A muscle is made of a bundle of muscle fibers. These fibers can be extremely long, up to about 14 inches in the leg, or very short, fractions of an inch. Within the muscle fiber are thousands of parallel structures called myofibrils. The myofibrils contain two types of filaments (actin and myosin) that cause the contractions of the muscle. These interlocking filaments can shorten by grabbing on to each other and pulling together; this is a muscle contraction.
Difference in Muscle Cells
Depending on the type of myosin in the mycofibril, a muscle can be either fast-twitch or slow-twitch. The slow-twitch or type I fibers use energy much more efficiently, but cannot contract as quickly. The fast-twitch or type II fibers use energy less efficiently, but contract up to ten times faster than the type I fibers. Both types of muscle fiber contract with about the same amount of force.
Variation in Fiber Composition
|Type of Person||Fast-twitch percent||Slow-twitch percent|