Originally Posted by Delta Flyer

The human body resembles the hybrid.

You have fast-twitch and slow-twitch muscles. Think: fast twich=hybrid assist or IMA - slow-twich=gas engine.

When you start to run or walk, both fast-twich and slow-twich muscles work, just like both the electric assist and gas engine work from a standstill. When you jog, just the slow-twich muscles work. Like the gas engine, they need oxygen, but have high endurance.

When you work at about 80% or more of your maximum heart rate, fast-twitch muscles go to work. It's a lot like the IMA engaging when passing. Both don't need oxygen and can last only a few minutes before exhaustion.

As a person exercises more, the fast-twitch muscles become more efficient - sort of like the Insight going into lean burn.

Glyocgen is what fuels the body most efficiently, as gas does a Honda hybrid. Fat can be burned with glyocgen, but never alone. It's like batteries will assist a Honda, but always with a little bit of gasoline....

Have to switch vehicles for "hitting the wall" on marathons. That is when a runner is completely out of glyocgen and their available energy dramatically drops off. This is sort of like a Prius running out of gas and having to limp to a gas station in EV mode.