The sliding filament theory describes the mechanism used by muscles to contract. Muscle contraction is initiated when a signal is sent to a motor neuron touching the sarcolemma of the muscle fibre. When a neuro signal reaches the neuromuscular junction, acetyl choline is released, which generates an action potential in the sacrolemma.
The action potential spreads to the muscle fibre, which causes the release of calcium ions in the sarcoplasm. The calcium ions further bind to a subunit of a troponin on actin filaments, thereby unmasking the active sites on actin for thick filaments. Myosin cross-bridges attached to the actin filament bring about a contraction. When calcium ions return to the sarcoplasmic reticulum, actin is deactivated.
When myosin releases ADP and Pi, cross-bridges are broken and muscles relax. Repeated activation of muscles causes the accumulation of lactic acid due to the anaerobic breakdown of glycogen, causing muscle fatigue. Muscles can be of two types: one, those that undergo aerobic respiration and two, those that depend on anaerobic process for energy.