“Note the way the overalls are bunched into deep half-lock folds in the back of the knee and at the hip joint. On the buttocks, the top of the thigh and the knee, the material stretches tightly over the underlying forms.”
I would add a couple more observations. The folds radiate from points of tension (shoulder, butt, knee, and elbow). Since most modern, fitted garments are constructed to be relatively wrinkle-free for a standing figure, a seated or kneeling figure creates areas of compression where the material has no choice but bunch up.
Folds also radiate from the seams where the arms meet the shoulders on both of these figures. Those folds often don’t continue across the seam because of the contrasts in thickness and fabric grain direction from one side of the seam to another.
All of these effects are softened in knit fabrics (such as T-shirts), which stretch in all directions, as opposed to woven fabrics, which stretch only on the bias.
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Famous Artist's School on Wikipedia
