Muscle is a soft tissue made of thousands of small fibers woven together, capable of contracting to produce movement, maintain posture, and power vital organs. It makes up roughly 40% of your total body weight and is one of the largest organ systems in the human body. Your muscles do far more than move your limbs: they store energy, generate most of your body heat, and release chemical signals that influence everything from blood sugar to fat storage.
Three Types of Muscle Tissue
Your body contains three distinct types of muscle, each built differently and serving a different role.
Skeletal muscle attaches to your bones and is the type you consciously control. When you lift a cup, walk across a room, or smile, skeletal muscle fibers are doing the work. Under a microscope, these fibers look striped, a pattern created by the repeating units inside each cell that allow it to shorten and lengthen.
Cardiac muscle exists only in the walls of your heart. It shares that same striped appearance as skeletal muscle, but you have no voluntary control over it. Cardiac muscle contracts rhythmically on its own, driven by your heart’s internal electrical system, pumping blood without any conscious effort on your part.
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 digestive tract, regulate blood pressure by tightening or relaxing vessel walls, and control dozens of other processes you never think about.
How a Muscle Is Built
A whole skeletal muscle, like your bicep, is considered an organ in its own right. It has a layered structure, a bit like a cable made of progressively smaller wires. The entire muscle is wrapped in a tough connective tissue sheath called the epimysium. Portions of that sheath extend inward, dividing the muscle into compartments. Each compartment holds a bundle of muscle fibers called a fascicle, wrapped in its own layer of connective tissue. Inside each fascicle, individual muscle cells (called muscle fibers) are each surrounded by yet another thin connective tissue layer.
This layered design serves a practical purpose. The connective tissue at every level carries blood vessels and nerves deep into the muscle, ensuring each fiber gets oxygen, fuel, and instructions from the nervous system. It also gives muscle its structural integrity, so forces generated by tiny fibers can be transmitted outward to tendons and bones without tearing apart.
How Muscles Contract
Every voluntary movement starts with a signal from your brain traveling down a nerve. Where the nerve meets a muscle fiber, there’s a tiny gap called the neuromuscular junction. The nerve ending releases a chemical messenger called acetylcholine into that gap. On the muscle fiber’s surface, receptors catch the acetylcholine and trigger an electrical impulse that races along the fiber, causing it to contract.
The whole process happens in milliseconds. An enzyme in the gap immediately breaks down the acetylcholine so the muscle doesn’t keep firing from a single signal. This rapid cleanup is what allows precise, controlled movements rather than uncontrolled spasms. Each time you want a muscle to contract again, a fresh burst of acetylcholine has to be released.
Muscle as an Energy Reserve
Your muscles are the body’s largest storage site for glycogen, the form of glucose your cells use for quick energy. About three-quarters of all the glycogen in your body is stored in skeletal muscle. This local fuel supply means your muscles can draw on energy during exercise without dramatically affecting your blood sugar levels.
During physical activity, only about 20 to 25% of the energy your muscles burn actually becomes movement. The rest is released as heat. This is why you warm up when you exercise, and it’s also why shivering works: rapid, involuntary muscle contractions generate heat to maintain your core temperature in cold environments.
Muscles as a Hormone-Like Organ
One of the more surprising discoveries in exercise science is that skeletal muscle functions as an endocrine organ. When muscles contract, they release signaling molecules called myokines into the bloodstream. These chemical messengers travel to other tissues and influence metabolism throughout the body.
One key myokine boosts glucose uptake and fat burning within the muscle itself, while also stimulating the liver to release glucose during exercise and prompting fat tissue to break down stored fat. It also creates an anti-inflammatory environment by suppressing inflammatory signals. Another myokine promotes the growth of new blood vessels in active muscles, improving their blood supply over time. A third acts as a muscle-building signal and, interestingly, appears to reduce body fat. When this molecule was given to rats for seven days, their white fat mass dropped by 33%.
This helps explain why regular exercise has benefits that extend far beyond the muscles you’re working. The signaling molecules muscles release during activity influence blood sugar regulation, inflammation, and body composition in ways that affect your entire system.
How Much Muscle You Carry
Skeletal muscle mass varies by age and sex. For men aged 18 to 35, muscle typically accounts for 40 to 44% of body weight. That drops to 36 to 40% between ages 36 and 55, and to 32 to 35% between 56 and 75. For women in the same age ranges, the averages are 31 to 33%, 29 to 31%, and 27 to 30%, respectively.
These numbers aren’t just trivia. The amount of muscle you carry directly affects your metabolic rate, your ability to regulate blood sugar, your bone density, and your physical resilience as you age.
Muscle Loss With Age
Starting around age 30, the body naturally loses about 3 to 5% of its muscle mass per decade, a process called sarcopenia. For the first couple of decades, the decline is gradual enough that most people don’t notice. By your 50s and 60s, the loss becomes more apparent: reduced strength, slower recovery from physical effort, and greater difficulty with tasks like climbing stairs or carrying groceries.
Sarcopenia isn’t just about looking less muscular. Because muscle tissue stores most of your glycogen, generates heat, releases anti-inflammatory signals, and supports your skeleton, losing it accelerates a cascade of health changes. Resistance training remains the most effective way to slow this decline, and starting earlier preserves a larger reserve to draw from later in life.