Specify a measured pressure profile for each shirt, not a word like "medium". Then match your marketing claims to what was measured. Sports compression is not medical compression. Medical compression garments are a separate, regulated product category with their own test methods, and a fashion or sports brand should not borrow medical language without that route.

What can be measured on a sports compression shirt

DIN SPEC 4868, initiated by the Hohenstein institute, is the first published standard for compressive sports textiles. According to Hohenstein, it covers measurement of local pressure, pressure profile, compression category, stiffness and material fatigue. That gives you a way to state, for each size, how much pressure the shirt exerts at defined body points, and how that changes as the fabric fatigues.

For two lines, the brief can say the gym shirt and recovery shirt must fall into different measured categories at named points, and that the difference must hold across the size range. Ask the factory to prototype both and have them measured, rather than estimating compression from fabric weight or spandex percentage.

Fabric, pattern and size all change pressure

Pressure comes from the fabric's tension at a given stretch and how much the pattern makes it stretch on the body. The same fabric can feel firm in a small and loose in a large if grading does not account for that. Fabric tension and elongation can be characterised with a method such as ASTM D4964, and that helps the factory choose fabric. But the garment measurement is what supports the product claim.

Keep the claims within the evidence

A measured category supports a statement such as "firmer compression than our training shirt". It does not by itself support claims about faster recovery, circulation or injury prevention. Those are performance or health claims and need their own evidence. If your market's advertising rules require substantiation, plan for it before the copy is written.

In short, define each shirt by a measured pressure profile per size, approve samples against it, and write marketing copy that only repeats what the test showed.

Keep a record of the tested sizes, measurement points and fabric reference with the approved sample, so the same profile can be checked again at reorder.