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The Muscular System: A Basic Guide

Tap a muscle type to see how it works.

Skeletal Muscle

Attached to bones by tendons, this muscle type is under voluntary, conscious control and is responsible for all deliberate body movements, such as walking, lifting, and facial expressions.

Smooth Muscle

Found in the walls of internal organs like the stomach, intestines, and blood vessels, this muscle type operates involuntarily, without conscious control, to move substances through the body.

Cardiac Muscle

Found exclusively in the heart, this uniquely durable and involuntary muscle type contracts rhythmically and continuously throughout a person's entire life without ever fully resting.

How Muscle Contraction Works

At a basic level, muscle contraction occurs when nerve signals trigger microscopic protein filaments within muscle fibers to slide past each other, physically shortening the muscle and generating force.

Why Muscles Get Tired

Prolonged or intense muscle activity depletes the energy stores and oxygen a muscle needs to keep contracting effectively, along with a buildup of metabolic byproducts, resulting in the sensation of fatigue.

The body's largest tissue system by weight

This page is for general educational information only and is not a substitute for professional medical advice. Muscle tissue makes up roughly 30 to 40 percent of total body weight in most adults, making the muscular system the single largest tissue type in the human body by mass.

Frequently Asked Questions

Why does the heart never seem to get tired like other muscles do?

Cardiac muscle cells are packed with an especially high number of energy-producing structures called mitochondria and have a unique cellular structure that resists fatigue far better than skeletal muscle, allowing the heart to contract continuously for an entire lifetime.

Can smooth muscle be consciously controlled at all?

Generally, no β€” smooth muscle is regulated automatically by the autonomic nervous system and hormones rather than conscious thought, which is why a person cannot deliberately speed up or slow down their own digestion the way they can move a skeletal muscle like an arm.