About Muscles
The body contains three major types of muscle: skeletal, cardiac, and smooth. Skeletal muscles move bones and help maintain posture. Cardiac muscle powers the heart. Smooth muscle moves materials through organs and changes the diameter of structures such as blood vessels.
Three Muscle Types
Skeletal, cardiac, and smooth muscle perform different jobs.
Learn MoreMuscle Fibers
A skeletal muscle fiber is a long cell packed with contractile machinery.
Learn MoreNerve Control
Skeletal muscles contract when motor neurons activate them.
Learn MoreTendons
Tendons connect many muscles to bones.
Learn MoreEnergy
Muscle contraction requires a constant supply of ATP.
Learn MorePosture and Balance
Muscles work even when you appear to be standing still.
Learn MoreAdaptation
Muscles can adapt to repeated physical demands.
Learn MoreRepair
Muscles can repair limited damage with help from specialized cells.
Learn MoreThree Muscle Types
Skeletal muscle is usually under voluntary control. Cardiac muscle contracts rhythmically in the heart. Smooth muscle works automatically in organs such as the intestines, bladder, uterus, airways, and blood vessels.
Muscle Fibers
Inside are many myofibrils containing repeating units called sarcomeres. Protein filaments slide past one another to shorten these units and generate force.
Nerve Control
A motor neuron can control several muscle fibers. The nervous system recruits different motor units and adjusts their firing rate to produce precise movements or stronger contractions.
Tendons
When a muscle shortens, the tendon transmits force to the skeleton. Because muscles usually pull rather than push, joints often use opposing muscle groups to move in different directions.
Energy
Cells regenerate ATP using stored chemicals and nutrients. During sustained activity, oxygen delivery and energy metabolism increase. Different muscle fibers are specialized for endurance, speed, or a mixture of both.
Posture and Balance
Small continuous adjustments stabilize joints and keep the body balanced against gravity. The brain, spinal cord, inner ear, eyes, and sensory receptors in muscles all contribute.
Adaptation
Strength training can increase muscle fiber size and improve nervous-system coordination. Endurance activity can increase mitochondria, capillary supply, and the ability to use oxygen efficiently.
Repair
Skeletal muscle contains satellite cells that can contribute new nuclei during growth and repair. Severe injuries may still form scar tissue, so muscle recovery depends on the type and extent of damage.