Muscle contraction

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Muscle contraction


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USMLE® Step 1 questions

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Muscle contraction

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USMLE® Step 1 style questions USMLE

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A researcher is studying skeletal muscle physiology. Baseline characteristics of a muscle sample obtained from a mouse are recorded. Which combination of changes to the number of muscle fibers recruited, frequency of muscle stimulation and velocity of muscle contraction will enable this muscle sample to generate the greatest force?  

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Even when you’re sitting perfectly still, when meditating for example, your muscles are still contracting a bit to stabilize joints and bones. And this force that the muscles apply at rest is called muscle tone.

On the other hand, when you pick up a 10 pound sack of potatoes, the force generated by the muscle contraction is much higher than the normal muscle tone in your biceps.

The pulling force transmitted through the muscle fiber is called the muscle tension.

Now let’s dive into some basics of muscle physiology, starting with a single muscle cell or muscle fiber. Within the muscle fiber is the sarcoplasm, which is the cytoplasm of a muscle fiber.

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

And each myofibril consists of contractile proteins called thin actin and thick myosin filaments.

These filaments don’t extend through the entire length of the muscle fiber - instead they’re arranged into shorter segments called sarcomeres. Alright, now let’s zoom into a sarcomere.

At the center of the sarcomere is the M line made of myomesin proteins, where the thick filaments attach.

At the borders of the sarcomere are the two Z-discs made of alpha actin proteins, where the thin filaments attach.

For every thick filament, there are two thin filaments-one above and one below and the two types of filaments overlap.

The region with only thin filaments is called the I band and it appears light.

The region with thick filaments is called an A band and it appears dark.

Now, most of the A band has overlap between the thick and thin filaments, but there’s an area towards the center called the H zone where there are only thick filament, so it appears slightly lighter.

When the muscle contracts, the thick filaments pull the thin filaments above and below it towards the M line.

The Z discs attached to the thin filament also gets pulled towards the M line, and the whole sarcomere gets shorter.

Now, the A band does not change since it’s the length of the thick filament.

But the H band and I band shortens because as the overlap increases, the region that consists of only thick or thin filament decreases.

At maximal contraction, there’s an almost complete overlap of the thick and thin filament and the H band and I band are almost completely gone.

Thick myosin filament is made up of hundreds of myosin proteins, each with a tail and two small club-like extensions, which are called myosin heads.

On the other hand, the thin filament actually looks like a pearl necklace that’s gently twisted.


Muscle contraction is the process by which muscle fibers shorten to generate force. Muscle cells contain proteins called actin and myosin that interact with one another and form cross-bridges to produce a contraction. The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state.


  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Terminology for contractions of muscles during shortening, while isometric, and during lengthening" Journal of Applied Physiology (2003)
  6. "Effects of eccentric and concentric muscle actions in resistance training" Acta Physiologica Scandinavica (1990)
  7. "Excitation-contraction coupling in muscular response" Yale J Biol Med (1952)

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