Overtraining can also be labeled as "under-recovery."
It can be described as too much training for your ability to recover between workouts in a weekly, monthly, or quarterly period (micro or macro-cycle).
Your ability to recover can be hindered by lack of or poor sleep quality, hydration, and nutrition. If you are low on calories or vitamins/minerals/protein your body cannot replenish itself as fast. If you cannot breakdown such nutrients for any reason, same effect.
High stress levels contribute to under-recovery.
Massage, spa/sauna, contrast bathing/showers, ice baths can aid in allowing the muscles to recover.
In this day and age, many youths and athletes have too much going on that it can be an issue. Multiple sport/activities, lack of hydration/nutrition or proper rest.
Overtraining can be a big issue. Burn out can be a serious problem. Ideally when you gymnasts are not at training they should miss it a little, and wish they got to do a little more than they did. This is a sign they are loving and enjoying it and are happy being there as much as they are. Once they start to feel like they wish they could just have a day off, or they are glad there is no training today, then the risk of burn out is high. If the kids are asking for more hours and more days then you probably have the schedule about right, if they are hard to chase out of the gym at the end of training then you are doing well.
Parents also tend to suffer from burn out if we over train the kids. They must arrange their lives around driving their gymnast too and from training and meets, they must sacrifice a lot of their disposable income on this sport. We want to keep them happy too.
What you often see happening is that as a gymnast increases their days and their hours they seem to continue to improve faster, but once it gets past a certain point you often see the opposite and its almost as if they stop improving at all. I think when we are making corrections to the kids it takes a certain amount of time too sink in. But if the kid always trains they tend to practice their mistakes more and don't allow time for the corrections to sink in.
Building up hours and days too quickly also seems to cause a problem. You often see a child jump from 1 day a week straight to 3 when they join team, or from 2 days a week straight to 4 when moving up from level 3-4. Kids and parents need their time in the gym built up gradually.
I believe you should not increase a child training schedule by more than 1 day at a time for 6 months. So instead of jumping from 3 days a week to 5. They do 6 months at 4 days a week first. Ideally you would add no more than one extra day per year, but if you need to move kids up faster 6 months is still doable.
The same goes for the number of hours the gymnasts train. If your gymnast trains for 2 hours at a time and it suddenly increases to 4 she is at risk. She will become tired and lose focus and is at more risk of an injury and training becomes less effective. Its best to increase training times in 1/2 hour increments. Step up from 2 hours to 2 1/2 for six months and then to 3 hours for six months and you will get a lot more from your gymnasts.
Also remember that if you want kids to be effective athletes they need time away from the gym. The way some kids schedules are it would seem they spend all their time either at the gym, at school, studying and eating or sleeping. In order to have a balanced life they need rest and relaxation time and they need time to spend with friends otherwise you will get a lot less from them in the gym.
I know one of the tools that Coach Sommer has stated was that every 3rd week is a half intensity week. I don't think they just come in half the hours and it is merely they condition 1/2 as much. So instead of say 1-2 hours it could be 30m-1 hour total. Well you get what I am explaining since I don't exactly what his schedule for his boys are specifically.