There are two main groups of muscle fibers — slow-twitch (Type I, aerobic) and fast-twitch (Type II, anaerobic). Slow-twitch muscle fibers are efficient at using oxygen to produce energy for endurance activities (e.g., distance running). Fast-twitch fibers support short bursts of strength or speed (e.g., sprinting) and fatigue much faster than slow-twitch fibers.
Genetics determine your fiber type.
The average person is born with 50% slow-twitch fibers and 50% fast-twitch fibers, but the specific ratio for each individual is determined by genetics. Additionally, some muscles contain more of one type due to their role in body movement. For example, the muscles of the core, which are responsible for holding you upright and maintaining posture, are largely made up of slow-twitch fibers making them resistant to fatigue.
Training may do little to change your fiber type.
Your prominent muscle fiber type can predict how well you perform in endurance versus anaerobic activities. Research is mixed regarding how much fiber type can be changed through training to improve performance. Many muscle fibers show characteristics of both slow-twitch and fast-twitch (often called hybrid fibers). It appears that these hybrid fibers can be trained to favor either fast-twitch or slow-twitch, but a fiber changing completely from one type to another is not likely. Elite athletes have as much as 80% of one fiber type, but it's difficult to assess how much of this is due to training as opposed to genetic makeup.
Fiber type changes with age.
Age-related muscle loss becomes evident around the age of 50. Research shows that slow-twitch fibers are more resistant to this decline, and decreases often do not occur until the age of 60. Regular resistance training increases strength and grows both types of muscle fibers, which helps to combat age-related muscle loss.
Don't be overly concerned with muscle fiber type when selecting an activity. A 50/50 mix of muscle fibers is a good thing because it allows you to perform a variety of activities well, increasing your enjoyment. Choosing an activity that you enjoy remains the key to sticking with an exercise program.