The muscles in our bodies are comprised of several different types of fibers. They are generally lumped into two broad categories – slow twitch and fast twitch. The number of slow to fast in a particular muscle is referred to as a muscle’s fiber ratio. The number before the slash refers to the percentage of slow twitch; the number after the slash fast twitch. For example, a common Trapezius ratio is 53.7/46.2.
Slow twitch fibers are dominant in muscles that contract slowly, and release energy over a long period of time, such as when standing, walking or holding your head upright. Because these muscles consume oxygen over long periods of time they are termed aerobic. In muscles having an about equal slow twitch to fast twitch ratio, slow twitch fibers will be engaged when performing endurance-type activities because they are more efficient at doing the job over a longer period of time.
On the other hand, muscles dominated by fast twitch fibers react quickly with powerful contractions, use a lot of anaerobic energy in a short amount of time, but tire quickly. With the activities like sprinting and weightlifting, fast twitch are more efficient at doing the job.
While each person’s muscle fiber ratios are initially predetermined genetically, ratios can altered based on type of exercise and strength training activities. Generally endurance-type activities, such as biking and running, tend to increase the amount of slow twitch fibers in the muscles used in the activity.
However in workout routines such as powerlifting tend to increase the ratio of fast twitch fibers. And the amount of weight you lift and number of repetitions can also change a fiber ratio. Lifting heavier weights but with fewer repetitions targets slow twitch fibers while the opposite – lifting lighter weight but with more repetitions – builds fast twitch fibers.
Also which muscle fibers, the type of workout depends on your fitness goal. For example if your goal is to bulk up and build muscle, then do activities that work fast twitch fibers. Because fast twitch fibers are physically larger than slow twitch to begin with, they will show more of an increase in size. However, if your goal is endurance, then work slow twitch fibers. Smaller in size, they won’t show as much difference in size, but will increase muscle stamina.
Depending on your goal, knowing which fibers to stimulate for your desired effect is key to training efficiency. Next is dedicating the time to training that type of fiber to reach your goal.