Question

why must muscles work in pairs to move a bone at a joint?

Answer 1

biceps and triceps help moving the bones 

Answer 2

Answer:

Muscles work in pairs to move a bone.

Skeletal muscles only PULL in one direction. For this reason they always come in pairs. When one muscle in a pair contracts, to bend a joint for example, its counterpart then contracts and pulls in the opposite direction to straighten the joint out again.