The muscular system consists of over 600 muscles that enable movement, maintain posture, and produce heat. There are three types of muscle: skeletal muscle (attached to bones, under voluntary control), smooth muscle (in organ walls like the stomach and blood vessels, involuntary), and cardiac muscle (only in the heart, involuntary). Skeletal muscles work by contracting -- pulling on bones to create movement. They always work in pairs: when one muscle contracts (shortens) to bend a joint, its partner relaxes and stretches, and vice versa to straighten the joint. Muscles can only pull, never push, which is why they must work in opposing pairs.
Have students make a fist and feel their biceps contract, then straighten their arm and feel the triceps engage -- this demonstrates opposing muscle pairs directly. Use the arm as the primary example: biceps bend the elbow (flexion), triceps straighten it (extension). Distinguish the three muscle types by function: "skeletal = you choose to move it, smooth = your body runs it automatically, cardiac = your heart's special muscle." Show images of the three types under a microscope -- skeletal is striped (striated), smooth is spindle-shaped, cardiac is striped but branched.
You've learned about bones and how they form the framework of your body. But bones can't move on their own -- they need muscles to pull on them. The muscular system provides the force that makes movement possible, from running and jumping to blinking and smiling.
Your body has over 600 skeletal muscles -- the type attached to bones that you control voluntarily. When your brain sends a signal through a nerve to a skeletal muscle, the muscle contracts (shortens), pulling on the bone it's attached to. Muscles connect to bones through tough, cord-like tissues called tendons. When your biceps muscle contracts, it pulls on your forearm bones through a tendon, bending your elbow. But here's the critical detail: muscles can only pull, never push. So how does your elbow straighten back out?
The answer is opposing pairs. On the back of your upper arm, the triceps muscle does the opposite job of the biceps. When the triceps contracts, it pulls the forearm in the other direction, straightening the elbow. Meanwhile, the biceps relaxes and stretches. Every voluntary movement in your body involves at least one pair of muscles working in opposition. Your quadriceps (front of thigh) and hamstrings (back of thigh) are another major pair -- quads straighten the knee, hamstrings bend it.
But skeletal muscle is only one of three types. Smooth muscle lines the walls of your internal organs -- your stomach, intestines, blood vessels, and bladder. You can't control smooth muscle voluntarily; it operates on autopilot, contracting in slow, rhythmic waves to push food through your digestive system, regulate blood flow, and control other internal processes.
Cardiac muscle is found only in your heart. It shares some features with both other types: it's striated (striped) like skeletal muscle, but involuntary like smooth muscle. Its unique property is that it can generate its own electrical signals and contract rhythmically without any input from your brain -- which is why a heart can keep beating briefly even when removed from the body. Cardiac muscle is also extraordinarily fatigue-resistant: it contracts about 100,000 times a day for your entire life without resting.
Muscles also produce heat as a byproduct of contraction. In fact, muscle activity generates about 85% of your body heat. Shivering -- the involuntary rapid contraction of skeletal muscles -- is your body's emergency heating system when you're cold.