descriptions of allowable equipment (for example, wetsuits are allowed in USAT events in the swimming stage of some races when the water temperature is below ), and prohibitions against interference between athletes. Additionally, the use of flippers or other swim propulsion and flotation aids are illegal in triathlon and can result in disqualification. One rule involving the cycle leg is that the competitor's helmet must be donned before the competitor mounts (or even takes possession of, in certain jurisdictions) the bike, and it must remain on until the competitor has dismounted; the competitor may not be required to wear the helmet when not on the bicycle (e.g. while repairing a mechanical problem). Failure to comply with this rule will result in disqualification. Additionally, while on the bike course, all bicycles shall be propelled only by human force and human power. Other than pushing a bicycle, any propulsive action brought on by use of the hands is prohibited. Should a competitor's bike malfunction they can proceed with the race as long as they are doing so with their bicycle in tow. There are also strict rules regarding the 'bike mount' line. Competitors may not begin riding their bicycle out of transition until they are over a clearly marked line. Mounting the bike prior to this may incur a penalty (example: a 15-second time penalty at the London 2012 Olympics was imposed on Jonathan Brownlee, a competitor from Great Britain, for mounting his bike too early.) Other time penalties can be incurred during the race for, among other things, drafting on the bike in a non-drafting race, improper passing, littering on course, and unsportsmanlike conduct. Triathlon and fitness Participants in triathlon often use the sport to improve or maintain their physical fitness. With each sport being an endurance event, training for a triathlon provides cardiovascular exercise benefits. Additionally, triathletes encounter fewer injuries than those who only use running as part of their exercise routine due to the incorporation of low impact swim and bike training. Triathletes spend many hours training for competitions, like other endurance event participants. There are three components that have been researched to improve endurance sports performance; aerobic capacity, lactate threshold, and economy. Injuries that are incurred from long hours of a single activity are not as common in triathlon as they are in single sport events. The cross-training effect that athletes achieve from training for one sport by