Slow twitch and fast twitch muscle fibers

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Slow twitch and fast twitch muscle fibers


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USMLE® Step 1 questions

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Slow twitch and fast twitch muscle fibers

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USMLE® Step 1 style questions USMLE

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A 27-year-old woman participates in a marathon and completes the race in 5 hours. Which of the following best characterizes the muscle fibers primarily responsible for helping this individual complete the race?  

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Fast twitch muscle fibers p. 465

Slow twitch muscle fibers p. 465

Type 2 muscle fibers p. 465

Muscle fibers p. 465

Type 1 muscle fibers p. 465

Ragged red muscle fibers p. 57

Red muscle fibers p. 465

Skeletal muscles

ACh receptors in p. 237

blood flow regulation in p. 301

glycogen metabolism in p. 84

White muscle fibers p. 465


type 2 muscle fibers p. 465


in muscle fibers p. 465


muscle fibers p. 465


Skeletal muscle fibers are divided into two main types: slow-twitch, which are also called slow oxidative fibers, and fast-twitch muscle fibers.

Fast-twitch muscle fibers are further subdivided into fast oxidative and fast glycolytic fibers. This classification is based on the speed of contraction and the metabolic pathway that’s used to make ATP, a molecule that stores energy for muscle contraction.

Most muscles possess a mix of slow-twitch and fast-twitch fibers, but the predominant one determines the primary function of the muscle.

Alright, now let’s take a look at a muscle cell, or myocyte - and specifically it’s sarcoplasm, which is the cytoplasm of a muscle cell.

The sarcoplasm is filled with stacks of long filaments called myofibrils.

Each myofibril has thick myosin and thin actin filaments that don’t extend through the entire length of the muscle fiber, instead they’re arranged into shorter segments called sarcomeres.

The myosin filaments have these small club-like extensions, which are called myosin heads.

The thin actin filament look like a pearl necklace that’s gently twisted. Each pearl represents one G-actin protein, which has an active site where the myosin head binds to during contraction.

Now, before myosin can bind actin, myosin needs to power up. Part of the myosin head is an ATPase, meaning that it can cleave an ATP molecule to ADP and phosphate ion and release some energy. The energy is used to cock the myosin head backwards, into its high energy position.

Next, the myosin head binds to the active site, and this triggers the release of the stored energy in the myosin head. When that happens, the myosin head launches pulling the thin filament along with it. This is called the power stroke.

The combined power strokes of all the myosin heads lead to sliding of the thin filament along the thick filament, and this results in the contraction of the skeletal muscle fiber.


Slow twitch fibers, also known as "red fibers," are characterized by a high endurance capacity and a slow contraction speed. They contain more mitochondria and blood vessels, allowing them to use oxygen efficiently for energy production. Slow twitch fibers are used for activities that require sustained effort, such as long-distance running or cycling. They use oxidative phosphorylation for obtaining ATP.

On the flip side, fast-twitch fibers have a high power output and tire quickly. They contain fewer mitochondria and blood vessels and rely on stored glycogen as a fuel source. They are used for short, intense activities such as weightlifting or sprinting. They use anaerobic glycolysis for obtaining ATP.


  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Role of pericytes in skeletal muscle regeneration and fat accumulation" Stem Cells Dev (2013)
  6. "Vibrations and sounds from evoked muscle twitches" Electromyogr Clin Neurophysiol (1992)

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