A muscle contraction begins when a muscle is activated via a neural transmission sent from the brain. This triggers an action potential to be sent a motor unit that is responsible for the contraction of a number of muscular fibres part of one muscle group.
The contraction itself occurs when thin myosin filaments reach out and bind to thin actin filaments that have been opened previously through a series of triggers and signals from the brain to the active muscle. This forms what we know as a cross-bridge. When the cross-bridge is activated, the myosin heads tilt and drag the thin actin filament towards the centre of the sarcomere in a phase known as the ‘power stroke’. When a stroke is completed, the myosin heads detach from the active site, rotate back to their original position and attach to a new active site further along the actin filament. This will happen numerous times throughout the contraction causing the filaments to slide past one another, thus the given name ‘sliding filament theory’ (aka. Cross-Bridge cycling). As long as the energy source for the contraction (ATP) is present then myosin will continue to pull, detach and recock, shortening the muscle until it reaches its anatomical limit.