The muscular system in the human body is responsible for movement, stability, and posture. It consists of three types of muscle tissue: skeletal muscles, which are attached to bones and enable voluntary movements like walking and lifting; smooth muscles, found in the walls of internal organs like the stomach and intestines, controlling involuntary movements like digestion; and cardiac muscle, which makes up the heart and is responsible for its rhythmic contractions. Muscles work by contracting and relaxing in response to signals from the nervous system, enabling various physical activities and maintaining bodily functions.