Serratus Anterior can be divided into three parts. The uppermost fibers of this muscle attach from the upper surfaces of ribs 1 and 2 to the uppermost part of the inner border of the scapula.
The fibers from ribs 3 and 4 fan out to attach along nearly the entire length of the inner border.
The fibers from ribs 5, 6, 7, 8, and 9 (and sometimes 10) all tie together to attach to the bottom of the inner border of the scapula. The action of these lower most fibers is the action most likely to get confused with that of the teres major since both attach to the lower portion of the scapulae.
These lowermost fibers are important because they can be used to pull the bottom corner of the scapula forwards, causing it to rotate in such a way that the bottom tip moves outwards and the shoulder socket moves upwards. The feeling is like you are digging the inside edge of the shoulder blade upwards into the back of the ribcage.
Personally I think it is this particular muscle fiber activation (activating the lower fibers) that can help prevent winging of the shoulder blade when the arms and shoulders are reasonably relaxed while standing (or sitting) upright.
To let the arms hang down with causing the shoulder blades to wing, first lift and expand the chest. Then spread the bottom tips of the shoulder blades so that your upper back feels open. Then roll your upper arm outwards so that the front of your shoulder feels open.
Let your arms hang from your shoulders.
If you didn't already know, you can see the saw tooth shaped ends of serratus anterior at the sides of the ribcage just below the pectoralis major.
Because serratus ant. attaches to the inner edge of the scapula (the part of the scapula closest to the vertebral column) one of the easiest ways to deliberately activate it is to focus on drawing the inner edge of the scapula away from the spine. If you are moving both shoulder blades at the same time then you can focus on moving the inner edges of each shoulder blade away from each other.
If you can't get the hang of this action then focus on moving your shoulder forwards and backwards a few times. Then once you've got the feeling of that, do the same thing but while focused on feeling the inner edges of your shoulder blades.
Serratus anterior attaches to the inner border of the shoulder blade and passes forwards around the sides of the ribcage to attach to the "front corner" of the ribcage. It's sandwiched between the front surface of the scapula and the ribcage.
The teres major attaches to the back of the scapula, near the bottom. It attaches to the front of the arm bone just below the shoulder joint. While this muscles is located on the back of the scapulae, its fibers reach forwards (and upwards) to attach to the arm bone.
With the arm by the side of the body the bodies of these two muscles are relatively close in space. And they share a similar action. Where serratus can be used to pull the shoulder blade (and shoulder joint) forwards, the teres major pulls the bottom of the shoulder blade forwards also. The big difference is that the teres major acts from the upper arm bone while the serratus anterior acts from the ribcage.
Another muscle action that may get confused with that of the serratus anterior is the pectoralis major. This muscle can be used to pull the armbone forwards relative to the ribcage, pulling the scapula with it. You can notice on yourself (or your students) if this muscle is activating to pull the shoulder forwards.
Neither of these muscles is bad. The point is being aware enough to be able to differentiate their actions.
The optimal arm position for "best" activation of the serratus is forwards and up, so that the hand is at about eye height.
If you slowly lift your arm to this position you may be able to feel the bottom tip of your shoulder blade moving ouwards. In this position you can try to reach your shoulder blade further away from your spine so that your arm reaches further forwards. Focus on moving the inner edge of each shoulder outwards (do this one arm at a time or both together.)
Keep your chest lifted and ribs open!
See if you can memorize the feeling of this position. Then keep this position while slowly bending your elbows. You may find that if you pull your elbows back behind your body it's harder to keep the "spread" feeling. Instead it may feel better to retract the shoulder blades (pull then inwards.)
If you look at the front of the inside edge of the shoulder blade there are two roughly triangular areas at the top and bottom. The uppermost "triangle" is at the "medial angle of the scapula. The lower triangle is at the inferior angle of the scapula.
In between is a straight section with runs along the inner edge of the shoulder blade from top to bottom. The serratus has fibers attaching to both triangle and to the line joining them both.
The scapula is roughly triangular in shape with one tip pointing downwards.
This bottom point is called the inferior angle. The point closest to the center line of the body is called the medial angle and the point furthest away from the centerline of the body is called the lateral angle.
The lateral angle is approximately where the shoulder socket is located.
Working backwards from the ribcage, the uppermost fibers or finger, of serratus antrior starts at the ribs 1 and 2 and runs to the uppermost triangular area of the scapulae. From ribs 3 and 4 the third and fourth fingers fan out to connect along the entire inner border (the line joining the two triangles.) The fingers from ribs 5, 6, 7, 8, 9 (and 10) all bunch together to attach to the lowermost triangle.
It may be that with the scapula positioned as mentioned above, all of these fibers are at optimal length so that it is easy for them to all contract and pull the scapula forwards or prevent it being pushed backwards.
The normal function of the serratus is to pull the scapula forwards relative to the ribcage. Remember it acts on the inner edges of the shoulder blades. If the shoulder blades start close together on the back of the ribcage, then the serratus first have to pull the inner edges of the scapula outwards before they can pull them forwards.
And so you could think of the ribcage as pulley, with the serratus wrapping partially around that pulley to pull on the shoulder blades.
So that they can pull effeciently one thing that you can do is set up the ribcage so that it acts as a good foundation for your serratus anterior to act on the scapula. Using the aforementioned external intercostals, in combination with the internal intercostals, you can lift and expand the ribcage.
For an even firmer ribcage, lead this action by pulling the head back and up so that the back of your neck feels long. Pull your chin in slightly. As you do this action allow your chest (the front of your ribcage) to lift and expand. One way to practice this action is with easy breathing.
With your neck long and your ribcage lifted you can then practice activating your serratus by moving your shoulders forwards and then relaxing so that they return and move back.
Because the shoulder joint is located on the shoulder blade, and because the shoulder joint is located at the outside edge of the shoulder blade, the minute you move your shoulder blades away from each other the shoulder joint moves forwards. And when you move the shoulder blades towards each other the shoulder joint moves back.
So while sitting or standing, with your chest lifted, you can first practice moving your shoulders forwards and backwards to get the basic idea of moving your shoulder blades.
Even though you are moving your shoulders forwards (and then backwards) you may not necessarily be activating your serratus anterior. You may be activating your chest (pectoralis major) muscles or your teres major instead.
Note also, that you don't have to keep your chest lifted to activate serratus anterior. Try the same action with your chest down.
You can also synchronize shoulder and chest movements. Try lifting your chest and moving your shoulders forwards. Then try lowering your chest and moving your shoulders forwards. In each case, lead or start with the chest movement.
Once you are used to the basic action of moving your shoulders forwards and backwards (do it slowly so that you can feel your shoulder blades and even your collar bones moving) then focus on feeling the inside edges of your shoulder blades as your move your shoulders forwards. Focus on moving the inside edges of your shoulder blades away from each other. Then relax so that your shoulder blades move back to the starting position.
By putting your focus on the inside edges of your shoulder blades you are more likely to activate your serratus anterior muscle. Why? because that is it's point of attachment to the shoulder blade.
To add a little bit of resistance or weight, move your shoulder blades apart and reach your arms forwards. But first lengthen your neck and open your chest. Then spread your shoulder blades. Then keeping them spread move your arms forwards. When you spread your shoulder blades without lifting your arms you may notice that your upper back feels "open." Keep this same feeling as you move your arms forwards.
Do all actions smoothly and slowly.
You can then try lifting your arms forwards and then up over your head. If you keep the spread feeling at the back of your upper ribcage, you'll more than likely be keeping your serratus anterior activated though in this case they'll be assisted by the trapezius and levator scapulae to lift the shoulder blades as the arms lift up.
If you are having trouble with upward facing bow or wheel pose, this is an action that you can try to duplicate while pushing up into the pose. Focus on spreading your shoulder blades and moving them upwards and use this action to "drive" the arms into the floor. It may also feel like you are digging the inner edges of your shoulder blades into the back of your ribcage. In this case you are then using your serratus (and your trapezius) to move your arms upwards relative to your ribcage. It feels so much more powerful and easy that trying to use your arm muscles to lift your body upwards. Note that if you aren't used to using your legs in wheel pose then this action might not help. Learn how to use your legs first.
A more usual way to practice activating and exercising your serratus anterior is in either cat pose or push up position. With your elbows straight allow your ribcage to sink down so that your shoulder blades move towards each other (stretch your serratus.) Then use your serratus to push your ribcage up.
You may find it easier to start with your elbows bent and your forearms and pelvis on the floor. As you did while standing, focus on spreading the inside edges of your shoulder blades away from each other. Because this causes your shoulder joints to move forwards relative to your ribcage your elbows will press into the floor. Your chest will lift as a result.
Now once you've gotten used to the action, set up your foundation. Pull your head up (away from the floor) to lengthen your neck (look down) and pull your chest forwards, away from your pelvis. Then do the same exercise again. Spread your shoulder blades to activate serratus, relax to stretch them and allow your ribcage to move down towards the floor.
You can then do the same exercise with elbows straight. You can also do it while doing side plank, first on your elbow and then with arm straight. (Bend your knees and place your bottom shin on the floor if you have difficulty with side plank.) In each case, prior to lifting your hips, move your ribcage away from the floor in the same way you did with both arms on the floor. You'll be using your serratus anterior to move your spinal column away from the inside edge of your shoulder blades.
While doing a push up and pushing your body away from the floor or while bending your elbows and returning to the floor you can work at keeping your shoulder blades spread so that your serratus anterior stays active.
In a push up, whether doing them with the same position as chaturanga dandasana or a more usual push up with arms out to the sides, you aren't so much trying to spread your shoulder blades as you are trying to keep them from moving.
You are trying to stabilize them and the force that you are working against is caused by the weight of your own body.
So even if your shoulder blades are inwards slightly or a lot, you can give your arm and shoulder muscles a good foundation by first of all keeping your neck long and your chest open and second of all by keeping your serratus active. Keep them active by trying to spread your shoulder blades.
Note that you may find it helpful to try moving your shoulders a little towards your head or a little away from your head. Notice how your shoulder joints feel. If you feel any sharp pain then adjust shoulder position and even the shape of your ribcage so that the pain is no longer present.
For pressing up, you can focus on positioning your chest and shoulder blades prior to pressing up and then slowly apply pressure to press up while keeping your serratus active. Initially try lifting just a little and then relaxing. As you get used to the action lift subsequently higher and higher till you are doing a full press up. As you lift up and as you lower focus on keep the "spread" or "open" feeling in your upper back.
Starting with elbows straight and while lowering down, you can do the same thing, bend your elbows slowly and slightly while keeping your shoulder blades spread and your serratus active. Depending on the arm position that you use you may find that your shoulder blades move toward each other. That may be okay so long as you "work" at keeping the inner edge of your shoulder blades moving apart (even though they move together) so that you keep your serratus anterior active.