Dog sport
Weight pulling is a dog sport involving a dog pulling a cart or sled loaded with weight a short distance across dirt/gravel, grass, carpet, or snow. It is a modern adaptation of freighting, in which dogs were used as freight animals to move cargo.
History.
Canine weight pull has a long history dating to at least the Klondike Gold Rush, where it was used as means of entertainment while trialing sled dogs used for freighting gear and passengers through Arctic and sub-Arctic terrain in North America. Mail delivery was also conducted by dog sled in these remote settlements until the 1930s with the last postal dog sled team being retired in 1963. For both freighting and mail delivery, dogs were expected to meet minimum standards of strength and speed, ranging from per dog on the faster mail team to per dog on the slow freighting team. Jack London's 1903 book "The Call of the Wild" makes one of the first literary references to the sport where the fictional dog Buck pulls a heavily loaded sled while spectators place bets on his ability. London spent almost a year in the Yukon, and his observations form much of the material for the book.
Objectives.
Weight pull matches start with either an empty cart or sled or at low weights. Carts may be placed on dirt, carpet or even rails. Dogs are sorted into classes by weight and then fitted with a specially constructed freighting harness designed to distribute the weight and minimize the chance of injury. At the start of a round, dogs are asked to pull the car or 16 feet within a set time frame. Dogs who successfully complete the round are eligible to go on to the next round. At the completion of each round, additional weight is added to the vehicle. The winner of each class is the dog who pulls the most weight. 
While historically associated with freighting and carting dog breeds, today's weight pull competitions are open to any dog regardless of breed, size or gender. 
Sanctioning Organizations.
Canine weight pulling competitions are sanctioned by various organizations, each with their own rules. 
Controversy.
Proponents of weight pull cite the improved fitness and wellbeing of the dogs, especially working breeds. Handlers often cite improved bond with their dog and that no force or baiting with food is used to convince the dogs to pull. The activity has been criticized for cruel or harmful by animal rights activists who cite that dogs are at risk of physical injury including muscle strains and tears, allegations of doping the dogs or abusing them during training and that the sport may be used by some to prepare dogs for dog fighting. 
References.
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