Muscular System
Three muscle types — skeletal (voluntary, striated, moves bones), cardiac (involuntary, striated, heart only), and smooth (involuntary, lines organs and vessels) — that produce movement and heat.
Skeletal muscles work in opposing pairs across joints — the biceps flexes the elbow while the triceps extends it — and contraction occurs when actin and myosin filaments slide past each other, an energy-hungry process that also generates body heat (shivering).