Which word rhymes with doubt?

Muscular system enables movement. Types: skeletal (voluntary, attached to bones), smooth (involuntary, in organs), cardiac (heart only, involuntary). Skeletal muscles work in antagonistic pairs – one contracts while other relaxes.

Muscle contraction: nerve impulse triggers calcium release, actin and myosin filaments slide past each other, muscle shortens. ATP provides energy. Fatigue occurs when ATP depleted.

Exercise benefits: increased strength, endurance, flexibility, bone density, cardiovascular health. Muscles atrophy (shrink) with disuse. Regular activity maintains muscular health.