Training Myths

The early scientific community thought heavy loading (squatting) negatively impacted on growth potential (height). If squatted with heavy weight at a young age compared to body weight for years, then the spine gets compressed, which halts the growth of the long bones in the legs.
In 1983, the American Academy of Pediatrics produced a position statement that weightlifting has a high injury rate and should be avoided by young children and recommending that the practice of weight lifting, power lifting, and bodybuilding should be avoided until after skeletal maturity is achieved. This stand remains until 1990 recommended, due to the lack of good data. PMID: 27409556 PMID: 1861956
The American Academy of Pediatrics also stated that resistive training is a waste of time prior to puberty, due to insufficiency of androgens that neither girls nor
prepubertal boys will increase muscle mass and will have little or no gain in strength. Further, before the first menstrual period, females might gain some strength with resistive training, but will not increase muscle mass at any age without the use of steroids. 
In 1990, the American Academy of Pediatrics accepted somewhat, recommending that strength training could be recommended for children if conducted by well-trained adults.
Things started to change in 1996 when an 8-week study showed large and significant increases in strength from resistive training, with no injuries in 7-12-year old boys and girls.

Moreover, Studies have shown that squatting can cause up to 3.59 mm (citation needed) of spinal shrinkage.PMID: 1810615. The extent of compression can vary depending on factors such as the duration, intensity, as well as individual differences in spinal health and flexibility. But this happens while walking too, which is temporary, and this height effect is restored to normal after a night's sleep due to decompressing in a horizontal position. PMID: 15836943 PMID: 15676783 PMID: 3830126. This normal physiological phenomenon is often described as “spinal shrinkage” or “disc compression.” which is a response to gravitational forces.PMID: 6539504

Maturing bones can be 2–5 times weaker than the surrounding tissues. The proposed mechanism was that excessive external loading would cause fracture or damage to the growth plates (influenced by the rate of chondrocytic proliferation and the amount of enlargement) at the ends of the long bones, leading to shorter and narrower bones.(further research is needed) PMID: 5952286 PMID: 15758453 PMID: 15677737 PMID: 11502470 PMID: 19540500

Apart from this, during Bone formation, there is a process of development of the Epiphyseal Plate, also known as growth plate. These growth plates are made up of cartilage which harden into solid bone during the puberty process; hence they are responsible for the Increase in height, which is influenced by Growth Hormone. In puberty, increasing levels of estrogen, in both females and males, signals the Growth Plates to fuse. This is the end of the growth phase for humans. Beyond this point, there can be no increase in height. The growth of humans in terms of height is dependent on hormones, and genetics determines body hormonal status. Nearly 700 to 3,290 genetic variants that are involved in determining height.PMID: 14671005 PMID: 16286781 PMID: 21865753 PMID: 25282103 PMID: 30124842

Heavyweight training cannot stop this growth.

The time of transition from childhood to adolescence is critical for healthy Bone minerals. On average, Bone maturity starts after 11 and 13 in Girls and Boys, respectively. Estrogen has a greater impact on Bone Mass and TESTOSTERONE has a greater impact on Bone size. This is the “Window of opportunity” for bone response in early puberty.”PMID: 8374675 PMID: 14988470 PMID: 19434876 PMID: 9312174 PMID: 9718379

Lifting weights with proper form has been shown to have an osteogenic (Bone density and formation) effect (Osteoporotic Patients) PMID: 30671455 PMID: 19453205 PMID: 7750019
Starting physical activity before the pubertal growth spurt increases bone and skeletal muscle growth more than normal growth in children who are not physically active children.PMID: 16796394

In fact, there is substantial evidence that weight training in prepubertal age leads to an increase in higher areal bone mineral content and areal bone mineral density.PMID: 9844098 PMID: 9525351

Bone is a living tissue like Muscle, it changes in response to training.PMID: 18201116. At a young age, training is important to build strong bones, whereas in old age it is essential to maintain its strength.PMID: 16702776

When is the proper age to start gym, in childhood?

As we can clearly see in the graph, GROWTH HORMONE plays a very critical role in Bone development and sex Hormones like ESTROGEN and TESTOSTERONE also apply a remarkable effect on Bones during and after puberty.PMID: 12145113

IGF-1 (Insulin like growth factor) increases suddenly around 9-11 age and gets at its peak level around 12-18 age, this is the peak time to start loading exercise when free biological active IGF-1 levels are climbing and after that start to decline.PMID: 12876414 PMID: 8855791  PMID: 10443684 PMID: 10443679 

In an 18-month study, male gymnasts, age 9-11, showed improvement in Bone strength.PMID: 9115154

In an 8 Months Study, an Exercise group of Boys and Girls, aged 5-10, showed gains in Bone Mineral Density.PMID: 10657819

In a 10-Month study, weight training Girls, aged 9-10, showed gains in Bone Mineral Density and Lean Muscle.PMID: 9286762

The 11 subjects (3 female, 8 male) had been training for Olympic Weightlifting for the first time for a minimum of 22 months.
There were no significant differences between the sexes.
Body weight and all performance measures increased significantly over time.
The fact that these children are able to lift greater than their own body weight in both the snatch and the clean and jerk suggests that this sport is suitable for children.PMID: 14658250

The fear that resistance training (Squat, deadlift) may injure growth plates is not supported by scientific reports or clinical observations. Furthermore, it can lead to increased serum IGF-I, which is beneficial for bone formation and growth.PMID: 24393806 PMID: 16437017 PMID: 28447880.

Moreover, injury rates for young athletes participating in the sports of weightlifting and powerlifting are considerably lower than those of rugby, soccer, and football. When performing a one repetition maximum (1RM) squat, the force an athlete's body sustains is actually less than what they would be exposed to daily in most other sports.PMID: 19945973 PMID: 14658250

It is safe to find young athletes with a maximum of 1 repetition without fear of injury, provided they have proper form and technique.PMID: 12580672

Therefore, the mechanical stress placed on growth plates from weightlifting may actually be beneficial for bone formation and growth.
It is recommended that children should engage in high-load weight-bearing training activities like Squat, Deadlift, Bench Press, Strict Press, clean and Jerk, Snatch so long as they are supervised by qualified trainers and coaches.

If you are a personal Trainer, or want to heir a Personal Trainer to teach any movement (Squat, Deadlift, Bench Press, Strict Press, Clean jerk or snatch) the following question should get clear.
Is the kid mature enough to accept teaching instructions?
Does Kid itself have an intrinsic desire to learn and not be under someone's pressure?
Does the Personal Trainer have enough qualifications to supervise the kids?
Does the Personal Trainer emphasize on form and techniques and not the amount of weight lifted?
Does the Personal Trainer know how to understand the kid's energy level and modify the training program and vary the intensity of lifting to avoid overtraining?