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1. What are muscles?
2. How many muscles are there in the human body?
3. What are muscles responsible for?
4. What’s the difference between a flexor and an extensor?

Muscles
Muscles are a bundle of fibrous tissue that make up the muscular system. There are between 650–700 muscles in the human body. Muscles have
the ability to contract. When muscles contract, they become shorter. When
they become shorter, they pull. Muscles work in pairs. When one muscle pulls,
the other one – its antagonist – pulls in the opposite direction. When muscles
contract, they pull on the bones they are attached to, allowing the body to
move. The muscle that contracts to make a part of the body bend at the joint
is called the flexor. The muscle that contracts to make a part of the body
straighten at a joint is called the extensor. For example: to bend an elbow, the biceps muscle contracts, while the triceps
relaxes. To straighten an elbow, the triceps contracts, while the biceps relaxes.
Muscles are involved in the following functions:
• movement
Skeletal muscles move the bones, which in turn move the body. Muscles also help move fluids around the body (blood, urine, lymph).
• body coordination. 
By contracting, muscles help maintain the correct posture. Without muscles people would not be able to walk or sit down.
• heat generation
When muscles contract they generate heat and thereby help maintain a normal body temperature.