The Muscular System

The muscular system is the network of muscles that produces movement, maintains posture and generates heat in the human body. Muscle is a tissue made of cells that can contract, shortening to pull on the structures they are attached to. The body contains hundreds of muscles, and together they make up a large fraction of body weight, working constantly even when a person is at rest to keep the heart beating and the body upright.

There are three main types of muscle. Skeletal muscle attaches to bones by tendons and produces voluntary movement, contracting under conscious control to move the limbs, face and trunk. Cardiac muscle forms the walls of the heart and contracts rhythmically and automatically to pump blood. Smooth muscle lines hollow organs such as the stomach, intestines and blood vessels, working involuntarily to move their contents. Skeletal and cardiac muscle have a striped, or striated, appearance under the microscope, while smooth muscle does not.

Skeletal muscles are often described by their location and the joints they move. Facial muscles produce expressions, while large muscles of the shoulder, chest and back move the upper body; the pectoralis muscles of the chest and the rotator cuff of the shoulder are familiar examples. In the lower body the gluteal muscles, thigh muscles and calves power walking and standing. Muscles usually work in pairs or groups, one contracting to produce a movement while another relaxes or opposes it, giving the body smooth and controlled motion.

Frequently asked questions

What are the three types of muscle?

They are skeletal muscle, which moves the body voluntarily; cardiac muscle, which forms the heart and beats automatically; and smooth muscle, which lines internal organs and works involuntarily.

How do muscles move bones?

Skeletal muscles attach to bones through tendons. When a muscle contracts and shortens, it pulls on the bone, producing movement at a joint.

Why do muscles work in pairs?

Muscles can only pull, not push, so they are arranged in opposing pairs or groups: one contracts to move a part in one direction while another moves it back.