Both of these mechanisms of becoming horizontal require a small amount of energy from the swimmer. When a person wearing a thick wetsuit floats motionless in the water, their legs tend to float on the surface. Theoretically, this obviates the small energy expenditure mentioned above, although an additional small amount of energy is required to continually flex the wetsuit during swimming motions. High-end triathlon wetsuits have extra flexibility that provides easier range of motion than a surfing or diving wetsuit. Some triathlon wetsuits have varied thickness by way of panels that provide custom flotation that aids the wearer in keeping an efficient position in the water. Open water swimming is an Olympic-sanctioned discipline. It has been included in the Olympic Games since 2008 and the FINA World Aquatics Championships since 1991.