text: Muscles are composed of muscle fibres called actin and myosin. They are arranged overlapping, and can slide over each other. There are receptors on them, and when the central nervous system sends a signal, the muscle fibres slide over each other with the help of the "ratchet mechanism", which means some ratchet like molecules on actin sweeps the myosin forward, causing more overlapping.\nMore overlapping of muscle fibres cause the whole muscle to contract. Less overlapping cause it to relax.\nA movement is often done with two opposing sets of muscles (they are called antagonists). For example in your arm, when the biceps contract, the triceps relax, and vice versa. If they contract or relax at the same time, there is no movement seen.