Table 1 Definition of study groups according to objective measurement and guideline criteria

Group 1: Sport specialist, MVPA

1. Participation in intensive training and/or competition in organized sports greater than 8 months per year (essentially year round) [29]

2. Participation in 1 sport to the exclusion of participation in other sports (limited free play overall) [30]

3. Involving prepubertal (seventh grade or roughly age 12 years) children.

Greater than 180 accumulated minutes of MVPA [31, 32] during participation in one sport type across one week of activity tracking

Meets criteria within either one or both years of follow-up

Group 2: Non-sport specialist, any activity level

1. Participation in more than 1 sport at any physical activity level

OR

2. Participation in none or low training and/or competition in organized sports for any period of time.

3. Involving prepubertal children.

Greater than or less than 180 accumulated minutes of MVPA across one week of activity tracking

Meets criteria during both years of follow-up