Muscles are arranged in pairs on the skeleton in order to allow extension and contraction of body parts. According to the BBC, skeletal muscles can only pull in one direction.
The BBC explains that when one muscle in a pair contracts, the other muscle relaxes. This allows joints to bend and then return to their original positions. For example, when the biceps muscle contracts the upper arm, the lower arm is pulled closer to the body, and the triceps muscle relaxes. When the biceps muscle relaxes, the triceps muscle contracts to allow the lower arm to move further from the body. Without these muscle pairs working this way, it would not be possible to straighten one's legs to walk or bend one's fingers to grip an object.