Our gym shorts have a built-in liner. What fabric and seams should the liner use?
The liner sits against the skin, so comfort, stretch and drying matter more than for the outer shell. Specify liner fabric, waistband join and seam types separately.
We are developing running shorts with a built-in liner. Our first samples have a stiff mesh brief liner, and testers say it rubs. Should we use compression fabric instead, and what else should we specify?
Main answer
A built-in liner does the job of underwear, so it should be specified like underwear and activewear, not as a lining. There are two common types: a brief-style liner, often in a soft mesh or jersey, and a compression liner, which is like a short pair of compression shorts sewn into the outer shell. Brief liners are lighter and cooler; compression liners give more support and reduce chafing on the inner thigh. Choose based on the activity and the customer.
Liner fabric
Soft, fine knits with stretch usually feel better than stiff mesh. Many liners use polyester or nylon with elastane. If you claim moisture management or quick drying, test the liner fabric with suitable methods such as the AATCC moisture management methods. For a compression liner, check stretch and recovery with a method suited to the fabric, such as ASTM D4964 for elastic fabrics or ASTM D2594 for knits with low power.
Seams and joins
Seams on the liner sit against sensitive skin. Flat seams such as flatlock are common for comfort. Keep seams away from the inner thigh and gusset centre where possible, and use a gusset panel. The waistband join between liner and shell must be flat and stretch with the waistband; a bulky join can rub during running.
Fit and attachment
The liner should fit closely without digging in, and its leg openings need soft, stable elastic or a bonded edge. Check how the liner hangs inside the shell when the wearer moves: a liner that is too short in the rise can pull the shorts down.
Testing before bulk
Wash samples with a named procedure such as ISO 6330, since liner elastics and fabric can change. Run a wear trial with a few people doing the intended activity, and ask specifically about rubbing, heat and movement. Record what changes between sample rounds.
Specify it
In the tech pack, treat the liner as its own component: fabric, weight, colour, seams, elastic, gusset and attachment method, with photos of the approved sample. Keep a washed approved sample as the reference for bulk, since liners often change more in washing than the outer shell.
Sources and further reading
- AATCC — Moisture management methods (TM195, TM197, TM201, TM213)
- ASTM D4964 — Tension and Elongation of Elastic Fabrics (CRE)
- ASTM D2594 — Stretch Properties of Knitted Fabrics Having Low Power
- ISO 6330:2021 — Domestic washing and drying procedures for textile testing
Standards and rules are summarised from their published scope or official guidance. Use the current edition and confirm requirements for your market.
Three perspectives on this question
Purchasing perspective
I'd ask the factory to source liner fabric from a mill that already supplies activewear underwear, since that fabric is designed for skin contact.
Merchandising perspective
Liner type is a big buying decision for runners. I'd show the liner clearly in product photos and describe it in the first lines of copy.
QC perspective
We'd check liner seams, gusset and waistband join on every lot, stretching them by hand to look for cracked stitches.
Working on a similar program?Send your tech pack, fabric references and test requirements. We reply with a quote and the questions we need answered.
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