Plus, overtraining is real. Your body needs rest sometimes. Training for boxing, in off season, is a time when you should only do about 25% of specific training, meaning training for your sport. The rest should be conditioning and strenght AND coordination (plus flexibility). Specially for young athletes (before 10 years old), coordination is the key as it will help you later on, but should be done at all ages (GSP was doing gymnastic to help for his coordination when he was UFC champion). Lomachenko was doing dance early on. In North america gymnastic and dance is considered to feminin but it should be a core training with young athlete, well, to make the better athlete when the time comes.