This is a three-armed intervention study that evaluates the injury preventive effects of three different training interventions in youth and adult football players. Two intervention arms are randomized and one arm acts as a non-randomized comparison group. Half of randomized participants will receive a general injury prevention exercise program with emphasis on the lower extremities, and the other half a hip/groin focused injury prevention exercise. A third group of participants who already use an injury prevention exercise program at study inclusion are invited to participate as a non-randomized comparison group and continue their usual training practices.

The study is a cluster-randomized controlled trial including male and female youth and adult football players (≥14 years). Club is used as cluster unit, where all the teams from the same club will be randomized to the same intervention arm. The randomization is stratified by level (junior/senior) and sex (male/female).

The study has three intervention arms:

Teams in the randomized intervention group A receive education of the injury prevention exercise program Knee Control+ (Knäkontroll+) before the start of the competitive football season in April. Knee Control+ consists of 6 different exercises, with 10 different variations/progressions, and takes 10-15 minutes to complete. In addition, teams are instructed to perform a structured 5-minute running warm-up before the Knee Control+ exercises. Teams are to carry out the running warm-up and Knee Control+ at all training sessions, and the running warm-up before all matches, during the whole season.

Teams in the randomized intervention group B receive education of an Adductor strengthening program before the start of the competitive football season in April. The Adductor strengthening program consists of a single exercise with three levels of difficulty, as described by Harøy et al., 2019, British Journal of Sports Medicine. The exercise is based on the Copenhagen Adduction exercise. Teams are to carry out the Adductor strengthening program as part of their regular warm-up 2-3 times per week, one set per side, during the pre-season, and 1 time per week, one set per side during the in-season. One set takes about 5 minutes to complete for a pair of players.

Teams reporting that they are already using the injury prevention exercise program Knee Control (Knäkontroll) at recruitment are not eligible for inclusion in the randomization. They will be invited to participate in the study as a non-randomized intervention group C and act as a comparison group to the randomized groups A and B. These teams will carry on their normal training and warm-up routines with no intervention from the researchers. Knee Control consists of 6 different exercises, with 4 different variations/progressions and 1 pair-exercise.

Hypotheses:

Hypothesis 1: Teams/athletes in the randomized group A (Knee Control+) will have a lower rate of ankle, knee and hamstring injuries compared to teams/athletes in the randomized group B (Adductor strengthening program).

Hypothesis 2: Teams/athletes in the randomized group B (Adductor strengthening program) will have a lower rate of hip/groin injuries compared to teams/athletes in the randomized group A (Knee Control+).

Hypothesis 3: Teams/athletes in the randomized group A (Knee Control+) will have a higher intervention compliance compared to teams/athletes in the non-randomized comparison group (Knee Control).

Data collection:

Teams are followed for one competitive football season from April to October 2020. Data registered for the study include individual exposure to football training and matches, and injuries that occur as a result of participation in football. Exposure and injury data are athlete-reported prospectively every week using web-based questionnaires (the Oslo Sports Trauma Research Center questionnaire). Study designated physiotherapists will collect additional injury information for the four main injury outcomes via telephone interview using a standard injury card. Use of the intervention (compliance) is reported weekly by the team coach and the athletes. Coaches and players will report demographics at baseline, and at three time-points during the season (baseline, mid-season, end of season) will report use and experiences of the training interventions, injury risk perceptions and outcome expectancies.

Sample size for primary outcomes:

Hypothesis 1: required sample size for primary outcome "any football related injury occurring to the ankle, knee or hamstrings, regardless of need of care or absence from football training or matches" is 252 subjects (approximately 17 teams). To account for a 30% dropout a sample size of 328 subjects (approximately 22 teams) is planned at study start.

Hypothesis 2: required sample size for primary outcome "any football related injury occurring to the hip/groin, regardless of need of care or absence from football training or matches" is 301 subjects (approximately 20 teams). To account for a 30% dropout a sample size of 391 subjects (approximately 26 teams) is planned at study start.

Statistical analysis plan:

Generalised estimating equations will be used to analyse cluster-aggregated weekly data of injuries and exposure. A Poisson distribution with a log link function will be applied to calculate the rate ratio and corresponding 95% confidence intervals for the injury incidence and injury prevalence according to the intention to treat (ITT) principle. ITT analyses on a whole group level will be adjusted for sex, and analyses will also be performed by sex separately. The statistician who performs the ITT analysis will be blinded to group allocation. The number needed to treat will be calculated as the inverted absolute rate reduction for the primary outcomes.

Inclusion Criteria for randomized intervention arms A and B:

• Teams participating in youth or adult league series in the 2020 football season. All players in the eligible teams will be invited to participate (players from the age of 14 and older, born 2006 or earlier)
• Teams who have not used a structured training program aiming to prevent injuries that resembles the 3 intervention arms (e.g. the Knee Control program, Knee Control+ program, 11+ program, Adductor strengthening program) on a regular basis the last year
• Teams who have scheduled training at least twice per week

Inclusion criteria for the non-randomized intervention arm C:

• Teams participating in youth or adult league series in the 2020 football season. All players in the eligible teams will be invited to participate (players from the age of 14 and older, born 2006 or earlier)
• Teams who have used the Knee Control program on a regular basis the last year, but no other structured training program aiming to prevent injuries (e.g. the Knee Control+ program, 11+ program, Adductor strengthening program)
• Teams who have scheduled training at least twice per week

Exclusion Criteria:

• Teams who have used a structured groin injury prevention exercise program (e.g. Adductor strengthening program, Hölmich protocol) on a regular basis the last year

Individual participant data may be available upon reasonable request to the principal investigator

Preliminary in autumn 2020

Study protocol and statistical analysis plan is planned to be submitted in writing to an open access scientific journal

Physiotherapists who assess injuries are blinded to group belonging. Statistician performing outcome analyses are blinded to group belonging.