VMBO Year 3 · Support and movement · Section 5.4

Muscles

How do muscles work?

The muscular system includes skeletal muscles attached to bones through tendons. Muscle contraction produces tension and can cause movement. A muscle pulls; it cannot actively push. Muscles therefore often work in antagonistic pairs: one produces a movement and the other produces the opposite movement.

The structure of a muscle

A muscle contains bundles of muscle fibres. Each skeletal muscle fibre is a long cell with many nuclei, formed during development by the fusion of smaller cells. Connective tissue surrounds the fibres, bundles and whole muscle. The outer connective-tissue covering blends with the tendons, transmitting force to the bones.

Muscle contraction

A nerve signal can trigger contraction. When a muscle shortens, its fibres become shorter and the muscle usually becomes thicker. The distance between its attachment points decreases and the muscle pulls on bones. This requires energy supplied by cellular processes using fuels; aerobic respiration also requires oxygen. Tendons transmit the pull but do not actively contract. Muscles can also produce tension without shortening, for example when holding a weight still.

Bending and straightening

A flexor, such as the biceps at the elbow, helps bend a joint. An extensor, such as the triceps, helps straighten it. During elbow flexion, the biceps shortens while the triceps lengthens; during extension their roles reverse. Coordinated action allows controlled movement and stability.