Strength Training, Bone Mineral Density and Growth in Adolescents

Does strength training impair growth in adolescents? If you think so you are falling into an old myth.

It has been known for decades that junior competitive weightlifters have an increased bone mineral density (BMC) well above the age-matched controls' mean (1). The effect of this type exercise appears to overcome any race or age-related BMC differences (1).

In another old study, BMD values of the spine and femoral neck of junior weightlifters were found to be significantly greater when compared with adult reference data (i.e., 20-39 yr old men) (2). It has found that strength accounting for 30-65% of the variance, and that in elite junior weightlifters, muscle strength has a major influence on BMD due to the influence of the chronic overloads experienced in training (2).

Several studies found that strength training, with proper technique and strict supervision, can increase strength in preadolescents and adolescents. Studies have reported that resistance training can be effective in producing strength gains among prepubescents, as shown by a meta-analysis (3). 

The effectiveness of resistance training can be influenced by factors such as age and maturation, gender, as well as the frequency, duration and intensity of the training program (3). From this meta-analysis it appears that a training frequency of twice per week is sufficient to induce strength gains in children (3).

A subsequent meta-analysis confirmed that resistance training appears to enhance strength and muscular endurance in children and youth, and that the magnitude of the effect appears to be a function of gender, training method, and experimental design (4).

Gains in strength can be attributed to a neurologic mechanism that enables them to increase the number of motor neurons that are “recruited” to fire with each muscle contraction (5,6,7,8,9).

Strength training augments the muscle growth that normally occurs with puberty in boys and girls by actual muscle hypertrophy (5,8,10,11) .

Regular physical activity, sport participation, and training for sport have no effect on attained stature, timing of peak height velocity (PHV), rate of growth in stature (12), and no effect on the timing of menarche in swimmers, track athletes, and rowers (12).

No apparent adverse effect on linear growth, growth plates, or the cardiovascular system were observed (5,6,13,14,15,16). However, caution should be used for young athletes with preexisting hypertension (5), and youth who have received chemotherapy with anthracyclines (17).

Evidence is also available for the role of activity in modulating the external geometry and trabecular architecture during growth, potentially enhancing skeletal strength with the magnitude of the effect of a 7% to 8% increase in peak BMD (18). 

Strength can be increased by 30% to 50% in children and adolescents after 8 to 12 weeks of a well-designed strength training program, with at least 2 sessions per week to maintain strength (19).

After 15 months of resistance training female adolescents (aged 14 to 17 years) increased leg strength (40%) and femoral neck BMD (20). After a 2-month resistance-training program (6 exercises, 3 x 10 repetitions maximum [RM], 3 times per week) in 19 untrained preadolescent males (11-13 years old), significant posttraining isometric strength gains were observed (17.5%) and mean testosterone along with free androgen index value were increased (21). However, 2 months of detraining resulted in a significant loss (9.5%) of isometric strength. Authors concluded that resistance training induced strength changes independent of the changes in the anabolic and androgenic activity in preadolescent males (21).

Safety and injury

Since balance and postural control skills mature to adult levels by 7 to 8 years of age (22) it is not advisable to start strength programs before achievement of those skills (5). Specific skill proficiency in their sport should also be attained before embarking on a disciplined strength-training program (5).

Some research indicate that children have participated in weightlifting programs with few injuries (23,24,25), however strict supervision and adherence to proper technique are mandatory for reducing the risk for injury (5).

Several reports indicate that few injuries occur in carefully supervised programs (25) and the most common cause of injury appears to be loss of form when heavy weights are lifted (26). Proper technique, good supervision and training programs appropriate to the athlete's level of physical and emotional maturity are important in this regard (25).

Some of the case reports of injuries related to strength training, include epiphyseal plate fractures and lower back injuries, primarily attributed to the misuse of equipment, inappropriate weight, improper technique, or lack of qualified adult supervision (18). For this reason, trained fitness professionals play an essential role in ensuring proper technique, form, progression of exercises, and safety (18).

The incidence and severity of pain in 13 regions of the body, as well as the site and type of powerlifting injury, were investigated in one study (27). The low back region was shown to be the site with the greatest number of injuries (49) (27), and was also the region with the highest percent of subjects recording an elevated occurrence and level of pain associated with powerlifting (27).

Two cases of bilateral radius and ulnar fractures in adolescent weight lifters were described in another report, but the fractures healed without complications (28). The authors stressed that these injuries can be prevented if coaches, trainers, and sports medicine physicians are aware of their occurrence and undertake necessary preventative measures (28). 

Specifically, adequate supervision with spotters at the end of each bar, appropriate foot wear, and, above all, the maximum level of concentration are key factors in preventing these injuries were recommended (28).

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