Mental health symptoms (dependent variables)
Mental health symptoms were operationalised according to the International Olympic Committee (IOC) Sport Mental Health Assessment Tool 1 (SMHAT-1), which assessed symptoms of distress, anxiety, depression, sleep disturbance, alcohol misuse, drug misuse and disordered eating.12
Distress in the previous 4 weeks was assessed using the Athlete Psychological Strain Questionnaire (APSQ) based on 10 items (eg, ‘During the past 4 weeks, I could not stop worrying about injury or my performance’) scored on a 5-point scale (from ‘none of the time’ to ‘all of the time’).13 The APSQ has been recently validated in the athletic population (internal consistency: 0.5–0.9; criterion-related validity: area under receiver operating characteristic (ROC) curve >0.9).13 14 A total score ranging from 10 to 50 was obtained by summing up the answers on the 10 items, with a score of 17 or more indicating an elevated or high risk for (athletic) distress.13 14
The 7-item General Anxiety Disorder-7 (GAD-7) was used to assess symptoms related to anxiety in the previous 4 weeks (eg, ‘Have you been feeling nervous, anxious or on edge?’) scored on a 4-point scale (from ‘not at all’ to ‘nearly every day’).15 The GAD-7 has been validated in several populations and European languages (internal consistency: 0.9; test–retest reliability: 0.8; criterion-related validity: sensitivity 0.9, specificity 0.8, area under ROC curve >0.9).15 A total score ranging from 0 to 21 was calculated by summing up the answers on the 7 items, with a score of 10 or more indicating the presence of moderate anxiety.15
The Patient Health Questionnaire-9 (PHQ-9) was used to assess the presence of symptoms of depression in the previous 4 weeks (eg, ‘Have you been feeling down, depressed or hopeless?’) scored on a 4-point scale (from ‘not at all’ to ‘nearly every day’).16 17 The PHQ-9 has been validated in several populations and European languages (internal consistency: >0.8; criterion-related validity: sensitivity >0.8, specificity >0.8, area under ROC curve >0.9).16 17 A total score ranging from 0 to 27 was calculated by summing up the answers on the 9 items, with a score of 10 or more indicating the presence of moderate depression.16 17
Measured using the shortened Athlete Sleep Screening Questionnaire (ASSQ), sleep disturbance in the previous 4 weeks was assessed through 5 items (eg, ‘How satisfied/dissatisfied are you with the quality of your sleep?’) scored on 4-point and 5-point scales.18 19 The ASSQ has been validated in athletes (internal consistency: >0.7; test–retest reliability: >0.8; criterion-related validity: sensitivity >0.8, specificity >0.9).18 19 A total score ranging from 1 to 17 was obtained by summing up the answers to the 5 items, a score of 8 or more indicating the presence of moderate sleep disturbance.18 19
Level of alcohol consumption was detected using the validated 3-item Alcohol Use Disorders Identification Test (AUDIT-C; eg, ‘How many standard drinks containing alcohol do you have on a typical day?’).20 The AUDIT-C has been validated in several populations and European languages (test–retest coefficients: 0.6–0.9; criterion-related validity: area under ROC curve 0.70–<1.0).20 21 A total score ranging from 0 to 12 was obtained by summing up the answers on the 3 items, a score of 3 or more indicating the presence of alcohol misuse.20
Based on the Cutting down, Annoyance by criticism, Guilty feeling and Eye-openers Adapted to Include Drugs (CAGE-AID), drug(s) misuse in the previous 3 months was assessed through 4 items (eg, ‘In the last 3 months, has anyone annoyed you or gotten on your nerves by telling you to cut down or stop using drugs?’) scored as yes or no.22 23 The CAGE-AID has been validated in several populations and European languages (reliability: >0.9; sensitivity: >79%; specificity: >97%).22 23 A total score ranging from 0 to 4 was obtained by summing up the answers to the four questions, with a score of 2 or more indicating the presence of drug misuse.22 23
The Brief Eating Disorder in Athletes Questionnaire (BEDA-Q) was used to assess the presence of disordered eating in the previous 4 weeks through 9 items (eg, ‘I feel extremely guilty after overeating’) scored on several scales.24 The BEDA-Q has been validated in athletes (internal consistency: >0.8; criterion-related validity: sensitivity >0.8, specificity >0.8, area under ROC curve >0.7).24 A total score ranging from 0 to 18 was calculated by summing up the answers on the first 6 items, with a score of 2 or more indicating the presence of disordered eating.12
Severe injury and related surgery (independent variable)
History of severe injury and related surgery in hips, knees and ankles was examined through 12 questions (eg, ‘How many severe injuries in your left knee have you had so far as professional footballer?’) as used in previous studies.5 6 9 25 In our study, a severe injury was defined as an injury that involved the given joint (hip, knee and/or ankles), occurred during team activities (training or match) and led to either training or match absence for more than 28 days.26 For this question, participants were requested to consult either their medical record or their team physician. The total number of severe injuries and related surgeries was calculated.
Procedures
A baseline and two follow-up electronic questionnaires were set up in English and French (CastorEDC, CIWIT B.V., Amsterdam, the Netherlands), including all variables from the study. In addition, the following descriptive variables were added to the baseline questionnaire: age, height, body weight, level of education, parallel activity (eg, study, work), field position, level of football, number of seasons as professional footballer, exposure to training and matches, history of mental health disorders, smoking status and use of medications (sleeping tablets, antidepressants). Information about the study was sent per email to potential participants by FIFPRO and affiliated national unions, procedures being hidden from the principal researcher for privacy reasons. If interested in the study, all participants gave their informed consent and were given access to the baseline (T0) questionnaire. Follow-up questionnaires were sent 6 (T1) and 12 (T2) months later. Each questionnaire took about 15–20 min to complete. The questionnaire responses were coded and anonymised for privacy and confidentiality reasons. Once completed, the electronic questionnaires were saved automatically on a secured electronic server that only the principal researcher could access. Players participated voluntarily in the study and did not receive any reward for their participation.