It's also the ratio of fast-twitch to slow-twitch muscle fibers you have been given/born with.
http://sportsmedicine.about.com/cs/e.../aa080901a.htm
"The two fiber types generally produce the same amount of force per contraction, but fast twitch fibers produce that force at a higher rate (they fire more rapidly). So a lot of fast twitch fibers can be an asset to a sprinter when there is a limited amount of time to generate maximal force. The slow twitch fibers, on the other hand, fire less rapidly, but can go for a long time before they fatigue.
The genetic component of muscle fiber type can not be overlooked when it comes to performance. Olympic athletes tend to be genetically blessed with large variations in fast and slow twitch fibers that perfectly suit their sport. Olympic sprinters have been shown to possess about 80% fast twitch fibers while those who excel in the marathon may have 80% slow twitch fibers.
Can you change your muscle fiber type by training?
This is a hard question to answer because good studies are just now being conducted (3). Currently, there is limited evidence to show that human skeletal muscle switches fiber types from "fast" to "slow" due to training (4). Researchers have demonstrated a fast-to-slow fiber transformation in animal skeletal muscle, and the human studies are showing similar outcomes. There is decent evidence that pure fast (Type IIb) fibers can transition to "hybrid" (Type IIa) fibers with chronic endurance training."