have, from the jump's takeoff to its landing, a "very good body position". A jump combination, defined as "two (or more) jumps performed in immediate succession", is executed when a skater's landing foot of the first jump is also the takeoff foot of the following jump. All jumps are considered in the order they are completed. Pair teams, both juniors and seniors, must perform one solo jump during their short programs. Jumps are divided into eight parts: the set-up, load, transition, pivot, takeoff, flight, landing, and exit. All jumps except the Axel and Waltz jumps are taken off while skating backward; Axels and Waltz jumps are entered into by skating forward. A skater's body absorbs up to 13–14 g-forces each time he or she lands from a jump, which may contribute to overuse injuries and stress fractures. Skaters add variations or unusual entries and exits to jumps to increase difficulty. Factors such as angular momentum, the moment of inertia, angular acceleration, and the skater's center of mass determines if a jump is successfully completed. History According to figure skating historian James R. Hines, jumping in figure skating is "relatively recent". Jumps were viewed as "acrobatic tricks, not as a part of a skater's art" and "had no place" in the skating practices in England during the 19th century, although skaters experimented with jumps from the ice during the last 25 years of the 1800s. Hops, or jumps without rotations, were done for safety reasons, to avoid obstacles, such as hats, barrels, and tree logs, on natural ice. In 1881 Spuren Auf Dem Eise ("Tracing on the Ice"), "a monumental publication describing the state of skating in Vienna", briefly mentioned jumps, describing three jumps in two pages. Jumping on skates was a part of the athletic side of free skating, and was considered inappropriate for female skaters. Hines says free skating movements such as spirals, spread eagles, spins, and jumps were originally individual compulsory figures, and sometimes special figures. For example, Norwegian skater Axel Paulsen, whom Hines calls "progressive", performed the first jump in competition, the Axel, which was named after him, at the first international competition in 1882, as a special figure. Jumps were also related to their corresponding figure; for example, the loop jump. Other jumps, such as the Axel and the Salchow, were named after the skaters who invented them. It was not until the early part of