According to Kestbaum, jumps are divided into eight parts: the set-up, load, transition, pivot, takeoff, flight, landing, and exit. All jumps, except for the Axel, are taken off while skating backward; Axels are entered into by skating forward. Skaters travel in three directions simultaneously while executing a jump: vertically (up off the ice and back down); horizontally (continuing along the direction of travel before leaving the ice); and around. A figure skater travels in an up and across, arc-like path while executing a jump, much like the projectile motion of a pole-vaulter. A jump's height is determined by vertical velocity and its length is determined by vertical and horizontal velocity. The trajectory of the jump is established during takeoff, so the shape of the arc cannot be changed once a skater is in the air. A skater's body absorbs up to 13—14 g-forces each time he or she lands from a jump, which sports researchers Lee Cabell and Erica Bateman state contributes to overuse injuries and stress fractures.