We've all tried holding a plank. Sure it requires strong core muscles. The plank is also made possible by a strong Serratus Anterior muscle, aka the Superhero muscle.
The Serratus is the saw-like serrated muscle that you would see on the side of the ribcage of really well-defined men and women. It spans horizontally from the front of the first 8-9 ribs all the way back to attach at the medial border of the scapula (closest to the spine).
When it contracts, i.e. when you use your Serratus, your shoulder blades slide outward (retract). It's the opposite action of squeezing them together.
If you hold the plank pose, your Serratus has to work hard to stabilize your shoulder blades in their place.
You strengthen your Serratus by holding planks, as well as doing push-ups, holding downward-facing dogs, and holding side planks.
Why We Need a Strong Serratus Anterior
1. Shoulder injury prevention. The Serratus stabilizes the scapula and thereby prevents excessive movement that may cause injuries.
2. Better Range of Motion of the whole shoulder girdle.
3. Better performance. Whether you need it for a swim stroke, golf swing, or a tennis serve, a strong Serratus Anterior is going to help improve and free up that action.
4. Freedom from neck tension. When the Serratus is weak, it's not able to stabilize the shoulder blades and other muscles have to step in. Typically the Levator Scapula and the Trapezius try to compensate - neither of them are designed for stabilization, so they tire and tighten, causing serious neck and shoulder tension. Many people complain of their neck tension which in fact can be treated by strengthening their Serratus.