Fitness boot camp classes are growing in popularity at fitness facilities across the country. This type of group physical training, which may take place indoors or outside, employs personal trainers and sometimes former military personnel as instructors. The regimen is designed to build strength and endurance through intense group exercise intervals. Classes may run 30 minutes to one hour in length. Many classes include some form of fast-paced cardiovascular exercise, along with a series of strength-training elements, like using dumbbells or the resistance of one's own body weight. Flexibility exercises or stretching may be part of the classes, borrowing elements from yoga and Pilates. Although the boot camp class gets its name from military training drills, it's not about being yelled at or receiving negative reinforcement from instructors. Many people enjoy these classes because they allows participants to compete against one another, competition that many people feel encourages them to do their best.