Baseline description of the participants
| Intervention group | Control group | ||
| Youth athletes (n=142) | 63 | 79 | |
| Sex, n (%) | |||
| Female | 46 (73) | 46 (58) | |
| Club size, n (%) | |||
| Small | 24 (38) | 11 (14) | |
| Age (years), mean (SD) | 12.4 (2.8) | 13.2 (2.0) | |
| Height (cm), mean (SD) | 164.0 (9.8) | 164.5 (10.6) | |
| Weight (kg), mean (SD) | 51.0 (10.8) | 50.3 (12.1) | |
| Menarche (yes), n (%) | 26 (57) | 31 (67) | |
| Sleep per night (hours), mean (SD) | 8.5 (0.8) | 8.4 (1.0) | |
| Participating in more than one sport (yes), n (%) | 31 (49) | 34 (43) | |
| Previous training volume, hours per week (SD) | 4 (2) | 5 (3) | |
| Previous training sessions, times per week (SD) | 3 (1) | 3 (1) | |
| No of coaches, mean (SD) | 3.0 (1.0) | 3.0 (1.0) | |
| Previous 3 week injury, n (%) | 17 (27) | 21 (27) | |
| Previous 3 week illnesses, n (%) | 4 (6) | 5 (6) | |
| Parents/carers (n=107) | 40 | 67 | |
| Sex, n (%) | |||
| Female | 27 (67) | 40 (60) | |
| Age (years), mean (SD) | 47.2 (4.6) | 47.5 (3.9) | |
| Country of birth, n (%) | |||
| Nordic country | 38 (95) | 62 (93) | |
| Highest level of education, n (%) | |||
| University | 28 (70) | 50 (75) | |
| Residence size, no of inhabitants, n (%) | |||
| <20 000 | 7 (18) | 13 (19) | |
| 20 000–100 000 | 12 (30) | 18 (27) | |
| >100 000 | 20 (50) | 36 (54) | |
| Former athlete, n (%) | |||
| Yes | 7 (18) | 21 (31) | |
| Coach today, n (%) | |||
| Yes | 4 (10) | 12 (18) | |
| Athletics as primary sport | 0 (0) | 0 (0) | |
| Any coach education, n (%) | |||
| Yes | 37 (93) | 56 (84) | |
| Coaches (n=52) | 29 | 23 | |
| Sex, n (%) | |||
| Female | 15 (52) | 11 (48) | |
| Coach, and also parent for a youth participating in the study | 21 (72) | 18 (78) | |
| Age (years), mean (SD) | 47.5 (8.2) | 43.1 (12.0) | |
| Country of birth, n (%) | |||
| Nordic country | 31 (100) | 19 (90) | |
| Highest level of education, n (%) | |||
| University | 20 (65) | 14 (67) | |
| Residence size, no of inhabitants, n (%) | |||
| <20 000 | 11 (35) | 7 (33) | |
| 20 000–100 000 | 9 (29) | 5 (24) | |
| >100 000 | 11 (35) | 9 (43) | |
| Former athlete, n (%) | |||
| Yes | 31 (100) | 21 (100) | |
| Coach today, n (%) | |||
| Yes | 31 (100) | 21 (100) | |
| Athletics as primary sport | 30 (97) | 20 (95) | |
| Any coach education, n (%) | |||
| Yes | 26 (84) | 18 (86) | |