THIS HOW WE ROLL!
Skeletal muscles create gross or skeletal movement by pulling on a tendon, that in turn pulls on the bone it connects onto. Since skeletal can only pull on the tendon, and not push, we humans require at least two skeletal muscles to be able to influence every long bone in our body. Accordingly, one pulls in one direction, and the other pulls in the opposite direction (as if the first skeletal muscle was pushing; which it cannot).
In order to create movements of diverse types, skeletal muscles function in several different possible roles. The first role that a skeletal muscle can "take upon itself" is the role of the agonist muscle. Agonist muscle/s are the muscles that create the main movement wanted at a specific time, circumstances, and specific joint. For example, if we wish to flex the elbow (create flexion of the elbow), all skeletal muscles that can flex the elbow are agonists for flexion of the elbow.
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If there is a muscle that without its function, the wanted movement cannot occur, that muscle will be regarded to as the prime mover muscle. Although the prime mover is usually the biggest agonist, it is not always the case. A well known example of the biggest muscle not being the prime mover would the biceps brachii muscle (one of the elbow flexors).
Any skeletal muscle/s that create the opposite movement to the agonist/s is considered to be an antagonist/s. The antagonist/s act as a safety guard against the agonist/s doing whatever they want, and developing too much force/strength/power. The activation of the antagonist/s prevents injury to the joint and all of its tissues, structures, and muscles. It prevents overloading the agonist/s as well.
Another role that a skeletal muscle can take it the role of stabilizer. Stabilizer muscles apply their force onto a neighboring joint and increase its stability. A more stable joint is less sensitive to injury, and helps the agonist/s be more effective and efficient in its force application. Stability requires that the stabilizer muscle/s apply their force opposite the direction where the most instability can be created.
In order to understand the need for the next skeletal muscle role that a muscle might have to take upon itself, we must first establish the understanding that skeletal muscles will create any and all movements they can when neurally activated by the nervous system. For example, if a muscle can naturally create flexion and rotation, it will create both movements. The question is, what if we need only the flexion and not the rotation?
Since the muscle can't stop itself from creating both movements, the body will have to activate a neutralizer muscle/s that will cancel the rotation movement of the other muscle that is not wanted. If the the agonist muscle creates internal/medial rotation, the neutralizer will be a muscle that creates external/lateral rotation. The overall outcome of the example given is that only flexion will influence the joint since the rotation has be opposed.
Synergist muscles, also addressed as assistant muscle, are activated when the agonist/s cannot produce enough of the needed force for the wanted movement. Synergists are also activated as the movement passes through a range of motion (ROM) where the agonist/s cannot be effective enough in influencing the joint. In addition, the synergists might be activated if they offer a unique movement that can complement the agonist/s movements.
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