Strength training for children
Strength naturally increases with age because of body growth and development of the neuromuscular system, but can strength in children be increased through training?
As children grow in size and develop muscle mass, they also develop increased strength. These strength improvements are independent of training. In other words, children grow bigger and stronger until full maturity.
For example, the average six-year-old boy can do five press-ups, a 12-year-old boy can do 15 press-ups and 18-year-olds can do 25 press-ups. In contrast, the average six-year-old girl can do five press-ups, a 12-year-old girl can do 12 press-ups and an 18-year-old girl can still only do 12 press-ups. This is the usual pattern of development for boys and girls; they are both similar as young children, but post-puberty the boys' strength development accelerates while the girls reach a plateau.
This diversity between sexes is mostly due to the hormonal changes which occur at puberty. Testosterone, which increases rapidly in boys, programmes extra upper-body bone growth and muscular hypertrophy. In contrast, oestrogen, which increases in girls, programmes extra pelvic-bone development and increased body-fat storage. These changes mean that boys' strength will increase naturally until 18-20 years, whereas girls' strength, especially in the upper limbs, is unlikely to improve naturally beyond 14 years.
Not all the natural development of strength is due to gains in muscle bulk. Strength also improves because of maturation of the neural systems. One of the major changes that occurs throughout childhood is the myelination of the nerve fibres. Myelination, in lay terms, is the 'insulation' of the fibres to allow faster conductivity of the electrical impulse. Full myelination is completed in adolescence, and so until then coordination and reactions will be limited. There is some evidence to suggest that muscular recruitment also improves with age; adults are able to recruit more motor units when performing maximum efforts, compared to children. In addition, the coordination of synergistic and antagonistic muscles develops with age. For example, a child performing a press-up often has difficulty maintaining a straight back, stable pelvis and stable shoulder position during the up-and-down movement. This is the reason why children often perform press-ups with their bums sticking up, shoulders rounded and hands in front of their heads. It is not until all the stabilising muscle groups are developed and become correctly coordinated with the prime movers that good form can be achieved on bodyweight and free-weight exercises such as the press-up.
Strength training can work
Strength naturally increases with age because of body growth and development of the neuromuscular system, but can strength in children be increased through training? The majority of the existing research provides convincing evidence that it can. One of the most important studies investigating the strength-training potential of young children was completed by Ramsay et al in 1990. They studied the effects of a 20-week strength-training programme on 9-11-year-old boys - specifically, elbow-flexion and knee-extension strength. The training programme comprised sessions of three times a week, 3-5 sets per exercise, performed at 8-12 Repetition Maximum intensity. This refers to weights that can only be performed 8-12 times with good form. Therefore, the training programme these boys undertook involved sufficient duration, intensity, volume and frequency to ensure that it would be an effective training dose.
Ramsay et al found that elbow-flexion force increased by 37 per cent and knee-extension force increased by 21 per cent in comparison with a non-training control group who showed no improvement. These are very similar to the scale of improvements that an adult would see after a similar training programme. This result confirms what other, earlier studies had also shown - namely, that if intensity, volume, frequency and duration are sufficient, young children can significantly improve their strength by the same relative amount as adults.
But no hypertrophy
Further findings from the Ramsay study are also very interesting. While the boys in the study significantly increased their force production, computerised tomography showed no increase in muscle size in the arms and thighs over the 20-week training period. Thus, by inference, the increases in strength must have been due to improvements in the neuromuscular system. Ramsay et al provided evidence for this by showing that motor-unit activation improved 9-12 per cent after 10 weeks and a further 2-3 per cent by the end of 20 weeks of training. This means that the boys were able to recruit more muscle fibres after training and thus produce more force. It is accepted that in adults strength increases as a result of both hypertrophy and neuromuscular improvements. However, it appears, and other studies support this, that children increase strength in training solely from neuromuscular improvements.
The research describing how a child develops strength, both through natural growth and through training, helps us to design appropriate strength programmes for young athletes. Pre-puberty, both boys and girls have similar strength, and at this age children have developing neuromuscular systems. Strength training for pre-pubertal athletes should focus on skills and techniques; since all the improvements from strength training come from neuromuscular development, this is the ideal time to teach coordination and stability. Children should be taught all the big muscle-group, free-weight and bodyweight movements with light loads. For example, power clean, bench press, press-ups and squats. Any child taught these has an advantage because good technique is learned at a young age, which allows for high-intensity training to be performed safely and effectively as the child gets older. During the pre-puberty years, particular attention should be paid to posture and stability, since children need good strength in the trunk muscles to support the body correctly.
At puberty boys benefit from a massive acceleration in strength because of the large increase in testosterone, which leads to muscle hypertrophy. Girls do not enjoy the same gains in strength, with little muscle-mass development post-puberty, especially in the upper body. At 18, girls have 50 per cent of the upper-limb muscle of boys and 70 per cent of the lower limb muscle. Almost all the differences in strength between the sexes is due to differences in muscle mass, and if strength is calculated relative to limb volume, i.e., the force per size of muscle, then both sexes have equal strength. Girls need to compensate for this natural disadvantage by prioritising strength training from puberty onwards, otherwise strength will plateau. Particular attention to strength must be made by girls involved in sports with upper-body components. Strength programmes for girls from puberty onwards must be effective, with sufficient frequency, volume and intensity. This is why it makes sense to establish good technique pre-puberty, since from puberty onwards when young athletes need to push weights of 8-12 RM intensity they will already have good technique and enough strength in the stabilising muscles to perform the exercises safely and effectively. Remember, intensity below 12 RM will target muscular strength endurance and not maximum strength development. So if you want children to get stronger, they have to push enough weight just as adults would. I recommend that most female athletes visit the weights room 2-3 times a week from puberty onwards, because it is their lesser maximal strength that is the major factor limiting speed and power in females.
Is it bad for children?
Boys enjoy more natural development during puberty and for a longer time afterwards. In fact, their peak gains in strength last for 18 months after their peak gains in size. However, strength training from puberty onwards would still be highly beneficial for boys. Puberty provides a great window of opportunity for them to develop strength through training because of the high testosterone levels. If regular training is maintained, the large possible gains at this time can last into adulthood. (Without regular training, i.e. at least once a week, children show the same detraining effects as adults.) For this reason I would also recommend starting 'adult-like' strength training for boys from puberty, depending on the pre-puberty training status. I reiterate that the aim should be to use 8-12 RM loads safely and effectively with pubertal boys by establishing good technique before the time when high-intensity training needs to begin.
Many coaches and parents believe that strength training is bad for children and even potentially dangerous. For instance, a myth exists that heavy weight-lifting too young will stunt growth. There is little research to suggest that weight training for young children is unsafe - in fact, most of it confirms that weight training is one of the safest exercises they can do. A child is much more likely to be injured on the football pitch, tennis court or running track than in the gym. Weltman et al (1986) specifically studied the effects of heavy strength training on young boys. During the training period, one of the 16 boys suffered a mild muscle strain and none of the boys showed any damage to the growth plates. In fact, strength training in young children will thicken the bones by promoting increased bone mineral density, and do nothing to hinder growth in length. I repeat once more, weight training with heavy loads is very safe if technique is correct and posture and stability are maintained. Poorly performed weight exercises are just as dangerous for adults as for children.
One final point
When deciding when to start and progress weight training, it is best to use biological and not chronological age as your guideline; otherwise, certain individuals may be starting too late or too early for optimum development.
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