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Exercise and physical conditioning or physical fitness play an important role in disease prevention. The three primary components of physical conditioning are endurance, flexibility, and strength training. Like adults, adolescents can improve their aerobic fitness by doing aerobic activity with achievement of 60% to 90% of the maximum heart rate for a minimum of 20 minutes at least three times per week. Flexibility should involve major muscle groups, using static stretching only, and be individually tailored for different sports. Stretching does not prevent injuries in persons without preexisting injuries. Strength training is associated with strength gains in adolescents, and is safe when supervised by knowledgeable adults. Areas for future research in regard to physical conditioning and adolescents include 1) the effects of physical education programs on youth fitness, 2) the relationship between flexibility or hypermobility and injury, and 3) the tracking of physical fitness and conditioning from adolescence into adulthood.