How does shoulder muscle work in pairs?
Your teacher is probably asking about ‘antagonistic muscle pairs’. This describes the way that the action produced by one muscle (the ‘agonist’) is limited by another one (the ‘antagonist’) pulling in the opposite direction (so that its movement can be finely controlled). Because the shoulder (two joints: the Sternoclavicular joint and the Glenohumeral joint) moves in so many different directions (it’s a ‘multi-axial system’), different muscles are ‘paired’ as antagonist/agonist according to the position it’s in for each movement. For example, you can work out which muscles are acting against each other when you’re sawing a bit of wood, by looking at an anatomy diagram of the shoulder and imagining what would need to get longer (relaxing) and what would need to shorten (contracting) as you move the saw back and forth. But for other movements, the opposing pairs would be different. (The knee only moves in one ‘plane’, so the muscles that move it are always the same.) Try looking in a ‘B
