Skeletal muscle

Your muscles help you move.

They pull on your bones using tough strings called tendons. These muscles help you stay up straight. They also help keep your body warm.

Inside your muscles are tiny parts called fibers. These fibers look striped under a lens. They are made of many small bundles.

Muscles can change. They grow larger when you exercise. They can also get smaller if you do not use them. This is how your body stays strong.

Skeletal muscles help you move your body.

They are attached to your bones by tough strings called tendons. These tendons pull on the bones to create movement. Muscles also help you hold your posture. They even help control your body temperature.

Inside a muscle, there are many tiny cells. We call these muscle fibers. These fibers are very long. They are made of many small bundles called fascicles.

Each fiber has many nuclei. A nucleus is a tiny part that runs the cell. Because they have many nuclei, these cells are called multinucleated.

Muscle fibers have a striped look. This look comes from tiny parts called sarcomeres.

These sarcomeres are the basic units that make the muscle contract. To move, the muscle uses power from fats and sugars. It makes a molecule called ATP to power its work.

Muscles can change based on how you use them. They grow larger when you exercise. They can also shrink if you do not use them. This helps your body stay ready for what you do.

Skeletal muscles are a huge part of your body.

They make up about 35% of a human's total weight. These muscles are part of your voluntary system. This means you can choose to move them. They are mostly attached to your bones by tough strings called tendons.

These tendons help turn muscle power into skeletal movement. Muscles do much more than just move you around. They help you hold a steady posture. They also help control your body temperature and stabilize your joints.

Inside every muscle, there is a very organized system. Each muscle is made of bundles called fascicles. These fascicles contain many long cells called muscle fibers.

These fibers look striped under a microscope. This striped look comes from tiny units called sarcomeres.

Inside these sarcomeres, thin filaments called actin and thick filaments called myosin work together. When these filaments move, the muscle contracts. To make this happen, the cells use molecules called ATP.

This energy comes from breaking down fats and carbohydrates.

Scientists have learned much about how these cells grow. Muscle fibers are unique because they are multinucleated. This means one single fiber has many nuclei.

These nuclei are called myonuclei. They are found along the inside of the cell membrane. During development, small cells called myoblasts fuse together to make these long fibers. This process is called myogenesis. There are also special stem cells called myosatellite cells.

These cells stay quiet until you exercise or get hurt. Then, they wake up to help repair or grow the muscle.

There are more than 600 skeletal muscles in the human body.

In healthy young adults, muscles can make up 40% of body weight. Most muscles work in pairs on both sides of your body. Some muscles have special names based on their shape or size. For example, the gluteus maximus is a very large muscle.

The biceps has two heads, while the triceps has three. The quadriceps is named for having four heads. Scientists even name muscles by where they are located. A temporal muscle is found near the temporal bone.

Muscles are very good at changing to fit your life. This is called plasticity.

When you exercise, your muscle fibers grow larger. This happens because exercise builds more myofibrils inside the cells. You can also develop more mitochondria to make energy. However, your muscles can also shrink if you do not use them. Muscle cells do not divide to make new cells in adults. Instead, the existing cells just get bigger or smaller. This helps your body stay efficient for the tasks you do every day.

Skeletal muscle is a specialized tissue that allows for voluntary movement in vertebrates. It is one of three muscle types, alongside cardiac and smooth muscle.

These muscles are typically attached to the skeleton by tough, fibrous structures called tendons. In healthy young adults, skeletal muscle can account for approximately 40% of total body weight. In Western populations, men often have about 61% more skeletal muscle than women on average. Beyond movement, these muscles serve vital roles in maintaining posture and stabilizing joints. They also help control body temperature and function as an endocrine organ by secreting various substances.

The microscopic structure of skeletal muscle is highly organized. A muscle is composed of multiple bundles known as fascicles. Each fascicle contains many individual muscle fibers, which are also called myocytes. These fibers are much longer than cells in other muscle types. For example, a human biceps fiber can be 10 cm long.

Each fiber is surrounded by connective tissue layers called mysia. These include the endomysium for fibers, the perimysium for fascicles, and the epimysium for the entire muscle.

Skeletal muscle fibers are unique because they are multinucleated. This means a single cell contains hundreds to thousands of nuclei, known as myonuclei.

These nuclei are formed during myogenesis, a process where developmental cells called myoblasts fuse together. This fusion is driven by specific proteins called fusogens, such as myomaker and myomerger. The myonuclei are located along the inside of the cell membrane, or sarcolemma. To support the high energy needs of these large cells, they also contain many mitochondria. Specialized stem cells called myosatellite cells reside near the membrane to assist with repair or growth.

Muscle contraction happens at the level of the sarcomere. The sarcomere is the basic functional unit of the muscle fiber.

Inside these units, myofibrils consist of protein filaments called myofilaments. These include thin actin filaments and thick myosin filaments.

When the muscle is activated, the myosin heads use energy to move, causing the filaments to slide. This process requires adenosine triphosphate, or ATP. Muscles primarily generate this energy through the oxidation of fats and carbohydrates. However, fast-twitch fibers can also use anaerobic chemical reactions to produce ATP.

Muscles are arranged in different architectures to optimize their function. In parallel muscles, the fascicles run alongside the axis of force. These include shapes like fusiform or convergent muscles, which fan out at one end.

Conversely, pennate muscles have fibers that run at an angle to the axis of force. This angle allows for more fiber packing, which increases the physiological cross-sectional area (PCSA). While this allows the muscle to generate more force, it reduces the speed and distance of the contraction. Pennate muscles can be unipennate, bipennate, or multipennate.

Force is transmitted from the muscle to the bone through the musculotendinous junction. This is a specialized interface where the contractile part of the muscle meets the non-contractile tendon.

Because muscles and tendons develop together, they form a dynamic unit for movement. To monitor this activity, the body uses sensory receptors. Muscle spindles are stretch receptors located in the muscle belly. Golgi tendon organs are proprioceptors located at the junction that sense muscle tension. These systems help the body understand its own position and force.

Muscle tissue shows significant plasticity, meaning it changes in response to use. When you exercise, muscle fibers grow larger by increasing the number of myofibrils.

This training can also increase the density of mitochondria, myoglobin, and capillaries. However, adult muscle cells do not divide to create new cells. Instead, existing cells simply grow or shrink. If muscles are not used, they will decrease in size. Scientists use many terms to name these muscles based on their specific traits. They might name them by size, such as 'maximus' for largest, or by shape, such as 'deltoid' for triangular.