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RE: Sub 1:50 800m Training
My guess is that initially, a slight increase in temperature in the muscle will increase enzyme activity.

There is an optimum temperature range for enzymatic reactions and this is likely to be slightly higher than the bodies resting core temp of 37C perhaps the optimum range for mitochondrial (aerobic) enzyme reactions is maybe 39-41 (at a guess) above the optimum range the reactions slow down, this is inevitable in a race such as an 800m, so you pace yourself accordingly judgeing your effort on previous training and racing experience.

I don't think that training will alter the optimum temp range for aerobic enzymes, but in longer races we may be able to tolerate a higher core temp when we are in top condition. Noakes has a chapter on this in his Lore of running vol 4. His references are limited but one study suggests that marathon winners tolerate a core temp of 41C at the end of the race. This would require very good biomechanics however. Often in a marathon a runners legs tighten up before the end of the race and this is the limiting factor which slows them down, whereas someone having a better race doesn't get such stiff legs and they maintain pace better resulting in a higher core temp than the guy whose legs stiffen up.

Is there a mechanism which shuts down the aerobic system? I don't know for sure but would put money on my guess that excess heat production will have this effect. Also as this process happens gradually it is bound to limit the amount of hydrogen ions which can be shuttled into the mitochondria. So aerobic enzyme shut down causes rising acidity, not the other way round. I think that is the most logical explanation.

Also, bear in mind that we get tired as we lose concentration, so we work harder as we lose running form.

Now if you consider that the second lap of an 800 is more aerobic than the first, then this can give you insight into the fact that you can extend your second lap pace (sub 60 second pace hopefully) to longer and longer distances.

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