Being an athlete involves more than competing in athletic events. Athletes spend many hours each day practicing skills and improving teamwork under the guidance of a coach or a sports instructor. They view videotapes to critique their own performances and techniques and to learn their opponents tendencies and weaknesses to gain a competitive advantage. Some athletes work regularly with strength trainers to gain muscle and stamina and to prevent injury. Many athletes push their bodies to the limit during both practice and play, so career-ending injury always is a risk; even minor injuries may put a player at risk of replacement. Because competition at all levels is extremely intense and job security is always precarious, many athletes train year round to maintain excellent form and technique and peak physical condition. Very little downtime from the sport exists at the professional level. Athletes also must conform to regimented diets during their sports season to supplement any physical training program. Irregular work hours are the trademark of the athlete. People in these occupations often work Saturdays, Sundays, evenings, and holidays. Athletes usually begin competing in their sports while in elementary or middle school, and continue through high school and sometimes college.