Rebound tension is the force at which the fabric will want to return back to its original size after being stretched. Think of a rubber band, when the rubber band is stretched, it wants to return back to its original size. The more you stretch the rubber band, the more tension will build up in the rubber band. When the band has been stretched, tension builds and when released, the band will snap back to its original position. Knit fabrics with a core yarn of elastane have stretch memory, the ability to fully return back to its original shape after being stretched, giving the fabric its elastic properties. A knit fabric made with fibers that contain spandex/elastane will have a greater degree of stretch memory and higher rebound tension than a fabric whose fibers do not contain spandex.
A fabric with low rebound tension, is like slime. Slime will have a high degree of stretch, but once you let go of the slime, it will not return back to its original position. In sum, an elastic band has high rebound tension and good stretch memory. Slime has low rebound tension and poor stretch memory/recovery.
The stretch and recovery of fibers in a fabric is essential for final garments to maintain their shape. There is a certain degree of mechanical stretch with knits but once that level of mechanical stretch has been exceeded, the fibers themselves will then stretch rather than the knit loops (as mentioned in The Difference Between Positive Ease and Negative Ease).
Fabrics with excellent stretch memory/recovery will completely return back to their original shape, whereas fabrics with poor stretch memory/recovery will not return to their original shape and sag on the body. To reiterate, good stretch and recovery of a fabric will prevent bagginess in the final garment after being worn.
There are several factors that influence rebound tension in knit fabrics. The factors that influence the stretch and recovery of a fabric are the fiber components (elastomeric, microfiber, textured etc…), the type of yarn (ply yarn or core yarn), the fabric structure (woven or knitted) and fabric finishes.
Fibers with a high elastane content, such as spandex, have a greater ability to recover back to its original length after being stretched than fibers without elastane. Moreover, fabrics that contain natural fibers tend to have lower rebound tension and may collapse completely when wet since they are more absorbent to moisture. The second factor is the structure of a fabric. With knits, the looser the knit, the more a knit will stretch but the less rebound tension it will have. Looser knits have more stretch but less rebound tension than tighter knits. The third factor is weight, heavier fabrics have greater rebound tension but less stretch than lighter fabrics.
Some fabrics that have high rebound tension are polyester/spandex blends, and nylon/spandex blends.
Fiber content and structure will play a role in determining the correct type of fabric to be used for a garment. For example, synthetic fibers would be the ideal choice when designing for swimwear and activewear. One of the reasons for this is that syntethic fibers (spandex) can stretch father than natural fibers and can recover easily back to their original length. Another reason natural fibers have less rebound tension than synthetic fibers is because natural fibers are more absorbent to moisture while also not having the same degree of elasticity as fibers with spandex/elastane. Whereas, synthetic fibers are made from plastic, which do not absorb water as readily and are therefore more water resistant. When the fabric absorbs water, it becomes heavy and loses its rebound tension. As a result, synthetic fabrics are less likely to lose their rebound tension when wet than natural fabrics.
When drafting for swimwear or activewear, you would want to use a fabric that has high rebound tension so it can conform to the wearer's body and maintain its shape when wet.
The combination of rebound tension and stretch memory/recovery will affect how a garment made with knit fabrics will fit after it has been worn and laundered. Moreover, the level of rebound tension of a fabric will influence the type of garment to be made. When designing garments made with knit fabrics, it is important to keep these factors in mind in order to achieve the desired fit.