Overview

The sliding filament theory explains the microscopic mechanism behind muscle contraction. It describes how the structural proteins within muscle fibres interact to generate force and shorten the muscle, enabling movement. This theory is central to understanding how skeletal muscles function during all forms of physical activity, from everyday movement to elite athletic performance.

Skeletal muscle fibres are made up of repeating units called sarcomeres, which are the functional contractile units of the muscle. Each sarcomere contains two main types of protein filaments: actin (thin filaments) and myosin (thick filaments). These filaments are arranged in an overlapping pattern, and it is their interaction that produces contraction.

When a muscle receives a signal from a motor neuron, an electrical impulse travels along the muscle fibre and triggers the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum. The calcium binds to troponin, a regulatory protein on the actin filament. This causes a shift in tropomyosin, another protein that usually blocks the binding sites on actin. With the binding sites exposed, the myosin heads can attach to actin, forming what are known as cross-bridges.

Once attached, the myosin heads pivot and pull the actin filaments inward, toward the centre of the sarcomere. This movement is known as the power stroke and results in the shortening of the sarcomere—and thus the contraction of the muscle. The energy for this action comes from the breakdown of adenosine triphosphate (ATP), which binds to the myosin head. After the power stroke, a new ATP molecule binds to the myosin, causing it to detach from actin. The ATP is then broken down to re-energise the myosin head, allowing the cycle to repeat as long as calcium and ATP are available.

This repetitive cycle of cross-bridge formation and movement causes the actin filaments to slide over the myosin filaments, shortening the sarcomere and producing contraction. Importantly, the filaments themselves do not change length—they simply slide past each other, which is why the theory is named the "sliding filament theory."

When the nerve signal stops, calcium is actively pumped back into the sarcoplasmic reticulum. Tropomyosin then returns to its original position, blocking the actin binding sites, and the muscle relaxes.

The sliding filament theory provides a detailed explanation of how microscopic processes result in macroscopic movement. It highlights the importance of neural stimulation, calcium availability, and energy supply in muscular function, all of which are essential for effective physical performance and are key areas of focus in sport and exercise science.