A muscle is a bundle of fibrous tissue that contracts to produce movement, maintain posture, and generate force. Your body contains more than 650 muscles, and skeletal muscle alone accounts for roughly 40% of your total body weight. Every motion you make, from blinking to sprinting, depends on muscles shortening, lengthening, or holding steady under tension.
The Three Types of Muscle
Your body has three distinct kinds of muscle tissue, each built for a different job.
Skeletal muscle is the type most people picture. These muscles attach to bones through tendons, tough cords of connective tissue, and they move your skeleton when they contract. You control skeletal muscles voluntarily: picking up a cup, turning your head, or taking a step all require conscious signals from your brain. Skeletal muscles range from the gluteus maximus (the largest muscle in the body, forming most of your buttock) to tiny muscles deep inside the ear.
Cardiac muscle exists only in the heart. It contracts rhythmically without any conscious effort, pumping about 2,500 gallons of blood per day. Cardiac muscle cells are uniquely branched and electrically connected so they can fire in sync, producing a coordinated heartbeat.
Smooth muscle lines your blood vessels, digestive tract, bladder, and airways. Like cardiac muscle, it works automatically. When smooth muscle in your intestines contracts in waves, it pushes food along. When it tightens around blood vessels, your blood pressure rises. You never have to think about any of it.
How a Muscle Contracts
Muscle contraction starts with a signal from the nervous system. A nerve cell releases a chemical messenger called acetylcholine at the point where nerve meets muscle. That messenger binds to receptors on the muscle fiber, triggering a rapid electrical change that sweeps along the fiber’s surface. A single one of these electrical signals is enough to set the whole fiber in motion.
Inside the fiber, the electrical signal causes a flood of calcium to be released from internal storage compartments. Calcium acts like an “on” switch: it binds to regulatory proteins wrapped around the muscle’s thin internal filaments, exposing sites where the thick filaments can grab hold. The thick filaments then ratchet along the thin ones, pulling them inward and shortening the muscle. This is known as the sliding filament theory, and it requires a constant supply of energy in the form of ATP, the cell’s fuel molecule.
Once the nerve signal stops, calcium gets pumped back into storage, the binding sites close, and the muscle relaxes. The whole cycle, from signal to contraction to relaxation, can happen in a fraction of a second.
Fast-Twitch vs. Slow-Twitch Fibers
Not all muscle fibers behave the same way. Skeletal muscle contains a mix of fiber types tuned for different demands.
- Slow-twitch (Type I) fibers contract more slowly but resist fatigue well. They rely heavily on oxygen to produce energy, making them ideal for endurance activities like walking, cycling, or holding your posture upright all day.
- Fast oxidative (Type IIa) fibers contract faster than slow-twitch fibers but tire out more quickly. They use a blend of oxygen-based and sugar-based energy pathways, suiting moderate-intensity efforts like a brisk run.
- Fast glycolytic (Type IIx) fibers produce the fastest, most powerful contractions but fatigue rapidly. They dominate during short bursts of maximum effort: a heavy deadlift, a sprint, or a vertical jump.
Everyone has a genetic mix of these fiber types, but training can shift fibers along the spectrum. Endurance exercise nudges fibers toward slower, more fatigue-resistant profiles, while power and sprint training favors faster characteristics.
How Muscles Grow and Repair
When you exercise intensely or sustain minor muscle damage, specialized stem cells nestled along each muscle fiber wake up. These cells, called satellite cells, normally sit dormant between the fiber and its outer sheath. Once activated by exercise or micro-injury, they multiply, mature, and fuse into the existing fiber, donating fresh nuclei. Those extra nuclei ramp up protein production, and the fiber gradually thickens. This process is the biological basis of muscle growth, or hypertrophy.
Some of the activated satellite cells don’t fuse into fibers. Instead, they return to a resting state, replenishing the reserve pool so the muscle can repair itself again in the future. This self-renewing cycle is why consistent training produces cumulative gains over months and years, and why muscles can bounce back from injury.
Muscle Mass by Age and Sex
Skeletal muscle mass varies considerably depending on age and sex. For men between 18 and 35, muscle typically accounts for 40% to 44% of body weight. That range drops to 36% to 40% between ages 36 and 55, and falls further to 32% to 35% between 56 and 75. After 76, averages dip below 31%.
Women carry proportionally less skeletal muscle. Between 18 and 35, the average range is 31% to 33% of body weight, declining to 29% to 31% between 36 and 55, and 27% to 30% between 56 and 75. After 76, averages fall below 26%. This gradual loss of muscle with age, sometimes called sarcopenia, is one reason strength and balance tend to decline in later decades. Regular resistance exercise is the most effective way to slow that decline.
Notable Muscles in the Body
The gluteus maximus earns the title of largest muscle. It extends your hip every time you stand up from a chair, climb stairs, or push off into a run. The strongest muscle relative to its size is the masseter, your primary chewing muscle. Working together with the other jaw muscles, it can generate up to 200 pounds of force on the molars. The muscle capable of pulling with the greatest absolute force is the soleus, a broad calf muscle critical for standing and walking. And the hardest-working muscle overall is the heart, which pumps roughly 2 ounces of blood with every beat and never takes a break.
How Muscles Attach to Bone
Skeletal muscles connect to the skeleton through tendons. A tendon is a dense, fibrous cord that transmits the pulling force of a contracting muscle directly to a bone, producing movement at a joint. Most muscles have two attachment points: the origin, which is the more stationary end, and the insertion, which is the end that moves. When your biceps contracts, for example, its origin stays anchored near your shoulder while its insertion pulls your forearm upward. Ligaments, by contrast, connect bone to bone and stabilize joints. They don’t generate movement on their own but keep bones aligned so muscles can do their job effectively.