Muscles

Muscles are attached to bones, and shortening (contracting) them makes bones move. The muscle in the musculoskeletal system (skeletal muscle) is made up of lots of muscle fibres and is controlled voluntarily.

Skeletal muscles can only contract in one direction, so are always present in pairs to allow two-way movement. A good example is in the upper arm, where there is a biceps muscle and a triceps muscle. To bend the arm at the elbow towards the body, the biceps contracts, which causes the triceps to relax. To straighten the arm, the triceps contracts and the biceps relaxes.

Muscles are attached to bones by robust bands of connective tissue called tendons. While they are strong, tendons can rupture if too much force is applied, either suddenly as is the case with an injury, or over a period of time due to overuse.