Test the underband elastic and the main fabric separately, before and after washing. In many sports bra constructions the underband provides much of the support, and its tension falling after laundering would explain a band that feels looser and rides up. The main fabric's recovery also contributes, so test both.

Test the underband elastic

ASTM D4964 measures tension and elongation of elastic fabrics on a constant-rate-of-extension tester. The buyer and supplier agree the elongation or tension points. Test the underband elastic new and after an agreed number of cycles under a named laundering procedure such as ISO 6330, including the drying method on your label. A noticeable drop in tension after washing points to the elastic.

Test the main fabric

Stretch fabrics with higher power behave differently from light knits, so choose a method suited to the fabric with your laboratory. Measure stretch and recovery new and after the same laundering. For lower-power knits, ASTM D2594 covers stretch properties including fabric growth. If the main fabric grows a lot, the bra loses compression even if the band holds.

Check the construction

How the elastic is enclosed and attached matters. Elastic stretched heavily during sewing, or covered with a fabric that relaxes, can lose tension sooner. Ask the factory to confirm the elastic reference, width, cut length per size and attaching method match the approved sample.

Match the care label to the evidence

Heat can reduce elastic performance. If your tests show that tumble drying causes the loss, the care label should say so; ISO 3758 symbols indicate the most severe treatment that does not cause irreversible damage. But if the loss happens even under gentle care, change the elastic or fabric rather than relying on the label.

Write the tested elastic and fabric references, their washed results and the care conditions into the spec, so the reorder can be checked against them.

Size grading matters too. If complaints come mainly from one band size, check that size's underband length and elastic cut length against the grading rules. A small grading error can feel like a loss of support even when the materials are fine.