Humans are meant to move, and children are no exceptions.
Children need 60 minutes of physical activity every day. This is a minimum recommendation, and increasing this amount offers additional benefits. Daily moderate to vigorous physical activity helps build the child’s bones and muscles, promotes psychological well-being, and lays the foundation for positive habits as an adult, leading to a stronger, healthier life.1
Traditionally, promoting physical activity for children has focused on cardiovascular activities. However, the past few decades worth of research has showed that resistance training offers unique benefits. Strong muscles allow children to function better, both in everyday activities and when participating in various sports during adolescence and the teenage years.
Beyond the muscular and performance-enhancing benefits, resistance training also offers young individuals psychological benefits like improved self-confidence and body image.
Strength training at a young age also lays the groundwork to prevent future health issues by improving a number of health markers and reducing the risk of various non-communicable diseases.
The purpose of this article is to provide a thorough overview of strength training for children and adolescents. It will cover both physiological and psychological effects, as well as potential risks. Additionally, it will give science-based recommendations on how to introduce children to strength training and how to design a safe and effective training program.
Physiological effects of resistance training for children and adolescents
Resistance training in childhood and youth leads to significant increases in strength. These increases are similar to those observed in untrained adults who take up strength training. Children can expect strength gains of up to 30 to 50 %.2 One study showed that 10-year old children who participate in regular bench press training can increase their exercise-specific strength up to 35%.3
The increased strength experienced in adolescents because of resistance training can be attributed to neuromuscular adaptations rather than muscular hypertrophy.4 Strength training does not produce a robust hypertrophic response in children, in part because of insufficient levels of circulating testosterone.
The exact mechanisms behind the neural adaptations related to increased strength in strength-training children are not completely explored, but the main driver is increased activation of muscle motor units. A motor unit consists of a motor neuron and the muscle fibers innervated by that neuron. The central nervous system activates these motor units as the muscular workload demands it, and they work in unison to coordinate muscle contractions.
With regular resistance training, a child’s central nervous system becomes more efficient at recruiting these motor units, resulting in increased muscle coordination and more frequent motor unit nerve firing. This, in combination with the fact that the central nervous system of a child adapts more quickly to stimuli than that of an adult, means that not only do strength-training children show increases in strength, they also improve their motor skills in all explosive exercises and balance-based form of activities compared to their peers.5
Obesity has become more and more prevalent among children and adolescents. Inactivity can often lead to a vicious circle, where overweight children and adolescents actively avoid exercise, as they find physical activity to be uncomfortable, boring, and embarrassing, leading to even greater risk of obesity.
Traditionally, overweight children have been encouraged to take up aerobic activities. An overweight child often lacks the motivation for this type of physical activity, or finds it less than fun. Resistance training places different physiological demands on the body and can be perceived as a more easily accessible type of exercise. By regularly participating in strength training, children and adolescents can acquire the self-confidence required for a more active lifestyle and enjoy said lifestyle.
Juvenile obesity is associated with increased metabolic risk, and resistance training can be part of the solution to improve muscle health and body composition.
An old misconception argues that weightlifting and strength training are harmful to the young skeleton and should be avoided. This misconception has since been replaced by evidence that not only is strength training in adolescence not harmful, youth is also the most beneficial time to strengthen the skeleton and increase bone mineral density through weight-bearing exercise.
Physical exercise, including resistance training, can help ensure that peak bone pass reaches optimal levels by the time the child enters adulthood and the skeleton becomes more resistant to manipulation through outside factors like exercise.8 Low peak bone mass is a significant risk factor for osteoporosis, and participating in weight bearing activities during adolescence is an effective way to counter this risk.
Resistance training has a positive effect on the skeleton even during adulthood, but it is primarily before the skeleton has developed completely and peak bone mass has been reached that the mechanical stress weight training puts on it acts in synergy with natural growth to enhance bone mass.
Introducing weight-bearing activities during adolescence can improve life-long bone health. It is, however, important to maintain this practice throughout adulthood and into old age, since training-related improvements in skeletal health will be lost upon cessation of exercise, just like muscle mass and strength is lost with detraining.11
Lower rates of injury in sports
Even if the main purpose is not to get bigger and stronger for the sake of getting bigger and stronger, strength training in adolescence can provide significant benefits for other sport activities.
Not only does the increased strength and muscle power mean better performance in almost any sport, research has also shown that young athletes who combine their sport-specific training with resistance training show lower rates of injury. In addition, when they are injured, resistance training as part of the overall training program shortens the rehabilitation.
One study showed that high school football athletes who were introduced to weight training as part of their pre-season training experienced lower rates of knee injuries season after season, both less complicated injuries and injuries requiring surgery.12
Another study demonstrated that high school athletes who regularly participated in strength training showed lower rates of injury than control groups who did not perform any dedicated weight training. Those who were injured managed to return to training and competition faster than their non-weight-training peers did.13
A meta-analysis concluded that children and adolescents who participate in strength training could reduce the rate of injury during other sports by up to 66 %.14
Hypertension in teens and even during childhood has become more prevalent as the obesity epidemic has spread to younger populations. There is no clear association between participation in physical exercise on a regular basis and blood pressure in normotensive youth; however, resistance training could help prevent high blood pressure from re-occurring after a successful blood pressure-lowering intervention in previously hypertensive youth.15
Positive changes in diet and body composition are likely the main drivers behind improvements in blood pressure in hypertensive youth, but the notion that physical exercise, a combination of weight-bearing endurance training and resistance training in particular, could help enhance the positive effects is far from unlikely.
Several studies have suggested that resistance training can have a favorable influence on blood lipids and lipoproteins in adolescents.16 17 Genes and diet are probably the main factors, but adding resistance training is likely to have further beneficial effects.
The risk most commonly associated with weight lifting in youth is probably the risk of injuries to the epiphyseal plates. This is a risk exclusive to children and adolescents. In adults, with a fully developed skeleton, the growth plates have already closed.
Epiphyseal plate fractures are common. Up to 30% of all skeletal fractures in children are epiphyseal fractures. These injuries usually heal without problems, but the risk for more severe complications is always a consideration. Serious trauma to these areas could lead to premature physeal closure in a worst-case scenario, resulting in stunted growth and limb length abnormalities.
Doctors and some scientists began advising against resistance training in the late 1970s and the early 1980s after case reports documented increasing trends of epiphyseal injuries and cartilage damage because of weight training in high school.18
National Electronic Injury Surveillance System (NEISS) is a US database where emergency department visit information is collected. Later evaluations of these reports and case studies have since revealed that the weight training injuries from this time mostly occurred because of improper form and poorly designed training programs, not because of strength training per se. The causes of the injuries were likely tied to the use of too heavy loads, improper technique, unsupervised training, and faulty equipment.
Both regular resistance training and weight lifting are safer and less injury-prone alternatives than the majority of sport activities school-age adolescents participate in.19
There is a lack of prospective studies on resistance training in youth and injuries to skeletal epiphyseal plates. However, three studies have shown acute injuries in strength training adolescents, but none of these were skeletal injuries. A review article found that the risk of injury in these three studies were 0.176, 0.053, and 0.055 per 100 hours of training.20 These number are significantly lower than most sports children and teenagers participate in, including all team sports. For example, playing football is almost 1000 times more likely to lead to injury.
Resistance training can theoretically lead to growth plate fractures, thus increasing the risk of permanent damage to the skeleton. This risk is only prevalent when the child is using improper form and more weight than he or she can properly handle. This makes it important to ensure that all children and adolescents who take up strength training do so under proper guidance and after thorough instructions, using correct lifting technique and selecting safely manageable loads.
The current opinion of the majority of the experts in the field is that participating in resistance training before epiphyseal closure is not in itself harmful.21Health care and fitness professionals from all organizations in the field agree that weight training is both safe and effective for children and adolescents, when done in a supervised setting and when recommended guidelines and precautions are followed.
Soft tissue injuries
Soft tissue injuries are the most common strength training-related injuries, in both adults and adolescents. There are numerous reports of injuries related to resistance training in youth, but that is inherent in all forms of physical exercise. When doing something physical, there is always a risk of injury. The only way to eliminate the risk of injury from exercise is to stay sedentary, but that means a much higher risk of negative health outcomes instead.
Luckily, the reported incidence of serious injuries in youth due to resistance training is quite low, especially when compared to sports like football or hockey. Apart from back injuries, the reported type of injury is usually along the lines of shoulder injuries leading to a week away from training.
Back injuries, however, are not uncommon. Of all reported strength training-related injuries in adolescents, about a third of those are injuries to the core, mostly back related. Most of these reported injuries have not been severe, but some have required surgical interventions. Compared to most other sports, the rate of injury is still quite low, and several studies have independently shown that the number of injuries per 100 hours of training is somewhere between 0.053 and 0.055.22 23
One study showed an injury rate of 0.29 per 100 hours of training.24 This study did show a significantly higher injury rate than previously mentioned, but the participants were competitive young power lifters. These are lifters using heavier loads and really pushing themselves, maybe not always using proper form, and injury rates can be expected to rise under such conditions, regardless of the lifters age. Still, compared to competitive sports like rugby, it is still a very low injury rate. The injury rate for adolescents participating in rugby is 275% higher.
The majority of back injuries reported in strength training youth could be the result of muscular imbalance from training for the mirror. Training programs focused on building visually appealing muscles and on looking good in the mirror could lead to neglected core, trunk and back training with resulting strength imbalances.25 Over time, these muscles become a weak link and the risk of injury increases. In this case, the injuries would not be a result of strength training per se, but the result of improperly constructed training programs leading to muscular imbalances over time.
Resistance training is one of the safest types of exercise for children and adolescents
The already low injury rate of youth resistance training might in fact be even lower. In databases for resistance training-related injuries reported in National Electronic Injury Surveillance System (NEISS) there is a code for “weight training”, but that code does not give any indication of how that injury occurred. It could very well be unsupervised children playing in a gym and injuring themselves on training equipment without participating in any structured, actual “training”. Already good statistics could therefore actually be even better.
In conclusion, there is a risk of injury when children and adolescents participate in weight training. Physical exercise always means an increased risk of physical injury compared to sedentary life. However, the risk of physical injury from resistance training is minor compared to the risk from most other types of physical exercise. The existing risk can easily be minimized and almost eliminated with supervised training using proper form and precautions.
The age-specific risks, like epiphyseal plate injuries, are not related to resistance training per se, but to not following recommended practices.
Psychological effects of resistance training in youth
Empirical evidence shows that resistance training in childhood and adolescence is an effective way to strengthen the young body and to lay the foundations for a strong and healthy adult life. However, there is a limited amount of research done on the psychological effects of resistance training, especially in children and adolescents. The available research indicate exclusively positive effects, however.
A 2013 randomized controlled trial demonstrated that overweight teenagers not only showed improved physical parameters like strength, but also improved self-evaluation and behavioral control after six months of regular strength training.26
Half a year after the study concluded, the positive effects had been eliminated, after the participants stopped training. This indicates that it is of utmost importance to make physical exercise, including strength training, a life-long endeavor rather than a temporary activity. The psychological benefits of weight training will slowly be lost with detraining, much like physical gains.
Another study showed that weight-training female schoolchildren rated their own bodies as more attractive and expressed more satisfaction with their own physical appearance compared to age-matched controls.27 These positive effects occurred without any actual physical changes in the girls’ bodies. Resistance training likely increases the satisfaction with the own body, even without any actual physical changes. The same effects were not evident in boys the same age, however.
Yet another study demonstrated that 12 weeks of resistance training resulted in significant increases in muscular strength, decreased amount of body fat, and increased self-esteem, both in overweight and normal-weight teenagers.28
Children and adolescents can benefit from regular resistance training to not only improve various physical health markers and muscle strength, but also to improve self-confidence and body image.
Resistance training in youth lays the foundation for a healthy adult life
A recent meta-analysis, which examined the associations between muscle strength and muscle health in young individuals and health markers later in life, found evidence that resistance training in youth increases the chance to stay strong and healthy in adulthood and old age.29
The better the muscle health in youth, the lower the risk of obesity as an adult. Partly because behavior and habits that promote physical activity carry over into adulthood, partly because of a more effective energy turnover and metabolism.
Isolated studies investigated in the meta-analysis revealed that strong muscles in adolescence were associated with lower risk of cardiovascular disease and metabolic syndrome later in life. The statistical significance of these implications were not enough for a conclusive finding, but this was more due to the amount of available research rather than suspicions that resistance training is useless in this regard.
The meta-analysis did find a significant association between low muscle strength and the risk of insulin resistance. Insulin resistance is a driving factor that leads to type 2 diabetes, and resistance training improves insulin sensitivity and the function of the beta cells, thus demonstrating a protective effect.
Another finding was the previously mentioned association between weight-bearing exercise in adolescence and a stronger adult skeleton.
Childhood and adolescence are formative years for habits and behavior that carry over into adulthood and greatly increase or decrease the risk of a number of diseases and health problems. In some cases, like bone formation, there is no better opportunity in life to influence this risk in a positive way. Resistance training, starting in childhood, can be an important factor contributing to the physiological effects necessary to prevent having to treat these health issues later in life. It is always better to prevent health issues than having to treat them after the fact.
When can a child begin resistance training?
As old myths and misunderstandings concerning strength training in children have been busted, today’s science indicate that the sooner a child is introduced to resistance training in some form, the better.
A child can take up structured strength training when he or she has reached a mature enough stage to understand and follow instructions to ensure correct exercise form. The child must also have reached the neuromuscular maturity needed to maintain balance, control, and posture while performing an exercise.
Since children mature at individual rates and do not reach a pre-determined level of maturity at a certain age, general recommendations for when an individual child is ready for resistance training cannot be given. The most common age when all necessary parameters are reached is often somewhere between 6 and 7 years of age, although variations from this interval are common.
Before puberty, the central nervous system is more pliable and able to develop basic motor skills and base levels of strength, which means that resistance training can give a child a life-long advantage compared to age-matched, sedentary peers. A number of studies have shown that children as young as 5 years old can benefit from and enjoy playful resistance training, as long as individual maturity allows for a proper understanding of instructions and the physical capacity to perform exercises with proper form, under supervision.30
There are recommendations on how to structure a proper, science based resistance-training program for children and adolescents readily available.31
In order to prepare properly, any resistance-training program intended for children and adolescents should include a warm-up protocol. A dynamic warm-up improves neuromuscular function and improves performance in force-demanding activities. In addition, such a warm-up routine acts as an injury-preventive measure, especially for the lower body.32
An effective warm-up protocol can last for 5 to 10 minutes and involve various dynamic movement-based exercises for the whole body, like hops, skips, jumping jacks, push-ups, sprints, crunches, and standing leg raises with toe touches.33
Choice of exercises
There is an almost unlimited selection of exercises to choose from, and there are no special exercises inherently bad for children because of their age. The choice of exercises should rather be based on the size of the child, training level and experience, and individual motor skills.
The exercise selection should ensure muscular balance over the joints and between muscle antagonists, like the quadriceps and the hamstrings, or the biceps and the triceps musculature, to eliminate the risk of over- or under-stimulating certain body parts or muscle groups.
All types of exercise protocols, from training programs using free weights like barbells and dumbbells to body weight-based programs to elastic bands and machine training have been successful in controlled studies with young participants. For a child taking up strength training for the first time, an introduction through machine-based training might be prudent, although not necessary. Free weights place higher demands on coordination and muscle control, and since these factors also improve effectively through machine-based training, such an introductory choice of equipment could make sense.
Regardless of the choice of exercise, performing both the concentric and the eccentric phases of the movements in a controlled way with proper form and complete range of motion is important.
The most common strength training protocols for adolescents are ones based on whole body training, in which every muscle group is trained multiple times per week. Exercises activating large muscle groups should be performed in the beginning of the workout, followed by exercises for smaller muscles. Compound movements should be performed before any isolation work within a workout.
This way, more demanding exercises using heavier loads are placed in the beginning of the workout, when the child’s muscles have not reached a state of fatigue. If the child is learning the technique of more demanding exercises, these exercises should be performed first, when the muscles are least affected by fatigue which could compromise coordination and form.
Training with an intensity of 6RM or heavier produces the greatest gains in strength in adults. Loads allowing for 20 or more repetitions have greater effect on developing muscular endurance.
Studies involving resistance-training youth show that this age group responds better to higher ranges of repetitions. Especially during the initial months of strength training, a repetition interval of 10 to 15 offers the best training adaptations in the form of strength increases.
An average intensity of 75% of 1RM, where the sets are not performed to failure, is a common and prudent recommendation regarding repetition intervals for strength-training adolescents.
Current recommendations presented by The National Strength and Conditioning Association suggest that young resistance-training individuals perform 1 to 3 sets per exercise for muscle strength and muscle health.
Previously untrained children introduced to strength training should start with one set per exercise and training session. After a few weeks, the child can increase this amount to two sets per exercise and training session. This can be repeated yet another few weeks later for a third set per exercise and training session. Performing more than one set per muscle group is superior in resistance-trained adults, and nothing indicates that the same does not hold true for younger people.
Resistance training youth should train 2 to 3 times per week on nonconsecutive days. Training only once a week provides only 67% of the strength gains that can be expected with two training sessions a week.34 A training frequency of once a week has been shown to eliminate any potential strength gains in some exercises.
Two to three days of strength training a week ensure sufficient time for recovery, for both muscles and the central nervous system, while still being high enough a training frequency for robust results.
If the resistance training child stops training, the strength gains he or she has achieved will gradually disappear and return to base levels. An average strength loss of 3% a week can be expected upon cessation of training, and eight weeks of detraining can eliminate all results the strength training has produced.35 This loss of strength occurs even if the child keeps physically active with other forms of exercise. To keep the strength gains, strength training children and adolescents need two weekly training sessions.
Strength gains is not something relevant to adults only. A return to sedentary behavior, or just a layoff from strength training itself, will result in a loss of the training-related gains. It is therefore important that the child enjoy training. It has to be something to look forward to, or it will soon turn into something boring and abandoned. When training becomes an enjoyable part of everyday life, chances the activity continues into adulthood are much greater.
General recommendations for resting in between sets are usually between 2 to 3 minutes for adults. These recommendations are not necessarily optimal for children and adolescents.
Current scientific evidence indicates that young athletes do not reach a fatigued state in between sets the same way adults do. One minute of rest between sets seems to be enough for adolescents to recover maximal performance.36 Resting longer will not hurt, but it might be considered a waste of time, if that is a factor.
A novice weight lifter should perform repetitions with a moderate tempo, with full control of the movement at all times, to get used to the exercise and establish a firm mind-muscle connection. This is a prudent practice regardless of age, but might be especially beneficial for strength training youth.
With experience comes the need to adapt the repetition speed to the goals of the lifter. A young lifter who trains for Olympic lifting purposes will have to adapt the repetition speed of the majority of the training to suit those goals, for example.
It is possible that varying the repetition speed regularly will improve the training adaptations of children and adolescents, as this forces the muscle to adapt to various tempos. As long as the child is in control of the movement and not the opposite way around, there are no disadvantages to gradually introducing fast, explosive moments into the training program.
A number of controlled studies have demonstrated that resistance training has significant and life-long positive effects on health, self-confidence and muscular strength for children and adolescents. As long as recommendations and precautions are followed, there are no inherent risks with strength training in youth. Many years of research have shown that resistance not only can, but also should, be part of a well-considered strategy to lay the foundation of a healthy and strong adult life.
Not only does strength training improve strength and muscular performance, it also provides young people with documented improvements of health markers like skeletal health, body composition, risk of cardiovascular disease, and psychological health.
Children and adolescents who regularly participate in strength training demonstrate performance advantages in other sports compared to non-strength training and age-matched peers. Resistance training also significantly lowers the risk of injury when participating in other sports.
Unlike many other forms of physical exercise, strength training can be adapted to suit any and all individuals, regardless of sex, social background, and handicaps.
Adolescents, including those who for various reasons find it hard to participate in more traditional youth sports activities can, in strength training, find an alternative that not only provides positive effects on almost everything, but that also can be tailored so suit everyone.
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