Hundreds of voluntary muscles make up your moving chassis
Skeletal muscle is one of three vertebrate muscle tissues beside cardiac and smooth. Fascicles of multinucleated fibers, wrapped in fascia, shorten via sarcomeres of actin and myosin. In humans it is roughly a third to two-fifths of body weight and powers movement, posture, heat, and joint stability.
The human body has more than 600 skeletal muscles, mostly in mirror-image pairs, and together they account for around 35 to 40 percent of a healthy young adult's weight. Among Western populations, men carry on average about 61 percent more of it than women. Muscle moves the skeleton through tendons, holds posture, generates heat and steadies joints. It also acts as an endocrine organ, releasing subsets of 654 different proteins along with lipids, metabolites and small RNAs depending on conditions.
A muscle is a bundle of bundles. Fascicles hold the fibres, each wrapped in connective tissue, and each fibre is a giant cell roughly 2 to 3 cm long and 100 micrometres wide, formed in development when many myoblasts fuse, a process driven by proteins called myomaker and myomerger. That fusion explains why a single fibre holds hundreds to thousands of nuclei; a 10 cm fibre in the biceps may have 3,000. A young man's biceps contains around 253,000 fibres. Inside, myofibrils are strings of sarcomeres, where actin and myosin filaments do the actual contracting, fuelled mainly by burning fats and carbohydrates.
Only about half the nuclei in muscle belong to fibres. The rest sit in other cells: capillary lining cells make up 45 percent of these, fibro-adipogenic progenitors 20 percent, pericytes 14 percent and stem cells 9 percent. The stem cells, called satellite cells, lie dormant under the fibre's membrane until exercise or injury wakes them to donate new nuclei for growth and repair. Muscle spindles in the belly sense stretch, while Golgi tendon organs at the muscle-tendon junction report tension.
Fibre layout matters. In parallel muscles the fascicles follow the line of pull; in pennate muscles they angle across it, which costs each fibre some force but packs more fibres into the same space. Exercise thickens fibres by adding myofibrils, mitochondria and capillaries, and disuse shrinks them. Muscle names encode Latin descriptions: maximus means largest, deltoid triangular, and biceps and quadriceps count two and four heads.
Source: Skeletal muscle
