In skydiving, Spectra is one of the most common materials used for suspension lines, largely supplanting the earlier-used Dacron, being lighter and less bulky. Spectra has excellent strength and wear-resistance, but is not dimensionally stable (i.e. shrinks) when exposed to heat, which leads to gradual and uneven shrinkage of different lines as they are subject to differing amounts of friction during canopy deployment, necessitating periodic line replacement. It is also almost completely inelastic, which can exacerbate the opening shock. For that reason, Dacron (PET) lines continue to be used in student and some tandem systems, where the added bulk is less of a concern than the potential for an injurious opening. In turn, in high performance parachutes used for swooping, Spectra is replaced with Vectran and HMA (high-modulus aramid), which are even thinner and dimensionally stable, but exhibit greater wear and require much more frequent maintenance to prevent catastrophic failure. Spectra / Dyneema are also used for reserve parachute closing loops when used with automatic activation devices, where their extremely low coefficient of friction is critical for proper operation in the event of cutter activation.