Human bones are composed of a dense outer layer called compact bone and an inner spongy layer known as trabecular bone. Bones contain bone marrow, which produces blood cells. They are vital for structural support, protection of organs, mineral storage, and movement.

Joints are classified into three main types: Fibrous joints (e.g., sutures in the skull) are immovable. Cartilaginous joints (e.g., the spine) allow limited movement. Synovial joints (e.g., the knee and elbow) are freely movable and contain synovial fluid to reduce friction.

Muscle contraction occurs when a muscle is stimulated by a nerve impulse, causing calcium ions to be released in the muscle fiber. This triggers the interaction of actin and myosin filaments, leading to the shortening of the muscle fiber. ATP is required for this process.