Question: Answer the following question given this paragraph: Muscles can only contract. They can't actively lengthen. Therefore, to move bones back and forth at a joint, skeletal muscles must work in pairs. For example, the bicep and triceps muscles of the upper arm work as a pair. You can see how this pair of muscles works in Figure below . When the bicep muscle contracts, it bends the arm at the elbow. When the triceps muscle contracts, it straightens the arm. Q: Skeletal muscles must work in pairs to move bones back and forth at a joint because they can only do what? Choices: - vibrate - pull - expand - contract A:
Answer:
contract