Spandex, also known as elastane or Lycra, is the single fiber that turned rigid athletic clothing into performance wear. Yet it is also the most misunderstood, and the most commonly mis-specified, component in our order books. After twenty years of sourcing, knitting, and testing stretch fabrics, I can tell you that the percentage on the label tells only part of the story, and often not the part that matters.
The first principle every engineer must internalize is that spandex content does not directly set stretch percentage. Stretch is a product of yarn count, knit structure, fabric weight, and the amount of spandex fed into the ground. We can build a 5 percent spandex fabric that stretches 120 percent and a 15 percent spandex fabric that stretches only 60 percent, simply by changing how tightly the spandex is stretched, or draft, during knitting. Specifying “5% spandex” without specifying draft and structure tells our factory almost nothing.
What spandex truly contributes is power, recovery, and shape retention. Power is the resistance the fabric offers as it stretches, the pressure a compression garment applies. Recovery is how completely and how quickly the fabric returns to its original dimensions after the stretch is released. A high-draft spandex knit delivers high power and brisk recovery; a low-draft construction feels softer, drapier, and less compressive. Choosing the right power profile is a design decision that changes the entire fit experience.
Recovery is measured, not guessed. In our laboratory we run a fixed-cycle stretch test, stretching the fabric to a defined elongation, holding, releasing, and measuring residual deformation after rest. A quality performance fabric recovers more than 90 percent within minutes and over 95 percent after several hours. The garments that “sag in the knees” are almost always fabrics that failed this recovery curve, regardless of what percentage was printed on the care label.
There are different classes of spandex, and they are not equal. Standard spandex, polyether-type, is fine for most apparel. Polyester-type spandex, and the newer so-called “hard” elastane grades, resist chlorine and high temperatures far better, which is why swimwear uses them. In sportswear, the choice also affects dyeing: spandex does not dye like polyester or nylon, and if the fabric is not carefully heat-set, exposed spandex fibers can appear as white streaks, what we call “grinning,” after stretching.
The construction of the spandex yarn matters as much as the polymer. Bare spandex is raw and tacky; covered spandex is wrapped in nylon or polyester filament, giving it strength, a smooth surface, and dyeability. For outer layers, a single- or double-covered spandex prevents the “shiny bare fiber” look and improves snag resistance. In our standard athletic leggings, we use bare spandex in a plated knit so it sits on the technical back, and covered spandex in the face for durability and a clean finish.
Heat-setting is the step where most quality problems hide. After knitting, the greige fabric is set on a stenter at elevated temperature to lock in the width, the stretch, and the recovery. Get the temperature or time wrong, and you either melt-set the stretch out of the fabric or fail to lock the recovery in, and the finished garment grows and sags within the first few washes. A competent supplier should be able to show you the heat-set temperature and the shrinkage and recovery data that came out of it. If they cannot, be suspicious.
My final advice to any brand developing stretch activewear: write stretch specifications in performance terms, not percentages. Specify the stretch percentage at a given load, the recovery after cycling, and the force the fabric exerts at a defined elongation. Then let the factory engineer the spandex content, type, and draft to hit those numbers. The label percentage is chemistry; the numbers you care about are behavior. Twenty years of testing has taught me to trust the behavior every time.
