An overlock seam joins and edges two layers in one operation, leaving a compact seam allowance that sits as a small ridge on the inside. It is fast, stretchy if set up correctly and widely used. A flatlock seam joins the fabric edges side by side or slightly overlapped, with stitching across both faces, so the seam lies flat with little or no ridge. That is why it is common on activewear and base layers where seams press against the skin during movement.

Where each works

Flatlock suits seams under pressure or friction: inner leg, gusset, and sometimes side seams on tight leggings. Overlock is often fine on less sensitive seams and is sometimes covered with a cover stitch on the outside for a cleaner look. Many leggings use both. The thigh rubbing your runners mention is a good reason to try flatlock on the inner leg.

Look and cost

Flatlock stitching is visible on the outside as a flat band, which affects the look; some brands use it as a design detail. It needs a specific machine and skilled set-up, and it can be slower, so it may cost a little more. Ask the factory to quote both.

Stretch and durability

Every seam on leggings must stretch at least as far as the fabric does in use, or the stitches will break. Test fabric stretch with a method suited to the fabric, such as ASTM D2594 for knits with low power, or ASTM D4964 for elastic fabrics, and compare seam stretch on the garment. Pull seams by hand to the extension the body needs and check for cracked stitches. Wash samples with a named procedure such as ISO 6330 and repeat.

Thread and fabric

Flatlock seams on fine fabrics need suitable thread and tension. Too tight, and the seam puckers; too loose, and it can ladder or show gaps. Approve the seam on the actual fabric and colour.

Specify it

In the tech pack, name the stitch type for each seam, the stitch density and thread, and include a close-up photo of the approved seam. Add a wear test on the inner leg for active styles.