Is strength training safe for teens?

Dear Mayo Clinic: My daughter is 15 and lifts weights regularly to stay in shape. Lately, she's complained about stretch marks on her legs as a result of weight lifting. She doesn't like how they look, but I'm more concerned that she is doing too much. Is weight training healthy for girls who are still growing? How much is too much?

A: Muscle strengthening activities, such as lifting weights, can be beneficial for teens. But, weight lifting is not the only way to increase strength. Encourage your daughter to consider varying her workouts to include other kinds of strength training. That may reduce the risk of unwanted side effects, such as stretch marks, that can come from doing just one activity. Also, muscle strengthening shouldn't be the only activity a teen uses to stay in shape. It needs to be part of an overall fitness program that includes aerobic activity as well.

In general, a safe and effective workout routine for teens involves strength training three times a week on nonconsecutive days. Your daughter can change up her activities throughout the week and still continue to build strength.

For example, with some activities, she can use her own body weight for resistance - a technique called body weight training. Examples include exercises such as rope or tree climbing, swinging on bars or other playground equipment, games such as tug-of-war, pushups, squats, lunges, abdominal crunches, pullups or step-ups.

Another option is to work muscles using resistance tubing. A lightweight, portable, inexpensive strength-training tool, the tubing provides resistance when stretched. Resistance tubing can be used to strengthen almost any muscle group.

When your daughter lifts weights, they can be free weights or part of weight machines. It's important that a trained professional supervise weightlifting to ensure teens use proper technique and lift the appropriate amount of weight.

Overall, strength training is safe for teens. The rate of injuries is low, with the most common injuries related to inadequate supervision or instruction, using improper technique, or trying to lift too much weight. In the past, there was some concern that muscle strengthening may have a negative impact on a teen's growth, but recent studies have found that growth is not affected by strength training.

To reduce the risk of injury, it's best to do a 10- to 15-minute warmup of light aerobic exercise before strength training. Stretching is not necessarily needed before strengthening; however, stretching can be performed afterward with at least 30 seconds of stretch per muscle group.

In addition to increased strength, teens can gain a variety of benefits from regular strength training, such as better physical endurance, enhanced self-esteem and higher self-confidence. Teens who engage in regular strength training often see improvement in their cholesterol levels, blood pressure, blood sugar and body weight. Research has found they tend to perform better in school, and they have lower levels of depression and anxiety than other teens. Muscle strengthening also contributes to building bone strength. That's important for teens, because 95 percent of a person's bone mass is accumulated by the end of the teenage years.

To achieve overall fitness, aerobic activity should be part of your daughter's routine. High-impact aerobic activity also provides the added benefit of building bone strength in teens. A good goal for teens is at least one hour of moderate to vigorous physical activity every day, with at least three days a week of aerobic activity at a vigorous level.

Moderate aerobic exercise includes brisk walking; games that require catching and throwing, such as baseball and softball; and active recreation, such as canoeing, hiking, skateboarding or inline skating. Examples of vigorous aerobic exercise are jumping rope; running; cross-country skiing; games that involve running and chasing, such as flag football or tag; and sports such as soccer, hockey, basketball, swimming and tennis.

©2016 Mayo Foundation for Medical Education and Research Distributed by Tribune Content Agency, LLC.