Weight training programs are designed to progressively overload the system to improve muscular strength and endurance (Malina, 2006). Positive training outcomes can include increased bone mineral density, improved body composition, motor pattern learning, increased lean tissue muscle mass, and injury prevention (Nordström et al., 1998). Although weight training is considered safe and connected to a cascade of positive health outcomes for adults, it has been advised against in children (Malina, 2006). In the past, it was thought to be harmful to children or adolescents due to potential risk for injury, damage to growing bones and growth plates, or even premature closure of growth plates. Also, it was thought that in young boys, a lack of circulating androgenic hormones (testosterone) would not allow for any strength increases, if any were safe in the first place (Malina, 2006). A model has been developed for long term athlete development that takes into account sensitive periods of a child's development that are affected by strength development, aerobic development and speed development, as well as the long term implications of strength training on children and adolescents (Balyi et al., 2006; Canadian Sport for Life, 2014; Ford et al., 2011). It is invaluable to know how and when to begin strength training programs in youth; especially if it can be detrimental at too young an age, or unproductive if introduced too late.
For the purpose of this blog entry, 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 weight lifting and power lifting which should be reserved to describe high intensity techniques used to lift maximal amounts of weights (Guy & Micheli, 2001). It is further important to distinguish between children (not yet developed secondary sex characteristics; 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).
In children, a multitude of studies have been conducted on the effects of strength training. 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 were observed. Many studies were also cited in this review, eliciting similar strength improvements in as little as 8 week interventions (Faigenbaum, 2000). Overall, the report suggested that various training modes including weight machines, free weights, body-weight, sport-conditioning drills and even various sets and reps provided adequate stimuli for strength enhancement in youth (Faigenbaum, 2000). Furthermore, when examining the safety of weight training programs on children, they found no greater incidence of injury in training groups compared to the control groups. Correspondingly, inadequate levels of androgens prevented 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 would not necessarily be of more benefit than allowing children to play a variety of sports developing movement patterns and familiarity with different sports. This position statement is reflected in the LTAD model (Balyi, 2003).
Although strength training doesn’t seem to have any negative implications for muscular development in children, 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). However, contrary to the belief that “strength training” could negatively affect childhood development, no evidence directly supports this claim that resistance training would cease the growth of centres of ossification or affect growth plates (Guy & Micheli, 2001). Not only that, exercise has been shown to have clear benefits for the skeleton (Turner & Robling, 2003). Studies on competitive tennis and squash players bone mineral content (BMC), have shown that the primary arm used to hold the racket not only had much higher BMC than non-used arm, but also that this difference is observed significantly more pre-menarchal than in adults (Turner & Robling, 2003). This is very important in understanding skeletal health because the mechanism of addition of osteocytes to the periosteal surface that occurs from loading the bone improves bending and torsional strength of the bone. As well, resorption (loss) of bone from the periosteal surface is extremely rare in adults (it is usually trabecular bone that gets reabsorbed), so this additional layer can add a protective effect later into life (Robling, Hinant, Burr, & Turner, 2002). Experiments by Robling et al. (2002), have found that when mechanical compression of the ulna bone was tested on rats in lab for 16 weeks, tests revealed that although BMD and content only improved by 5-6%, there was a 64% increase in ultimate force (max amount of force the bone could support before failing) and a 94% increase in energy failure (amount of energy absorbed by the bone before failure). Not only that, 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. 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, studies have since confirmed this response in humans (Turner & Robling, 2003). Importantly, engaging in exercise during skeletal growth is shown to be more osteogenic than exercise during skeletal maturation (Turner & Robling, 2003), so loading children’s bones during times of growth, even despite large hypertrophic “strength” increases in their musculature, 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 olds 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 strength due to it’s 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 strength because not only is the bone fully fused, and has reached it’s full potential for longitudinal and perhaps also breadth of growth (Turner & Robling, 2003).
CSEP’s recommendations for resistance training in youth and adolescents include low- to moderate-intensity resistance exercise done 2–3 times/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, can 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 population (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 development. As per the LTAD model, it is advantageous to cash in on the sensitive periods of trainability. Strength gains are primarily increased 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 criteria could not be adapted to, then better alignment of strength and conditioning programs with growth could be a future solution. Preventing dropout from sport in early adolescent years could be one of the most important take-aways to this information because there is strong evidence to suggest that 95-99% of peak bone mass is gained in the first 2 decades of life (Haapasalo et al., 1998). Although physiologically, the sensitive period for bone development can and should be “cashed in on”, cross sectional studies confirm that maximization of movement and strength during adolescence is not the reality seen in males and females of this generation (Bonjour, J. P., Theintz, G., 1991).
Strength and Fitness is VITAL to your health and well being. #VitalStrength
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