Methods Participants Male football players 16 years or older, who are participating in full team training in Greece, will be included in the study. The study will be conducted pre-season. During that time each player will have at least 4 training sessions and 1 match per week.
Exclusion criteria were
* any systematic resistance training of the hip adductors (\> 1/week) in the preceding 1 month before study initiation.
* groin pain resulting in loss of one or more football matches/training sessions in the preceding 1 month prior to study initiation.
* any longstanding injury (≥ 6 weeks) in the lower extremities or in the hip/groin in the preceding 6 months prior to study initiation.
* pain of more than 2/10 on a numeric rating scale (NRS) when performing the adduction strength tests or exercises were also excluded.
* For analysis, players will be excluded if there was an adherence of less than 75% of the exercise sessions.
Ethics Ethical approval was obtained from University of West Attica ethics committee and all players will give their informed consent prior to inclusion in accordance with the Declaration of Helsinki.
Randomisation Randomisation will be performed after baseline strength testing. Participants will be individually randomized to perform either the CA or the SQ exercise protocol. The randomisation sequence will be obtained using an online software (e.g. randomizer.org), and sealed, opaque envelopes containing group allocation will be prepared by an independent person not otherwise involved in the study, and who will be unaware of the baseline strength test results. All participants will be instructed not to perform any additional adductor exercise other than their allocated exercise and to keep their regular training as usual.
Blinding The person performing the strength tests will be blinded to group allocation, and participants will be instructed not to reveal any information about their group allocation. In addition, the physiotherapist supervising the intervention procedures will not be allowed to know the results of the strength measurements of each player.
Training intervention Both groups will perform an 8-week training protocol with either the CA or the SQ exercise performed before or after the normal football training sessions.
Copenhagen Adduction exercise The CA is a high-intensity partner exercise where the player is lying on their side using the elbow of the lower forearm to support their body and the other arm placed along their body. The upper arm is supported by the partner who places their one hand under the knee and the other under the ankle, holding the leg approximately in the height of their hip. The player performs a 3-second concentric movement lifting their body until it reaches a straight line. At the same time, the other leg is adducted so that it touches the other leg. A 3-second eccentric adduction then follows with the body lowered halfway to the ground and the foot of the lower leg touching the ground without supporting the body. To ensure a correct performance the therapist instructs the players that there should not be any side flexion of the trunk or hip during the exercise. Each player performs one set on each side and then the players change roles. Each couple will be comprised of players of similar height and weight to ensure optimal conditions.
Adductor Squeeze exercise The SQ exercise is an isometric hip adduction exercise with the player holding a ball between their knees. The player lies supine with the ball placed between the knees with the knees and hips flexed and the feet flat on the surface with the first toe is pointed straight forward. Then the player is asked to press against the ball as hard as they can. The contraction is held for 10 seconds.
Load progression The CA will be performed with increasing number of repetitions during the intervention period as the only load progression variable. The number of sets will be kept at 2 sets per side throughout the study. Additionally, the total time under tension (TUT) of the CA protocol is adjusted to that of the SQ exercise. The SQ exercise has previously been suggested to be performed in two positions with 5 repetitions of 10 seconds each. This study will use only one exercise position for comparison, the SQ exercise will be performed for 10 repetitions.
Outcome measures The primary outcome measure of the study will be maximal eccentric adduction strength (EHAD). Secondary outcome measures will be maximal unilateral isometric adduction strength (IHAD), and DOMS. The distance between anterior superior iliac spine (ASIS) and the point of the handheld dynamometer placement will be measured to calculate torque (Nm/kg). Both the side-lying eccentric adduction and unilateral supine isometric strength measurements will be conducted on the dominant as well as non-dominant limb. Testing will be conducted in the club's training facility. The tester will use a handheld dynamometer following standardized procedures which have been shown to have a good intra-tester reliability: eccentric adduction strength ICC 0.91 (0.70-0.98), SEM: 6.3% and isometric adduction strength ICC 0.78 (0.30-0.95), SEM: 7.3%
Compliance Compliance and perceived loading will be recorded after every training session with the respective intervention exercise, while hip adductor DOMS will be recorded in respect of the previous training session. Perceived loading will be defined via the Borg CR10 scale and DOMS via a numerical ranking scale (NRS 0-10). All players will be instructed to avoid any type of training 24h before baseline and follow-up testing. Moreover, the follow-up testing will be conducted 3 days after the intervention to avoid DOMS potentially influencing the measures.
Statistical analyses Baseline and follow-up values as well as anthropometric data will be presented as mean SD, unless otherwise stated. A mixed analysis of variance (ANOVA) will be used to assess the interactions between groups (CA vs SQ) and time (baseline vs follow-up) on the dependent variables (EHAD, IHAD and DOMS). Additionally, the simple main effect of time will be analyzed using a dependent t-test. Cohen's d will be used for effect size (ES) calculation and will be set as small (0.2), moderate (0.5) and large (0.8) . Statistical significance level will be set at p 0.05. The data analysis will be performed using SPSS Statistics (v 26, IBM Corporation).
Sample size A sample size calculation was performed prior to study initiation using G\*power software (v. 3.1, Heinrich-Heine-Universität, Düsseldorf, Germany). Based on previous studies the expected mean normalized eccentric adduction strength was set at 3.0 Nm/kg with a standard deviation (SD) of 0.6 Nm/kg. The minimal relevant difference in EHAD strength between groups was chosen to be 20%, and correlation was arbitrarily set at 0.5. With 80% power and a type-1 error risk of 5%, and a correlation a total sample size of 42 participants was adequate to detect a large moderate effect size. To account for dropouts during the intervention a minimum of 60 participants are included.
