Weight-training programs may be designed to improve muscular strength and endurance through the application of progressive overload. (Malina, 2006). Progressive overload is the training principle that states that the body needs progressively greater demands placed on it for training adaptations to take place, like strength and fitness increases (Baechle, 2000). For the purpose of this article, strength training (or resistance training) will refer to a special method of conditioning used to increase ability to exert force or resist force (Faigenbaum, 2000). This should be distinguished from weightlifting and powerlifting which is reserved to describe high intensity techniques used to lift maximal amounts of weights (Guy & Micheli, 2001). Additionally, it is important to distinguish between children (pre-pubescent; up to age 11 girls and 13 boys), and adolescents (typically girls aged 12 to 18 and boys aged 14 to 18) in that the transition from one to the other has a relatively large range (Faigenbaum, 2000).
Positive training outcomes associated with weightlifting include increased bone mineral density, motor pattern learning, increased muscle mass, decreased fat mass, and injury prevention (Nordström et al., 1998). Although it is practiced safely and with positive and beneficial outcomes for healthy adults, it has, at times, been advised against in children (Malina, 2006). Weight-training was thought to be harmful to children and adolescents due to potential risks of injury which may include damage to bones (i.e., growth plates) which may result in premature closure of growth plates and discrepancies in limb length. In young boys, it was believed that a lack of circulating androgenic hormones (i.e., testosterone) would not permit improvements in strength (Malina, 2006). However, there is evidence to suggest increases in force generating capacity is primarily a neuromuscular adaptation that is not dependent on muscle size (Lyristakis et al, 2022), even in prepubescent children. (Bijeh & Shahrabadi, 2018).
Participation in organized youth sports is increasing and opportunities to participate in competitive environments are happening at younger ages, for more months of the year, and is becoming more and more prominent in all sports. This increase in year-round sport participation has fostered a concomitant growth in the strength and conditioning industry, with efforts to enhance performance and prevent injuries. These recent changes have led many parents to ask the valid questions, “Is it safe for my kid to lift weights?” or “When is the best age for them to start? (Myer et al, 2013). It is invaluable to know how and when to begin strength training programs in youth; especially if it is detrimental too young or past the point of being beneficial if introduced too late. So, is it beneficial to start young? Is it detrimental to start late? Let’s review some of the evidence.
A multitude of studies have been conducted on the effects of strength training in children. In a study by Ramsay (1990), thirteen boys (age 9-11) participated in a 20-week resistance training program three times per week. Significant improvements in 1-RM bench press, leg press, isometric elbow flexion, knee extension strength was observed. Many studies were also cited in this review, eliciting similar strength improvements in as little as 8-week interventions (Faigenbaum, 2000). The overarching conclusions across these studies suggest that various training modes such as weight machines, free- weights, body weight, sport-conditioning drills and even various sets and reps provide adequate stimuli for strength enhancement in youth (Faigenbaum, 2000). As such, participation in any form of resistance exercise is far better than not participating at all. Furthermore, when examining the safety of weight training programs in children, a review by Faigenbaum (2000), found no greater incidence of injury in training groups compared to the control groups. Correspondingly, inadequate levels of circulating androgens attenuated increases in cross-sectional area, so mechanisms of strength gains appeared to be related to neurological learning effects rather than hypertrophy (increase in the cross-sectional area of muscle) (Bernhardt, 2000).
This would suggest that “resistance training” per se could not only be beneficial, but it could be very beneficial, even if started at young ages. There is a misconception that resistance training is all about barbells and dumbbells – but that is not true. Resistance training in the early stages involves learning movement patterns and coordination, and light resistance before the youth can progressively overload the patterns with more load or volume. Resistance training is fundamental for increasing bone density, aiding in the development of movement patters specific for training, improving cognitive health, and overall wellbeing (Faigenbaum, 2013). Both participation in resistance training and exposure to different sports are important – and resistance training can act to help develop youth athletes in a more well-rounded way, changing up the areas of the body that are loaded, and helping to prevent repetitive injuries associated with certain sports. There is a massive misconception that the participation in resistance exercise is purely to put on muscle. The activation of muscle groups, increasing the mind-muscle connection, movement pattern development such as when performing deadlifts and squats, will only benefit the young athlete when the coaches are trained in appropriate starting points for the lifts and appropriate rates of overload for children and youth. This position statement is reflected in the Long Term Athletic Development (LTAD) model by Balyi (2003). “How young is too young?” is likely a question about the ability of the child to be stay focused and be motivated to try new movement skills involved in strength training and conditioning. For youth, games are likely the most motivating which is why the position statement calls training between the ages of 6-9 the “FUNdamentals” stage.
Strength training does not appear to have negative implications for muscular adaptations in children, however, the stunting of growth has been raised as a fear of strength training in children in the past (Kröger, Kotaniemi, Kröger, & Alhava, 1993).At the onset of puberty, ossification overruns the growth plate, fusing the primary growth centre to the secondary growth centres, resulting in a mostly unchanged length of bone thereafter (Mackie, Ahmed, Tatarczuch, Chen, & Mirams, 2008). But, contrary to the belief that “strength training” could negatively affect childhood development, no evidence directly supports the claim that resistance training would cease the growing of children’s growth plates (Guy & Micheli, 2001). In fact, dynamic exercise, especially resistance training, has been shown to have clear benefits for the skeleton (Turner & Robling, 2003). Studies examining the bone mineral content (BMC) of competitive tennis and squash players, have shown that the dominant arm used to hold the racket not only had much higher BMC than the non-dominant arm, but also that this difference is observed significantly more pre-menarcheal than in adults (Turner & Robling, 2003). This is important for skeletal health because the mechanism underpinning the addition of osteocytes to the periosteal surface occurs from loading the bone (Robling, Hinant, Burr, & Turner, 2002). In turn, this improves the bending and torsional strength of the bone. As well, resorption of bone from the periosteal surface (outside layer of our bone) is extremely rare in adults (it is usually trabecular bone that gets reabsorbed/lost), so this additional layer can add a protective effect later into life (Robling, Hinant, Burr, & Turner, 2002). When mechanical compressions of the ulna was evaluated in rodent models over 16 weeks, tests revealed that bone mineral density (BMD) and content improved by 5-6%, yet there was a 64% increase in ultimate force (i.e., maximal amount of force the bone could support before failing) and a 94% increase in energy failure (i.e., amount of energy absorbed by the bone before failure). Interestingly, mechanical loading was found to be more effective if loads were applied in discrete bouts separated by recovery periods rather than if loads were applied in a single session. Though this study was done on rats, this information can be translated to humans as bone formation and adaptations occur via similar mechanisms.
Additionally, dynamic loading as opposed to static loading appears to be more beneficial due to the sensitivity of bone cells to shear stresses (Hert & Landa, 1971). Although these studies were in rabbits or rats, recent studies that have since confirmed this response in humans (Turner & Robling, 2003). Importantly, engaging in exercise during skeletal growth is shown to be more osteogenic (bone-forming) than exercise after skeletal maturation (Turner & Robling, 2003), so loading children’s bones during times of growth, even without the corresponding large hypertrophic “strength” increases in their musculature that we typically associate with “strength training”, could result in long term bone health implications!
How should we know what age to transition children and adolescents into weight training? Chronological and developmental ages can vary drastically in that two twelve-year-old’s could be the same age, but in completely different stages of development of bone and puberty. It appears that peak BMD occurs around menarche in females (around age 12) and around ages 13-17 in males (adolescence) (Kröger et al., 1993), which is typically right after peak height velocity (PHV) occurs. PHV landmarks the start of the growth spurt and peak of the spurt, and they are key landmarks for design of training and competition programs. Soon after PHV, the growth plates will fuse, and this appears to be the best time to build muscular strength due to its sensitive period, outlined in the LTAD model, occurring 1-2 years after (Balyi et al., 2006; Kröger et al., 1993). This would be the optimal time to start building muscular strength because not only is the bone fully fused, but it has also reached its full potential for growth (Turner & Robling, 2003).
From ages 0-6, the recommendations are to have kids start general exploration of movement, on all environments and surfaces (ice, snow, gravel, grass, water) (Balyi, 2006). Structured gymnastics and swimming programs usually begin this young. Between the ages of 6-9, depending on the child’s mental cognitive and emotion development, we can start introducing the ABC’s of athleticism (agility, balance, coordination, and speed) (Balyi, 2006). This would also be an appropriate time to start doing body weight exercises and slightly more structured programs. Between the ages of 8-12, Balyi (2006) recommends that we should further introduce resistance training, for example through body weight, medicine ball, and swiss ball exercises to progressively overload the developing sport patterns and movement skills. Between 11-16, most children will hit puberty, and attain their PHV, where introducing free weights to continue the progressive overload would be appropriate (Balyi, 2006).
The Canadian Society for Exercise Physiology’s (CSEP) recommendations for resistance training in youth and adolescents include low- to moderate-intensity resistance exercise 2–3 times a week on non-consecutive days, with 1–2 sets initially, progressing to 4 sets of 8–15 repetitions for 8–12 exercises (Behm, Faigenbaum, Falk, & Klentrou, 2008). They recommend that exercises including more advanced movements such as Olympic-style lifting, plyometrics, and balance training, which can enhance strength, power, co-ordination, and balance, should be used with gradual progression under qualified instruction and supervision with appropriately sized equipment. Resistance training can safely lead to functional (i.e., muscular strength, endurance, power, balance, and co-ordination) and health benefits in these populations (Behm et al., 2008).
In conclusion, recommendations for strength training for children are to start with physical literacy and to let kids play as much as possible as often as possible while experiencing different planes of movement, upper and lower extremities, and different surfaces (ice, snow, gravel, grass, water). The goal here is to develop the neuromuscular system for a strong base of coordination as well as to enhance bone and cardiovascular development. The more fun a kid has with their “training”, in other words – how well we can disguise “training” as play – the more likely they will continue participating in the activity, even in the long term. As per the LTAD model, it is advantageous to cash in on the sensitive periods of trainability. Strength improvements are prominent after PHV is passed, because bones are at their peak lengths, are strong, and newly circulating hormones help develop the muscles as well as help lay down a protective layer of BMD during that sensitive period. Long term considerations of the LTAD model, although it is not completely evidence-based and is not an individualized approach, presents an advancement in the understanding of the developing athlete alongside their biological growth (Ford et al., 2011). If possible, improvements in sports systems would allow grouping of athletes according to developmental age rather than chronological age, and if this criterion could not be adapted to, then better alignment of strength and conditioning programs with growth could be a future solution. The take-home message is that resistance training in youth is safe and effective, and it can have many health-related benefits. Furthermore, the safety and effectiveness of training will be maximized under the supervision of a qualified strength and conditioning coach.
So, yes, it is safe for youth and children to lift weights, when introduced in a safe, progressive, and movement pattern focused way!
About the Author
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