The overload principle (see Chapter 4 of Fitness for Life Middles School, Second Edition, or Fitness for Life, Seventh Edition) 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 and the type of fitness you want to achieve. ACSM's Guidelines for Exercise Testing & Prescription, which provides recommendations for adults, recommends low repetitions with high resistance for strength and high repetitions with lower resistance for muscular endurance (ACSM 2022). For youth, the same is true. The information in both Fitness for Life Middles School, Second Edition, or Fitness for Life, Seventh Edition, is based on guidelines from the sources below.
American College of Sports Medicine. (2022). ACSM's guidelines for exercise testing & prescription (11th ed.). Philadelphia: Lippincott Williams & Wilkins.
Faigenbaum, A.D. et al. (2020). Essentials of Youth Fitness. Champaign, IL: Human Kinetics.