Terminology & Definitions
It is useful to learn your muscles in agonist and antagonist pairs.
In the image you have the brachioradialis, rhomboids, and teres major - these muscles can act as synergysts to the brachialis, trapezius, and latissimus dorsi respectively.
In the image if the antagonist muscle were to contract at the same time then it would oppose the agonist and there would be no movement - this increases stability around the joint. Imagine how you might brace the muscles around your knees, hips and core as you prepare to ski down a hill, basically in a quarter squat position maintaining strength around all the joints. This is called co contraction.