How do Muscles Contract?

By: Louise Rigby

Everyday your muscles contract again and again when completing daily activities. A simple sit to stand activity requires multiple muscle contraction from a variety of different muscle groups! Our muscles have the ability to contract in a number of ways such as:

  • Isometric contraction (The muscle is working but staying the same length).
  • Isotonic contraction (the muscle is working and changing length).
    • Isotonic eccentric contraction (the muscle lengthens as it works).
    • Isotonic concentric (the muscle shortens as it works).

Have you ever thought about how your muscles contract?

Within this blog, I will focus on how a muscle concentrically contracts using the sliding filament theory!

The sliding filament theory was introduced by Andrew Huxley and Niedergerke to explain how a muscle contracts. Before getting stuck into the sliding filament theory let’s first look into the basic anatomy of a muscle.

Each muscle is made up of small muscle fibres called Myofibrils which contain Actin and Myosin filaments. The actin and myosin filaments are able to slide in and out of each other. Hence the name “sliding filament theory”.




An additional structure you should be aware of is Tropomyosin, this is a structure that covers the actin binding site when there is no muscular contraction needed.

The sliding filament theory is complex but here is a brief explanation to introduce you to the concept.

The sliding filament theory step by step:

  1. A nerve impulse arrives at the neuromuscular junction, this causes the release of chemical Acetylcholine and calcium.
  2. The presence of calcium causes tropomyosin to move away from the actin binding site.
  3. The myosin head can then attach to the actin binding site to form a cross-bridge between the two filaments.
  4. The myosin heads then pull on the actin filaments causing them to slide and overlap each other which subsequently shortens the muscle length this is known as the “power stroke”.
  5. Using ATP energy the myosin head detaches from the actin binding site then reattached to a-actin binding site further down the actin filament to repeat the “power stroke”.
  6. Once the nerve impulse stops the calcium is removed causing the tropomyosin to move back onto the actin binding site preventing the myosin head attaching.
  7. Actin and myosin filaments return to their resting position.

The sliding filament theory is complex but hopefully, this blog has shed some light on the subject of muscular contractions!

Louise Rigby

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