How do muscles contract?

Muscles are pieces of soft tissues that can be found throughout your body. They facilitate everything you do that’s associated with movement and motion (Clinic, 2023). By contraction, muscles produce a force. However, you may wonder, how do muscles contract? This article will dive into the fundamentals of muscle contraction. 

The structure of muscles

Figure 1. Structure of muscles

Figure 2. Structure of muscle fibres

Before we dive into the detailed mechanisms of muscle contraction, it is important to know the basic structure of muscles. As you can see from the diagrams, the structure of muscles is very complicated. But essentially, the most basic muscle contraction takes place in the sarcomere (refer to the diagrams). 

The sliding filament theory 

Each sarcomere contains many parallel, overlapping thin (actin) and thick (myosin) filaments. The muscle contracts when these filaments slide past each other, resulting in a shortening of the sarcomere and thus the muscle (Morgan, 2022). This is known as the sliding filament theory, which is the fundamental mechanism of muscle contraction. 

The sliding filament theory can be more specifically broken down into the following steps:  

1. A motor neuron generates an action potential, which triggers the release of calcium ions from the sarcoplasmic reticulum

2. The calcium ions bind to troponin and cause tropomyosin to move (troponin and tropomyosin are blocking complex on actin covering the binding sites), exposing binding sites for the myosin heads

3. The actin filament and myosin heads form a cross-bridge, and the bridge is then broken by ATP 

4. ATP hydrolysis causes the myosin heads to swivel and change orientation

5. The swiveled myosin heads bind to the next binding site on the actin filament before returning to their original conformation, and the actin filament is moved towards the centre of the sarcomere due to this repositioning 

6. Therefore, the sarcomere shortens because of the sliding of actin along myosin, causing muscle contraction

Figure 3. The sliding filament theory

Works Cited 

Clinic, C. (2023). Muscles of the Body: Types, Groups, Anatomy & Functions. [online] Cleveland Clinic. Available at: https://my.clevelandclinic.org/health/body/21887-muscle [Accessed 8 Mar. 2025].

Morgan, K. (2022). Muscle Contraction The sliding filament theory. [online] Available at: https://anaesthetics.ukzn.ac.za/wp-content/uploads/2024/06/Muscle-contraction-Ref-2022.pdf.