples, if subject #1 & #2 start out with the same fitness, same MHR and LT levels. They can both generate about 200watts @ LT around 170bpm. But #2 does more training and increases his muscle-efficiency more to 400watts @ LT and same 170bpm. These two guys go for a ride later at the same 200watts output. Subject #1 is riding at 90% of his LT at that output, while #2 is riding at only 50%. Subject #2 will be burning more fat because his muscles are operating at a much lower percentage of their max and will be utilitizing a higher percentage of low-twitch aerobic muscles.
One of the most efficient athletes I ever saw at the OTC the summer I was there was a football player. He generated the most power per oxygen consumed they've ever tested... unfortunately his mass and bulk would limit his potential as a cyclist.
"Intramuscular fat? Does the body discriminate about what type of fat to burn?"
I haven't found anything on discriminating between intramuscular fat vs. adipose tissue. I would suspect that your body redistributes this anyway. What I read is that higher triglyceride-levels in the blood does increase fat-burning. This can be triggered by various glycogen-sparing techniques like coffee that stimulates glucagon production and triggers the fat-cells to dump their supply into the bloodstream to be used for fuel helping stretch out the glycogen supply and fending off the bonk for longer.
Last edited by DannoXYZ; 09-03-06 at 01:21 PM.