Surprisingly, lubrication was not one of them. "The first factor was sprocket size," Spicer says. "The larger the sprocket, the higher the efficiency we recorded." Between the front and rear sprockets, the chain links line up straight. But when the links reach the sprocket, they bend slightly as they curl around the gear. "When the sprocket is larger, the links bend at a smaller angle," Spicer explains. "There's less frictional work, and as a result, less energy is lost." The second factor that affected efficiency was tension in the chain. The higher the chain tension, the higher the efficiency score.

Engineers made another interesting discovery when they looked at the role of lubricants. The team purchased three popular products used to lubricate a bicycle chain: a wax-based lubricant, a synthetic oil and a "dry"-type lithium-based spray lubricant. In lab tests comparing the three products, there was no significant difference in energy efficiency. "Then we removed any lubricant from the chain and ran the test again," Spicer recalls, "we were surprised to find that the efficiency was essentially the same as when it was lubricated."

The researcher speculates that a bicycle lubricant does not play a critical role under clean lab conditions, using a brand new chain. But it may contribute to energy efficiency in the outdoor environment. "The role of the lubricant, as far as we can tell, is to take up space so that dirt doesn't get into the chain," Spicer says. "The lubricant is essentially a clean substance that fills up the spaces so that dirt doesn't get into the critical portions of the chain where the parts are very tightly meshed. But in lab conditions, where there is no dirt, it makes no difference. On the road, we believe the lubricant mostly assumes the role of keeping out dirt, which could very well affect friction in the drive train."

Spicer cautioned that the chain drive is not the only place on a bicycle where energy can be lost because of friction, but it is an important one. The Johns Hopkins engineer wonders why bicycle manufacturers don't advertise the energy efficiency of their products, especially considering that the source of this energy is a human rider. "When you walk into a store and look at appliances, there's usually an energy guide on them, telling you how much it will cost to run the machine for a year. That allows you to make comparisons," Spicer says. "And if you go to an automobile dealer, you can see how many miles per gallon a car is expected to get, and that's essentially a measure of efficiency. So why shouldn't bicycle manufacturers post their energy efficiency?" The tests were supported by a grant from Shimano Inc.

As a reaction to such demands, heart rate is higher in the heat (more blood is needed to go to the skin for cooling). If intensity continues to increase, as a result of such conflicting demands, most of blood volume gets directed to the skin (for cooling) limiting oxygen delivery to muscles (for exercise). At some point there will not be enough blood to meet both demands since there is a limit to cardiac output (blood volume pumped per minute). Something has to give, self-preservation instinct takes over and your athletic performance decreases. But if you will continue pushing yourself in such environmental conditions, you may end up with a heat-related illness, involuntarily collapse or worst.