Pre-adolescent youths should not be exposed to excessive anaerobic lactic loads due to their immature physiology.
The stress of these types of loads is in excess of their ability to effectively tolerate them.
Possible and irreversible effects of this type of training may induce premature thickening of the left ventricular wall as well as finalizing the imprint on the percentage of muscle fibers that are, during the pre-adolescent stage, still plastic; thereby causing red fiber adaptations.
It is therefore in the interest of all pre-adolescent youths to participate in only anaerobic alactic training and sport game environments as well as low level aerobic environments.
Thus, many disciplines, over a wide range of sports, are contraindicated for youths at this stage (the 400 and 800m in T&F are on the top of that list as well as the cycling, ice, alpine, and Nordic events of equivalent bioenergetic structure)
A simple rule of thumb to follow with the training and sport game participation of pre-adolescents is to adhere to Charlie’s speed training philosophy in so far as minimal time spent in the medium intensity range.
Plenty of time to pursue anaerobic lactic disciplines once the requisite level of biological maturation is in place.