Final answer:
Muscles are arranged around joints in pairs and work together to produce movement by contracting against the skeleton. Flexor muscles contract to bend a joint, while extensor muscles contract to straighten a joint. This coordinated action of muscle groups allows for smooth and controlled movement.
Step-by-step explanation:
When it comes to movements of the body, muscles play a crucial role. Muscles are arranged around joints in pairs, working together to produce movement. They do this by contracting against the skeleton, which causes the bones to move. A muscle that contracts to bend a joint is called a flexor, while a muscle that contracts to straighten a joint is called an extensor.
For example, when you bend your arm at the elbow joint, the biceps muscle contracts as the flexor, causing the joint to bend. On the other hand, when you straighten your arm, the triceps muscle contracts as the extensor. This opposing action of muscles allows for smooth and controlled movement around joints.
It's important to note that muscles cannot actively extend, they can only contract. So, to move bones in opposite directions, pairs of muscles must work in opposition. One muscle in the pair contracts while the other elongates. This coordinated effort of muscle groups helps to produce a wide range of movements in the body.