ISU Judging System (IJS) was structured to reward difficult elements, so skaters earned more points despite falling on multi-rotational, complicated jumps than their competitors who skated "clean" programs with less difficult elements and did not lose points from falling. According to former American figure skater Katrina Hacker, falls during jumps occur for the following reasons: the skater makes an error during their takeoff; their jump is under-rotated, or not fully rotated while the skater is in the air; they execute a tilted jump and is unable to land upright on their feet; and they make an error during the first jump of a combination jump, resulting in not having enough smoothness, speed, and flow to complete the second jump. Injuries to the lower body (the knee, ankle, and back) are the most common for both single skaters and ice dancers. Single skaters experience 0.97 injuries per athlete, over the course of their careers. Single skaters also tend to have more injuries caused by chronic overuse of their lower limbs or backs. Researchers Jason Vescovi and Jaci VanHeest state that 50–75% of injuries can be prevented because they are caused from "training and/or performance issues". If there is an interruption while performing their program, skaters can lose one point if it lasts more than 10 seconds but not over 20 seconds. They can lose two points if the interruption lasts 20 seconds but not over 30 seconds, and three points if it lasts 30 seconds but not more than 40 seconds. They can lose five points if they do not resume their program until three minutes after the interruption begins. They can also lose five points if the interruption is caused by an "adverse condition" up to three minutes before the start of their program. If the quality or tempo of the music the skater is using in their program is deficient, or if there is a stop or interruption in their music, no matter the reason, they must stop skating when they become aware of the problem or when signaled to stop by a skating official, whichever occurs first. If any problems with the music happen within 20 seconds after they have begun their program, the skater can choose to either restart their program or to continue from the point where they have stopped performing. If they decide to continue from the point where they stopped, they are continued to