I agree: when I really want to make the distinction most folks make when they say "apples and oranges," I say "potatoes and coal."

But wait!

Potatoes and coal both come out of the ground.

Shucks and fudge!

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I worked last night and wrote my earlier post from work.

I have since ridden home seven miles, uphill, against a five to fifteen mile per hour wind.

On my longest and most difficult hill, into the wind, I thought that if on my geared bike I would gear down and go slower; which means, do the same amount of work over a longer period of time.

By stretching the same amount of work over a longer period of time, I can keep up aerobically.

For a given blood sugar situation and level of training, we have a predetermined amount of time in which we can function anaerobically before we need to recover with either rest or food, or both.
However, aerobically, we can function indefinitely if our body can convert our body's reserves into blood sugar fast enough.
Geared bikes buy the body time to convert body mass into blood sugar and then burn it with oxygen.

Fixed gear bikes, on the other hand, involve the same cadence issues as geared bikes, but because the rider cannot gear down on a hill, he has to do the same amount of work in less time, which often puts the fixed gear rider into an anaerobic situation in which his muscles burn the sugar/oxygen stored in the muscle cells themselves.

"Four different types of muscle fibers ... However, only the aerobic slow-twitch fiber and the anaerobic fast-twitch fiber are found in human skeletal muscle."

http://www.chiroweb.com/archives/18/07/06.html

Aerobic, meaning with oxygen, involves the slow-twitch muscles; and, anaerobic, meaning without oxygen replacement and using only the energy stored in the muscle cells themselves, involves fast-twitch muscle.

By spreading the work out over a longer time period, geared bikes enable slow-twitch muscle to utilize the replacement oxygen provided by the lungs and cardio-vascular system.

In contrast, by constraining the work into a smaller time period, fixed gear bikes, on some hills and against some winds, force the body into an anaerobic situation which utilizes fast-twitch muscle.

So, on some hills I kick butt and on other hills my fixed-gear butt gets kicked, depending on the length and steepness of the hill.