|On initial inspection, Energetic has visual similarities to Kevlar or an aramid product, but the comparison ends there. Iso-Speed claims the multifilament Energetic 16 combines comfort, control and durability. This string is recognized in particular for its tension maintenance. A popular choice for players using larger racquets. Iso-Speed Energetic 16 gauge is a multifilament with an inner core comprised of 280 polyemide fibers with an outer core of 600 polyester fibers. Polyolefin ribbons are then applied to create the outer wrap.