why do skeletal muscles have to work in pairs
It is all done this way to produce smooth movement. Muscles work in pairs and sometimes in more than pairs (2) because it makes the movement smooth. The muscle that is making the move is called the prime mover while another is called the antagonist and it resists the move. It slowly "lets go". This way the movement is not jerky. There are often other muscles involved, especially if the joint is complex like the shoulder joint or the knee joint.
There are also helpers of the prime mover which are called on when more force is needed. Sometimes the joint will have to be held in place and muscles called fixators will have this job.
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. Without this arrangement you wouldn't be able to straighten your legs when you walk or bend your fingers to grip something. When your biceps muscle in your upper arm contracts, it pulls your lower arm in towards your shoulder.
However, when it relaxes, your biceps cannot push your arm back out. To do this, your triceps muscle, on the underside of your upper arm, contracts and straightens your arm out. If your triceps muscle wasn't there, your arm would stay drawn in permanently.
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