Now, a new study suggests that the greater efficiency may be related to the rapid rate at which glycogen is depleted in fast-twitch muscle fibers during slow, high-force pedaling.
To determine the actual effects of slow and fast pedaling on leg-muscle cells, scientists at the University of Wisconsin and the University of Wyoming asked eight experienced cyclists to cycle at an intensity of 85 percent V02max for 30 minutes under two different conditions.
In one case the cyclists pedaled their bikes at 50 revolutions per minute (rpm) while using a high gear. In the second case, the athletes pedaled in a low gear at 100 rpm. The athletes were traveling at identical speeds in the two instances, so their leg-muscle contractions were quite forceful at 50 rpm and moderate—but more frequent—at 100 rpm.
As it turned out, the athletes' oxygen consumption rates were nearly identical in the two cases, and heart and breathing rates, total rate of power production, and blood lactate levels were also similar.
However, athletes broke down the carbohydrate in their muscles at a greater rate when the 50 rpm strategy was used, while the 100 rpm cadence produced a greater reliance on fat.
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The greater glycogen depletion at 50 rpm occurred only in fast-twitch muscle cells. Slow-twitch muscle cells lost comparable amounts of their glycogen at 50 and 100 rpm, but fast-twitch cells lost almost 50 percent of their glycogen at 50 rpm and only 33 percent at 100 rpm, even though the exercise bouts lasted for 30 minutes in each case.
This rapid loss of carbohydrate in the fast-twitch cells during slow, high-force pedaling probably explains why slow pedaling is less efficient than faster cadences of 80 to 85 rpm.
Basically, as the fast fibers quickly deplete their glycogen during slow, high-strength pedaling, their contractions become less forceful, so more muscle cells must be activated to maintain a particular speed. This activation of a larger number of muscle cells then leads to higher oxygen consumption rates and reduced economy.