LihAS

Lihas is the Estonian term for a muscle, a tissue that contracts to generate force. A single lihas, or lihas, is composed of muscle fibers bound by connective tissue, nerves, and blood vessels; collectively, the muscular system produces movement, supports posture, and contributes to heat production.

There are three main types of muscles. Skeletal muscles are voluntary and attached to bones; they are striated and responsible for most body movements. Cardiac muscle forms the walls of the heart; it contracts involuntarily in a regular rhythm to pump blood and is also striated. Smooth muscles are involuntary and found in the walls of hollow organs and blood vessels, where they regulate flow and pressure.

Muscle structure includes fibers organized into bundles, wrapped in connective tissue. Within each fiber are myofibrils containing sarcomeres, the contractile units made of actin and myosin filaments. Contraction occurs via a sliding filament mechanism powered by ATP, initiated by electrical signals from motor neurons at the neuromuscular junction.

Physiology and adaptation: Muscles generate force to move limbs, stabilize joints, and maintain posture, while also producing heat. They rely on energy from aerobic and anaerobic metabolism. Training induces adaptations such as increased oxidative capacity, changes in fiber properties, and hypertrophy of muscle fibers, improving strength and endurance.

Clinical and health aspects: Muscles can be injured by strains or tears. Chronic conditions include myopathies and muscular dystrophies; aging commonly leads to sarcopenia, a decline in muscle mass and strength. Rehabilitation and conditioning aim to restore function and prevent decline.

Etymology and usage: In Estonian medical and scientific language, lihas is the standard term for a muscle, with lihased referring to the muscular system as a whole.