The literature is scarce when it comes to more comprehensive research project that investigates the differences between youth and adult elite football players in physical parameters and the risk of injury. There is a lack of prospective long-term studies examining the importance of increased training load, physical fitness and performance, training status, position and maturity on its relationship to injury and success in elite football.

The aim of the project is to evaluate different aspects of physical performance, training load and their role as potential risk or protective factors for injury in elite Swedish football. Another aim is to follow youth players at the football academy to explore which physical factors are important for success in elite football and how a possible injury affects the player's continued elite efforts. Finally, the aim is to develop a test battery that can be used to screen for deficits in physical fitness, monitor football players at risk of injury and for return to sport after injury. The main research question intended to be investigated is: How do internal and external load, wellness, and physical performance factors (including Acute Chronic Workload Ratio) differ between academy-level and professional football players, and how do these factors relate to injury risk and success in elite football? A cohort design will be used with ~200 academy football players (~50% women) between 15-19 years old and 50 professional football players participating. The study starts with the assessment of different aspects of physical performance at baseline during the players first year at the academy and internal and external training load and injuries will be registered prospectively for 3 years. The players will be assessed at follow-ups, with the same tests, 3 times a year, to evaluate performance changes. Players who will be transferred to elite play will be evaluated 3 times a year for additional 5 years. Professional football players will also be evaluated with tests with following registration of training load and injuries.

Purpose and aims

The aim of the project is to evaluate different aspects of physical fitness, muscle function, training load and their role as potential risk or protective factors for injury in elite Swedish football. Another aim is to follow youth players at the football academy (Swedish sports high schools for football) to explore which physical factors are important for success in elite football and how a possible injury affects the player's continued elite efforts. Finally, the aim is to develop a test battery that can be used to screen for deficits in physical fitness, monitor football players at risk of injury and for return to sport after injury. The following research questions are intended to be investigated:

1. What is the internal and external load and Acute Chronic Workload Ratio in football players at academy level compared to professional level?
2. What are the levels of wellness in football players at academy level compared to professional level?
3. Is impaired physical fitness and muscle function (physical performance) a risk factor for injury in football?
4. Which physical factors are important for success in elite football?
5. What impact has an injury on the youth player's continued elite efforts?
6. Can a test battery be used to screen for deficits in physical performance at the same time monitor football players at risk of injury?
7. Which physical factors are important for return to sport after injury in football?

Procedure and time frame The study is expected to begin with pilot testing in November 2024 and then set off in January 2025 with assessment of different aspects of physical performance at baseline during the players first year at the academy and internal and external training load and injuries will be registered prospectively for 3 years. The players will be assessed at follow-ups, with the same tests, 3 times a year, to evaluate performance changes. Players who will be transferred to elite play will be evaluated 3 times a year for additional 5 years. New academy players will take part in 2026 and 2027 following the same procedure. The professional football players will also be evaluated with tests with following registration of training load and injuries.

Measurements Injury registration Data on injury will be collected using a form7 distributed by the team's RPT. All players will report injuries occurring during sport activity diagnosed by a physical examination, MRI, or arthroscopy. Details on injury situation, and diagnosis will be reported by the team's RPT.

External and internal training load For the academy players, external training load will be collected weekly by reporting hours of match and training using a web-based form5 distributed through SurveyMonkey®. For the professional players external training load will be analyzed using data from the GPS system Polar Team Pro tracking devices. All players will also estimate session ratings of perceived exertion (sRPE) after each football session and Freshness score; muscle soreness, sleep quality and mental stress, before every session. Internal training load and Acute Chronic Workload Ratio (ACWR) will be calculated using sRPE multiplied by training minutes2.

Anthropometry Body height (cm) will be measured with a stadiometer, and body mass (kg) will be obtained from a bioelectrical impedance analysis (BIA) measurement (InBody 770, InBody Co, Ltd, Seoul, South Korea) together with total body lean body mass (LBM).

Physical Performance assessment The tests will be instructed and supervised by educated test leaders, and will take place at Slottslabbet, Radix, at Linnaeus University beginning with a warm-up protocol.

Jump performance Jump performance will be evaluated by the vertical jump test and performed as a counter-movement jump with the computerized system (MuscleLab, Ergotest Technology), as described8.

Squat strength Multiple joint maximal muscle strength of the lower extremity will be measured with the Isometric squat test, as described9. Peak isometric force (N) and avgRFD (N·s-1) are recorded and assessed using a force plate and analyzed with commercial software (MuscleLab, Ergotest Technology AS, Langesund, Norway).

Test of Nordic hamstring exercise (NHE) performance Both the fast stretch-shortening cycle and slow eccentric NHE will be measured using the protocol by Augustsson et al.9. During the NHEs, a linear encoder will be used to measure the position where the peak knee flexor force is recorded.

Isokinetic strength testing Strength testing of the quadriceps and hamstring muscles will be conducted unilaterally with players seated on an isokinetic dynamometer (Biodex System 4; Biodex Medical System), as described8 assessing both eccentric and concentric muscle actions. Isokinetic strength measurements of the hip abductor and adductor will also be assessed in a standing position.

Isometric midthigh pull (IMTP) strength test The IMTP is an isometric assessment used to determine maximal force in various athletic populations11. We will use a custom-built IMTP device that measures strength via load cells, following the principles described by Augustsson et al.10.

Lateral agility and muscular endurance Lateral agility, muscular endurance and any lower between-limb differences will be evaluated with the 30-sec Side hop test for each leg, as described5.

VO2max Peak VO2 will be performed with a graded treadmill (Rodby RL2000 60) protocol and with the player equipped with a heart rate monitor (Polar RS400; Polar, Kempele, Finland) and face mask (mask 7450 V2). Cardiorespiratory variables will be measured continuously breath-by-breath using ergospirometry (Vyntus CPX, CareFusion, Hochberg, Germany).

Speed Linear sprinting speed is assessed by three a 20-m sprint attempts using two timing gates (Witty, Microgate, Italy) with 2 minutes of recovery between each sprint.

Change of direction speed The 505 agility test will be used to assesses 180-degree turning ability using e timing gate (Witty, Microgate, Italy) as described12.

Zig-zag (running and dribbling) test The Zig-zag (running) test will be used to assess running speed with a constant change of direction using two-timing gates (Witty, Microgate, Italy) as described13.

Reactive Agility To assess football-specific agility, the reactive agility (RAG) test is used using an infrared (IR) sensor that is positioned 1.5 m from the line and a hardware module (ATMEL Corp, San Jose, CA, United States) that activates one of the four LEDs inside the 30-cm-high cones (labeled A, B, C, and D) as described14.

Neuromuscular knee control Neuromuscular knee control will be assessed and scored with one functional performance tests, the drop jump test (DJ) as described15 and video-recorded using the Coach Eye app for smart phone. To facilitate analysis and to be able to measure knee valgus coach tape will be used to mark landmarks on the SIAS, mid-patella, and mid-centered between the lateral and medial malleolus. Analysis and calculation of the valgus angle will be performed using the software program Kinovea (version 0.9.5), which is a well-established software program designed to investigate and analyze kinematic parameters.

Inclusion Criteria:

• female and male academy football players aged 15-19 years and
• female and male professional football players that plays the one of the two top division in Sweden.

Exclusion Criteria:

• injuries at baseline assessment, that may affect test results

Swedish male and female players that are playing at professional level or at academy level (from Swedish sport high schools).

The data will contain highly personal data from professional football players which could be traced to a specific person. These players are very sensitive because of their market value. Although de-identified, a compilation of the outcomes can reveal the person. Thus, these ethical dilemma together with the regulation on the protection of natural persons by the General Data Protection Regulation (GDPR) (2018:218), we opt not to share IPD.