Q: Skeletal muscles must work in pairs to move bones back and forth at a joint because they can only do what?


Read this paragraph and choose the correct option from the provided answers:

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.

 Choices:

- vibrate

- pull

- expand

- contract


A:
contract