Identify the kind of stretch
Stretch the garment gently in both directions and see whether it recovers. A jersey, rib, stretch denim and loosely woven fabric each need a different level of support.
Check whether the damage is from abrasion, a seam opening or fabric distortion. Sashiko rows alone may not solve a seam or a knit ladder.
Stabilise without freezing
A lightweight patch or backing can create a stable stitching zone, but too much structure removes the garment’s comfort. Use the smallest effective support and test the stretch after basting.
Keep the stitch field open and avoid pulling the thread tight. The cloth needs room to expand between rows without the thread cutting into it.
Set realistic expectations
A stretchy garment may need a flexible mending stitch, a repair designed for knit fabric or professional alteration rather than a dense traditional sashiko field.
If a sample remains comfortable through the full range of movement and wash, continue with the same method. If it puckers or restricts the garment, choose a different repair plan.