💪 Muscle Contraction: The Sliding Filament Theory

About this simulation

A sarcomere is the basic contractile unit of skeletal muscle, bounded by two Z-discs. Thin actin filaments anchor to each Z-disc and interdigitate with a central thick myosin filament. Muscle does not shorten because the filaments themselves contract — they slide past one another while staying the same length. This is the sliding filament theory, first proposed by Huxley and Hanson in 1954.

How it works

• Excitation–contraction coupling — an action potential triggers the sarcoplasmic reticulum to release Ca²⁺, which binds troponin and shifts tropomyosin off the actin binding sites.
• Cross-bridge cycling — exposed sites let myosin heads bind actin, perform a power stroke, then release using energy from ATP hydrolysis, repeating many times per second.
• Sliding, not shortening — each cycle ratchets actin a small step toward the M-line; filament lengths stay fixed while the sarcomere as a whole shortens.
• Rigor — without ATP, myosin heads cannot detach from actin after binding. The filaments lock in place — the same mechanism behind rigor mortis.

Controls

• Ca²⁺ release — sustained calcium concentration bathing the filaments (0–100%).
• ATP availability — fuel for cross-bridge detachment and cycling speed (0–100%).
• ⚡ Stimulate twitch — fires a brief calcium transient on top of the baseline, like a single motor-neuron action potential.
• ↺ Reset sarcomere — returns the model to its resting length. Drag to rotate, scroll to zoom.

Did you know?

A single muscle fiber can shorten by roughly 30–40% of its resting sarcomere length during a full contraction — beyond that, the thick and thin filaments start to collide and overlap inefficiently, and tension actually falls, exactly as described by the classic length–tension curve.