Each frame in this sequence is about two-tenths of a second apart.
Here is a swimmer with pretty good posture in the water. In the first frame she is extending her right arm forward nicely, finding quiet water.
But things go downhill quickly. Literally. In the second frame she is beginning her catch by levering her straight right arm down into the water, using her shoulder muscles.
By the third frame her hand and forearm are perpendicular to her direction of travel, providing the best hold on the water. She has moved her arm through an arc of 90 degrees in the water (hard work), and is only now getting a good hold on the water, so that she can lift her body over that spot.
Still in the third frame, notice how deep in the water her hand and forearm are. She cannot move forward without lifting the entire arm up towards the surface. As she lifts it up, she will be slipping. Also, it will take a lot of lifting energy to get her arm out from that deep in the water. While she lifts that weight out of the water, her body will respond by sinking, which she counteracts by kicking rather hard.
Think of her hand and forearm as the blade of a paddle. As such, it is well anchored in the third frame. But to get to where it is in the fourth frame, she has lifted her entire arm toward the surface. Her "paddle" is slipping vertically.
So show should this look?
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