When I was in high-tech sales, I owned several St. John Knits suits. They were great for travel. One day I needed to re-stitch the center back seam on a pair of pants, and I made a discovery. They sewed a piece of clear elastic in the seam to stabilize it. I thought that was brilliant! Ever since, I have used clear elastic to stabilize the crotch curve on all my knit pants.
I prefer to use 1/4 inch clear elastic, but I was out of it, so I used 3/8 inch instead here.
You can either serge the seam, sandwiching the elastic between the back pieces (carefully, making sure that the elastic doesn’t show on the outside of the garment), or you can apply the elastic to the seam after you have sewn it. I usually do the latter, and that’s what I show here. It’s super easy. Just zigzag a length of clear elastic within your seam on one side. Using the zigzag stitch allows the elastic to stretch as needed when you wear the pants. Don’t stretch the elastic as you sew it – you don’t want to have your pants bunching up.
I attach the elastic starting in the front about an inch above where the curve starts, going around to at least an inch above the end of the curve in the back. If I’m working with a heavy knit (rayon doubleknit or the like), I will sew the elastic all the way to the waistline in the back. The elastic keeps the seam from sagging over time. It adds no bulk, and it’s comfortable to wear.
All in all, an easy addition that will make a great difference in comfort and wearability.