The concept of windows of adaptability, opportunity or optimal periods of trainability has been popularised by Balyi’s work. This is partly because of the popularity and success of his models but also due to the fact his statements around the area have a little more sting in the tail i.e. if you do not train the motor skills optimally during this time then you will never allow your athletes to reach their genetic potential . This is a fairly strong message for training specific motor qualities at the appropriate stage in each athlete’s development.
These statements by Balyi are largely unfounded in academic literature ; the following is a brief review of the work investigating the area, concluding with the author’s opinion on the subject and practical considerations when dealing with athletes who are developing through this critical stage in their development towards adulthood.
Istvan Balyi has popularised the logical and safe progression of athlete development using biological age-specific stages. His model of LTAD is excellent and has brought about a real change in NSO, NGB, professional sport structures because the language and practical applications used have been able to be transferred into a variety of sports. He has also differentiated between early and late-specialisation sports. However, when you go to search for his published work in peer-reviewed academic journals, unfortunately there is a dearth. Most of the work is via presentations and coaching journals. I am not saying this to knock using these avenues, I wish I was as popular as him and aspire to get my messages out as well as he has but what has happened here is he has left his concepts and model up to severe criticism from the higher end of the professional scale. I say higher end because I do not want to differentiate between coaches, academics and scientists, at some level we all need evidence that is reliable and valid. No matter what some people may say.
Balyi’s model is based around 6 stages (this is in late specialisation sports) starting with FUNdamental stage going through to Training to Win and Retirement/Retainment stage. The model is generic in nature and requires adjustment for different sports. Each stage is paired with definitive periods of age and is differentiated for by male and female. As chronological age progresses so does the complexity
Many of the controversies revolve around the assumptions made with no background evidence. For example quoting “critical periods of trainability”, implying that if fundamental skills are not developed at the appropriate time, with massive age gaps, that significant windows will be lost, leading to compromising the ability of the young player/athlete to reach their full potential . Throughout the work there is no mention of specifics of the motor and coordination skills or measurable values to indicate the development. This severely depletes the credence that this model has because although pertaining to give practical applications the directions are far too broad to be useful to a coach, and gives enough leeway to be dangerous in the wrong hands, of a poorly educated coach. What is meant by “specific activities” with regards to agility, quickness and change of direction? Do high knees, a cone drill and butt kickers! Are you serious! If you said that in an interview for a job or at a coaching presentation you are going to get slaughtered by my peers!
Another example of this are the recommendations of types of Periodisation, training implements and methods to be used. How can someone who is not a coach make these assumptions?
Therefore the common LTAD Model by Balyi is adequate for the general public, parents and people who want to take an interest but these principles must not be applied to any level of competitive sport that has the potential to move people through to a high performance level. Although the principles and suggestions are fine and you cannot argue with someone taking these and using with their kids, my athletes deserve better and I will deliver better.
Maybe this is the difference? Balyi’s LTAD model is not for professional strength and conditioning coaches to use.
Atko Viru 1999  puts together an excellent review of his and fellow researchers attempt to discover the development of motor abilities during childhood and adolescence. Viru summarises the accelerated periods of development of aerobic endurance, muscle strength, explosive strength and sprint velocity.
To evidence the critical periods, four groups have been defined in this work
- The appearance of critical events in ontogenesis which influence the further course of growth, maturation and development;
- Periods of accelerated growth or development of the total body or its parts, organs, tissues, cells;
- Increased sensitivity to factors stimulating development;
- Enhanced vulnerability for harmful effects
In summary, there are 2 periods were development is accelerated in our development, the first around the ages of 5-9 in Boys and Girls and around 10-15 in Girls and 13-16 in boys . Viru has established that there are periods of accelerated improvement in various motor abilities. During the first decade of life there is little change in muscle tissue with most improvements being accounted for mainly by improved coordination mechanisms. For sprint speed and explosive strength between the ages of 5-9, alterations to coordination result in enhanced simultaneous and fast recruitment of the optimal number of motor units necessary for explosive strength, and rapid contraction-relaxation cycles. An essential role may also belong to the improvement of intramuscular mechanisms for excitation-contraction coupling and fast relaxation. For accelerated muscle endurance during these crucial stages an important development is the increased functional capacity of the oxygen transport system, including modest cardiac hypertrophy .
The accelerated improvement around the maturation stages 3 to 5 (second peak) is undoubtedly related to the enhanced anabolic effects on skeletal muscle. In males, the main role belongs to testosterone in association with the anabolic action of growth hormone, tissue growth factors and thyroid hormones. In girls the period of accelerated improvement in motor performance appears approximately 2 years earlier than boys, reflecting their earlier sexual maturation. Hormonal stimulation of skeletal muscle is suggested to be related to not only muscle strength, but sprint speed and power in the pubertal period. This idea that not only is strength affected by maturation, and training will enhance this but consequentially this will positively affect speed and power. This is an obvious assumption, if you were to state this around the athletic development of an adult athlete, namely that they got stronger, then more powerful and therefore ran faster you would not be saying anything new here. In fact, this is basic human physiology and biomechanics. I could quote anyone here from Stone to McBride to Cormie. You cannot be powerful or fast without some degree of strength, if the ability to be strong is transferred correctly an athlete will become more powerful and sprint speed will increase. The only discrepancy to this development is during the first decade of postnatal life when the coordination of all systems may positively affect mechanical efficiency therefore increasing power and running speed more so than global strength gains.
What is interesting is that Viru’s work has considered the notion of trainability, unlike Balyi who simply states that due to natural ontogenetic development’s effect on certain motor skills this is the optimal time to train these skills. Although some facts suggest that critical periods alter trainability, the relationship between critical events and increases in rate of improved motor performance and trainability need further research.
Age-dependent differences in trainability should be related to the same factors which determine the rate of ontogenetic development in motor performance. As I summarised previously, at the end of the first decade of postnatal life and in the early teens the main factor determining trainability of muscle strength, speed and power is improved coordination of muscle activity. Changes in the oxygen transport system, appearing at the end of the first decade of postnatal life, make the effect of aerobic endurance training at the age of 10-12years possible. In the last stages of sexual maturity the training effect is achieved through testosterone-dependent hypertrophy. In female adolescents, the low level of testosterone may not restrict the training effects since the oestrogen-dependent increase in muscle tissue sensitivity together with IGF-1 compensates for it.
In this review on the periods of accelerated development, there appears to be little evidence to support either the notion of optimal windows of adaptation or trainability, or does there appear to be any work suggesting that by missing a critical period of development of motor performance that this skill is lost and unable to reach its potential ever again. I can only imagine these statements in Balyi’s work are scaremongering tactics to reinforce the importance of using his model and clearly have no more evidence to support. Ford  agrees, showing that the LTAD is based on a lack of empirical evidence and any evidence the model is based on rely on questionable assumptions and erroneous methodologies. For a comprehensive review of these assumptions follow the work of Ford 
The take home message for me here is to work on the coordination of muscles and energy systems consistently and thoroughly during ontogenetic development, this will enable all aspects of motor performance to be prepared for high performance workloads and demands, when fully matured. Coaches should consider that at different times, some motor skills will improve more than others, these should be identified and be embraced; however this must not take priority over the long term development of the athlete.
Future research directions
The improvements through trainability must be further investigated. I would hasten to suggest that any improvements made during adolescence, compared to that of a well coached and trained athlete over a long period of time would be negligible. I would also suggest that this will always be a theoretical discussion, the variables involved over such a long period time, coupled with the individual nature of development and performance, are too great. For me, I will take the concepts on board but will always strive to coach and develop each athlete by analysing what is in front of me today and not leaning too much on a conceptual model for guidance.
1. Balyi, I. and A. Hamilton, Long-term athlete development : Trainability in childhood and adolescence. Coaching Update. 20(2): p. 10-13. 2005
2. Balyi, I. and A. Hamilton, Long-term athlete development: Trainability in childhood and adolescence "windows of opportunity, optimal trainability". Performance Conditioning Soccer. 12(1): p. 8-10. 2005
3. Ford, P., M.D.S. Croix, R. Lloyd, R. Meyers, M. Moosavi, J. Oliver, K. Till, and C. Williams, The long-term athlete development model: Physiological evidence and application. Journal of Sports Sciences. iFirst article: p. 1–14. 2010
4. Viru, A., J. Loko, M. Harro, A. Volver, L. Laaneots, and M. Viru, Critical periods in the development of performance capacity during childhood and adolescence. European Journal of Physical Education. 4(1): p. 75-119. 1999
5. Viru, A., J. Loko, A. Volver, L. Laaneots, K. Karelson, and M. Viru, Age periods of accelerated improvement of muscle strength, power, speed and endurance in the age interval 6-18 years. Biology of Sport. 15(4): p. 211-227. 1998