About twenty years ago, I was enrolled in an undergrad course led by a professor who had a theory that children could safely and effectively engage in strength training. A requirement of the class was to help him collect data for a study he was conducting to test this hypothesis. At the time, his thinking was a bit progressive. The school of thought then was that lifting weights could injure the growth plates and stunt growth in the developing child. Today, evidence indicates the contrary--strength training in youth can be safe and beneficial for overall health.

Benefits of a Youth Resistance Training Program

• Increased muscular strength
• Improved motor skills
• Enhanced sports performance
• Increased bone mineral density
• Improved body composition/maintenance of a healthy body weight
• Reduced risk for sports injuries
• Increased self-esteem, confidence and body image
• Improved social skills

At What Age Can Strength Training Begin?

• Strength training for youth can begin around the ages of 7 to 8 years--when a child is able to listen to, understand, and follow directions. In general, if the child is able to participate in other athletic activities, such as Little League baseball or soccer, then he is ready to engage in strength training.

Getting Children Started

• Constant and competent supervision is required while the child is resistance training because of the potential for serious injury. The instructor/coach should have knowledge regarding the physical and emotional development of children and an understanding of the principles of strength training. Safety guidelines, such as proper spotting and correct equipment size, also need to be followed. 
• The initial emphasis should be on helping the child to understand the concepts of basic strength training and developing proper form.
• When choosing resistance level, it is best to underestimate the child's strength. This serves three purposes: first, it reduces the risk for acute and overuse injury; second, it ensures that the correct lifting technique will be learned; and, third, it allows progression to be readily perceptible to the child. In other words, the child will be able to see a quicker advancement through workload levels during the initial stages of the program.  This will help to maintain motivation and create a sense of self-efficacy.
• The American College of Sports Medicine contends that the resistance training guidelines recommended for adults can be implemented in a program for children. The child should perform 1 to 3 sets of 8 to 15 repetitions at a workload that elicits moderate muscular fatigue without compromising proper mechanical form (in general, an intensity that is equal to 60-80% of the child's 1RM). Strength training should be performed 2-3 days per week on nonconsecutive days.

Points to Consider

• The resistance training session should begin with a warm-up and end with a cool down of about 5-10 minutes each.
• Progression should occur gradually.
• Strength training exercises should be varied to develop full-body benefits.
• Prepubescent children increase strength through neuromuscular adaptations. As a result, muscular hypertrophy in this group is unlikely; however, postpubescent youth can expect to see increases in muscle size due to adequate levels of anabolic hormones.
• Strength training activities do not have to be limited to the use of weight  machines and free weights, but can also include medicine balls, resistance bands, and even the child's own body weight (e.g., pull-ups, push-ups, etc.,).

References
ACSM's Resource Manual for Guidelines for Exercise Testing and Prescription; Sixth Edition.

American Council on Exercise