Alright, looking at the left arm, on our female athlete, I want you to once again, imagine it’s the only force acting on her body. So, nothing else is going on. The other arm isn’t involved and neither are her legs. See Figure 1.
We see, then, that because it has been thrust behind her, which is a function of the shoulder, or, arm extensor muscles, it will still exert a pulling force on the left shoulder joint, just like the right shoulder did, except this time it’s backward in the direction of the white horizontal arrow. Again, see Figure 1 above.
And yes, there is a vertical component to this force, as well, that I have shown by the red arrow. See Figure 2 below.
But as before, I am going to illustrate the net result of these two components with just one simple horizontal white arrow. Now, again, if this was the only force acting on the body, do you think the effect of it would be to just pull her straight forward, or, do you think there might also be some additional reaction?
Well, once more, the only way this force causes her to move straight ahead would be if it were acting along her midline, as seen in Figure 3, but that’s not the case.
So, because it’s been displaced away from it, It would not only pull her backward, but, would also cause her upper body to twist, or, rotate. See Figure 4.
It would have to because it does not fall along her midline where her center of gravity lies. So, if she does rotate, the question again, is, which direction would it be? Clockwise, or, counterclockwise? To help you see this better, let’s look at this from above on our male athlete.
The same force on our female athlete pulling her left shoulder backward can also be shown here on our male athlete in Figure 5 below.
So, as the force pulls backward on this vertical line that now represents the level of his shoulders, we can see where it will try and rotate it in the… what direction? Care to take a guess? If you said the counterclockwise direction, then you are correct yet again, and let’s show it by the direction of this red arrow. See Figure 6.
And let’s also go ahead and place the corresponding counterclockwise red arrow showing this rotational force, or, torque on our female athlete and perhaps now you can envision this one taking place a little better, as well. See Figure 7.