The overload principle (described on page 79 of the middle school textbook and on page 93 of the high school textbook) provides the basis for determining the amount of exercise necessary for building muscle fitness. The amount of resistance exercise recommended for building muscle fitness varies depending on age. ACSM's Guidelines for Exercise Testing & Prescription, which provides recommendations for adults, notes that 3 to 6 repetitions of heavy resistance can be used at one extreme, and more repetitions at lighter resistance (8 to 12) can be used at the other extreme (ACSM 2014). The guidelines suggest that to elicit improvement in both muscular strength and endurance, 8 to 12 repetitions at an intensity that results in muscular fatigue is recommended for healthy adults. For youth, the general recommendation is that lighter resistance (weight) be used, especially for younger teens. A summary of the literature by Faigenbaum resulted in the recommendation of 10 to 15 repetitions for youth rather than 8 to 12 as recommended for adults. For exercises that use body weight as resistance, such as curl-ups and push-ups, or exercises with very light resistance, up to 25 repetitions can be used (with a focus more on muscular endurance than strength). More than 25 repetitions of an exercise would be possible only with very low resistance and would be of limited value for building muscle fitness. If muscle fitness is the goal, fewer repetitions with a higher resistance is recommended.
Of course, depending on a person's level of fitness, the amount of resistance training will vary. For example, it is recommended that teens who are beginning resistance training start with lower resistance and a higher number of repetitions. As individuals gradually improve (principle of progression; see page 80 of the middle school textbook), the amount of resistance and the number of repetitions will typically change. More advanced exercisers and older teens will typically use greater resistance and fewer repetitions (see the sixth edition of Fitness for Life for high school students).
American College of Sports Medicine. (2014). ACSM's guidelines for exercise testing & prescription (9th ed.). Philadelphia: Lippincott Williams & Wilkins.
Corbin, C.B., and R. Lindsey. (2013). Fitness for life (6th edition). Champaign, IL: Human Kinetics.
Faigenbaum, A.D. (2003). Youth resistance training. President's Council on Physical Fitness and Sports Research Digest 4(3): 1-8. www.fitness.gov.