Have you ever wondered why one person at the gym can lift heavy weights with ease, while another person of a similar build can barely eke out a few sets? Some of it has to do with training, but genetics and muscle fibers play a role too.
The Two Types of Muscle Fibers
Muscles are made up of two main types of fibers. Type 1 or slow-twitch muscle fibers are designed for endurance exercise. They have more capillaries and larger mitochondria for generating energy aerobically as well as more myoglobin. This makes them more resistant to fatigue, but they also contract more slowly. This makes them ideal for runners and people who engage in other sports requiring endurance but less ideal for activities that require speed and power.
The second group of muscle fibers is fast-twitch types. There are two types of fast-twitch muscle fibers called type 2A and type 2B. Fast-twitch muscle fibers generate more force than slow-twitch ones, but they also fatigue more quickly.
There are two types of fast-twitch muscle fibers. Type 2B fibers generate the most force of the two fast-twitch muscle fibers, but they also fatigue more quickly. They’re ideal for lifting heavy weights where large amounts of force are generated. Type 2A fast-twitch muscle fibers are a little less powerful than type 2B fibers, but they’re also more resistant to fatigue, though not as much as slow-twitch fibers.
Muscle Fiber Types and How They Affect Strength
Research shows that athletes that are successful in anaerobic sports such as sprinting and heavy weight lifting have a greater proportion of fast-twitch muscle fibers relative to slow-twitch ones. Long distance runners and athletes that participate in endurance sports usually have a greater proportion of slow-twitch muscle fiber types. This serves them well in their sport since these fibers are more resistant to fatigue. In turn, weight lifters benefit from having more fast-twitch muscle fibers that generate force.
How many fast-twitch muscle fibers a person has relative to slow-twitch ones is determined primarily by genetics. At one time it was thought that the ratio of muscle fiber types couldn’t be altered by training. Since then, researchers have discovered that type 2A muscle fibers can be converted to slow-twitch muscle fibers through endurance training. It’s not clear whether slow-twitch muscle fibers can be changed to fast-twitch ones through strength training.
Other Factors Affect How Strong a Person Is Too
Having more fast-twitch muscle fibers is one factor that determines a person’s strength potential, but it’s not the only one. Neural factors involving the brain play a role in muscle strength too. With strength training, a person is able to activate more motor units during an exercise. This allows them to develop more force. Type 2 muscle fibers also become larger with strength training, allowing a person to lift more weight. In addition, factors such as a person’s psychological mindset and their hormonal status play a role too. A complex array of factors comes into play in determining how strong a person is, but the ratio of fast-twitch to slow-twitch fibers plays an important role.
Fortunately, no matter what ratio of muscle fiber types a person has, strength training can improve their overall strength.
Exercise Physiology. Theory and Application to Fitness and Performance. Seventh edition. 2009.
Idea Fitness. “What Makes Some People Faster or Stronger?
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