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About Weight Lifting and Fast-Twitch and Slow-Twitch Muscle Fibers

author image Lisa M. Wolfe
A mother of two and passionate fitness presenter, Lisa M. Wolfe had her first fitness article published in 2001. She is the author of six fitness books and holds an Associate of Arts in exercise science from Oakland Community College. When not writing, Wolfe is hula-hooping, kayaking, walking or cycling.
About Weight Lifting and Fast-Twitch and Slow-Twitch Muscle Fibers
Muscle fibers are either fast or slow twitch. Photo Credit: Minerva Studio/iStock/Getty Images

You pick up the weights and perform your exercises. You see and feel the results when you look in the mirror or lift heavier weights, but this is just a small result of weight training. The big changes occur beneath the skin's surface in your slow-twitch and fast-twitch muscle fibers.

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Two Types

Muscle fiber types are either fast-twitch or slow-twitch. Fibers are contained within motor units. Each motor unit contains only one fiber type. Lifting weights engages or recruits predominantly fast twitch fibers. When the fibers contract during weight training, damage occurs to the cells of the fibers. In response, your body sends satellite cells to repair the damage, strengthen the fibers and increase the size. This is how you gain strength, muscle tone, and muscular size.

Fast As You Can

The fast-twitch muscle fibers contract quickly, contract with force, but fatigue fast. The fast-twitch fibers are used when you weight train at a fast to moderate pace. You use a resistance level that stimulates the muscle contraction and causes muscular fatigue within minutes. Also known as white fibers, the fast-twitch muscles are located in areas that need strong contractions such as your triceps.

Slow And Steady

The proportion of fast-twitch and slow-twitch muscle fibers in your body is mainly dependent on genetics. The slow-twitch muscle fibers are highly fatigue-resistance. You use these mainly when performing aerobic activities such as cycling and walking. Also known as red fibers, slow-twitch muscles are highly concentrated in areas that you use for endurance activities such as your calves for walking. A classic use of slow twitch fibers is marathon running.

Built To Last

Fast-twitch fibers are typically called upon for a weightlifting session. However, you can change the fiber recruitment focus to shift to the slow-twitch fibers by training for endurance. For example, lift a light weight at a slow speed for a high number of repetitions to recruit more slow-twitch muscle fibers. The high resistance to fatigue of the slow-twitch fibers makes you able to perform the exercise for a longer duration.

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