Muscles are soft tissues found throughout your body that contract to produce movement, stabilize joints, pump blood, and move food through your digestive tract. The human body contains more than 600 named muscles, and skeletal muscle alone accounts for roughly 40% of your total body weight. At a chemical level, muscle tissue is about 75% water and 18.5% protein, with small amounts of fat, carbohydrates, and minerals making up the rest.
Three Types of Muscle Tissue
Your body has three distinct types of muscle, each built differently and serving a different purpose.
Skeletal muscle attaches to your bones and is the type you can consciously control. When you lift a cup, walk, or turn your head, skeletal muscles are doing the work. Under a microscope, these fibers look striped (a pattern called striation), which reflects the way their internal protein filaments are stacked in repeating units.
Cardiac muscle forms the walls of your heart. It’s also striated, but you have no voluntary control over it. Cardiac muscle contracts rhythmically on its own, driven by electrical signals that keep your heart beating around the clock.
Smooth muscle lines the walls of hollow organs like your stomach, intestines, blood vessels, and bladder. These spindle-shaped fibers work automatically to push food through your gut, regulate blood flow, and perform other background functions you never have to think about.
How a Muscle Is Built
A skeletal muscle is not just a slab of tissue. It’s organized in layers, like cables bundled inside thicker cables. The entire muscle is wrapped in a connective tissue sheath. Portions of that sheath project inward, dividing the muscle into compartments called fascicles, each containing a bundle of individual muscle fibers. Every single fiber is then wrapped in its own thin layer of connective tissue.
These layers do more than hold everything together. They protect the delicate fibers from the force of their own contractions, and they create channels for blood vessels and nerves to reach deep into the muscle. At either end of the muscle, these connective tissue layers merge to form tendons, the tough, rope-like cords that anchor muscle to bone.
How Muscles Contract
Every voluntary movement starts with a signal from your brain. That signal travels down a motor nerve until it reaches the junction where the nerve meets a muscle fiber. When the electrical signal arrives at the nerve ending, calcium floods in and triggers the release of a chemical messenger called acetylcholine into the tiny gap between the nerve and the muscle.
Acetylcholine latches onto receptors on the muscle fiber’s surface, opening channels that let charged particles rush in. This creates an electrical wave that spreads across the muscle fiber and causes the protein filaments inside to slide past each other, shortening the fiber. That shortening is what we call contraction. The whole process, from brain signal to muscle movement, happens in milliseconds.
Fast-Twitch vs. Slow-Twitch Fibers
Not all skeletal muscle fibers are identical. They come in two broad categories that determine whether a muscle is better suited for endurance or explosive power.
- Slow-twitch (Type I) fibers contract slowly and resist fatigue. They’re optimized for sustained, low-intensity activity like maintaining posture, walking long distances, or cycling at a steady pace. These fibers rely heavily on oxygen to produce energy, which is why they’re packed with blood vessels.
- Fast-twitch (Type II) fibers contract quickly and generate much greater force, but they tire out fast. Sprinting, jumping, and heavy lifting all depend on these fibers. They burn through energy rapidly and can’t sustain effort for long.
Every muscle in your body contains a mix of both fiber types. The ratio is partly genetic. Elite sprinters tend to have a higher proportion of fast-twitch fibers in their legs, while marathon runners lean toward slow-twitch. Training can shift how these fibers behave to some degree, but the basic ratio you’re born with sets a baseline.
How Muscles Grow and Repair
When you stress a muscle through resistance training or intense activity, you create microscopic damage to the fibers. The repair process is what makes muscles bigger and stronger over time.
Nestled against each muscle fiber are specialized stem cells called satellite cells. In normal conditions, they sit dormant. When a fiber is damaged, satellite cells wake up, multiply, and either fuse with the injured fiber (donating their nucleus to help it grow) or fuse with each other to build entirely new fibers. Some satellite cells return to their resting state to replenish the reserve for future repairs. This cycle of damage, repair, and growth is the biological basis of muscle hypertrophy, the increase in muscle size you see from consistent strength training. Research consistently shows that muscle fiber growth is accompanied by increases in both satellite cell numbers and the number of nuclei within each fiber.
Muscle Mass Changes With Age and Sex
How much muscle you carry depends on your age, sex, and activity level. For men between 18 and 35, skeletal muscle typically makes up 40% to 44% of body weight. For women in the same age range, it’s 31% to 33%, largely because of differences in testosterone levels.
From there, muscle mass gradually declines. Men between 56 and 75 average 32% to 35%, while women in the same bracket average 27% to 30%. By age 76 to 85, men drop below 31% and women below 26%. This age-related loss of muscle is a major reason why strength, balance, and mobility decline in older adults.
Muscles and Metabolism
Muscle tissue is one of the most metabolically active tissues in your body, meaning it burns a significant amount of energy just to maintain itself, even while you’re sitting still. This is why muscle mass is a key factor in your basal metabolic rate, the number of calories your body uses at rest.
The age-related decline in muscle mass is one of the main reasons metabolism slows as you get older. Hormonal shifts during menopause accelerate this process in women by reducing lean muscle. Building or preserving muscle through regular resistance training is one of the most effective ways to keep your resting metabolism higher. This is different from aerobic exercise like running or swimming, which burns calories during the activity but doesn’t add the same metabolic advantage at rest that lean muscle provides around the clock.