When sourcing Lycra, the most critical factor is the composition ratio, typically ranging from 5% to 20% Spandex blended with polyester, nylon, or cotton. You must verify the GSM (Grams per Square Meter) to ensure the fabric weight matches your application (e.g., 160-220 GSM for swimwear or 250+ GSM for compression leggings). Additionally, check the four-way stretch recovery rate; high-quality Lycra should return to its original shape without 'bagging' after being stretched to at least 50% of its length.