You can use one elastic for both lines if it meets the requirements of the more demanding use. Often, though, the two lines end up with different elastics because their priorities differ. The athletic style relies on the waistband and leg straps to hold position during movement. The fashion style often uses a wider, softer or decorative band where appearance and comfort matter more. Decide that per line, and write each choice down as a specification.

Describe each elastic by measured properties

For each line, specify width, construction, fibre content and the tension and elongation behaviour you want. 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. For an athletic style you may want higher tension at the agreed elongation. For a fashion band you may accept lower tension in exchange for comfort. Either is fine if it is stated and tested.

Leg straps carry a lot of the load in a jockstrap, so specify and test the strap elastic separately from the waistband. They are often different tapes.

Test after laundering

Elastic performance can change with washing, which is when many complaints start. ASTM D4964 states that laundering temperature and cycle counts are by agreement. Use a named home-laundering procedure such as ISO 6330, re-test after the agreed cycles, and compare with the new result. If one elastic is used across both lines, one test program covers both.

Keep the care label aligned

If the elastic only holds up when washed cool or line-dried, the care label must say so. In the U.S., the FTC Care Labeling Rule (16 CFR Part 423) requires care instructions to be supported by reliable evidence. Your wash test is part of that evidence, so test under the same conditions the label recommends.

The practical answer: decide whether one elastic can meet the athletic requirement and still look right on the fashion band. If not, run two specified elastics, both tested new and after washing.

Record the chosen elastic references and test results with the approved samples for both lines, so reorders and any elastic substitutions can be checked against them.