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Introduction: Training and match demands in women’s football have soared during the last decades and may have altered the risk for injury and illness players face. To develop effective preventive measures tailored to the needs of modern female football players, we need reliable, up-to-date surveillance data. Therefore, we aimed to describe the prevalence, incidence, and burden of all health problems in the Norwegian women’s premier league (Paper I). Because we hypothesised that the changes in women’s football had caused a high risk for muscle injuries, particularly hamstring injuries, we also conducted an in-depth study on hamstring injuries in the Norwegian women's premier league (Paper II). Low eccentric strength is considered a risk factor for hamstring injury, therefore accurate measurement of maximal eccentric hamstring strength is important. The NordBord is a commonly used for this purpose and measures maximal eccentric hamstring force through the Nordic hamstring exercise. To ensure that the test is supramaximal, as intended, several studies have made participants hold extra weights during the test if they are able to control the last 20° of the Nordic hamstring exercise’s range of motion. This approach has not been based on substantial evidence and may have introduced bias and overestimated the true change in strength. Therefore, we examined how performing NordBord-testing with added weight affected the force measured (Paper III). Although eccentric hamstring strengthening programmes can effectively reduce the risk of hamstring injuries, they are not adopted by football teams, possibly due to the high training volume and long duration of the programmes. Therefore, we compared the effect of a high- and low-volume Nordic hamstring programme on maximal eccentric hamstring strength in female football players (Paper IV). Methods: We conducted a two-season prospective cohort study in the Norwegian premier league (2020-2021). In Paper I, 294 players (22±4 yrs, 93% of all players invited) reported all health problems by responding to the updated version of the Oslo Sports Trauma Research Center Questionnaire on Health problems (OSTRC-H2) once a week. The reported health problems were diagnosed by team physiotherapists using the Sports Medicine Diagnostic Coding System (SMDCS). In Paper II, the hamstring injuries recorded in the prospective cohort study were examined by standardised clinical examinations performed by the team physiotherapists and with magnetic resonance imaging (MRI). In Paper III, we tested 84 of the female premier league players (22±4 yrs) and 56 male 1st division players (24±4 yrs) in the NordBord with 0 kg, 5 kg and 10 kg added weight and compared the results from the three testing conditions. In Paper IV, we conducted a randomised trial where 45 players (21±4 yrs) from two 1st division women’s teams (2nd tier) performed a high- or low-volume Nordic Hamstring training programme during the pre-season period, and compared the training programmes’ effect on maximal eccentric hamstring strength, jump height and sprint performance. Main results: The average weekly prevalence of substantial health problems was high (22%, 95% CI: 21-23%), mainly caused by injuries. The prevalence, incidence and burden of illness was low. Gradual-onset injuries were more common than previously reported (35% of all health problems), but sudden-onset injuries were still the most severe (68% of total days lost). Muscle injuries were the most frequent, while injuries to ligament/joint capsule were most severe. Hamstring injuries were the most common injury diagnosis and caused the 3rd highest injury burden (7.9 days lost/1000 h), behind anterior cruciate ligament (ACL) injuries (39.3 days lost/1000 h) and concussion (8.3 days lost/1000 h). In Paper II, 53 hamstring injuries were examined clinically and 31 of these with MRI. Most were non-contact and occurred during sprinting. Gradual-onset (53%) and sudden-onset injuries (47%) were evenly distributed. Of injuries displaying MRI changes, 60% were in the m. biceps femoris, most involving the muscletendon junction, and 40% in the m. semimembranosus, most in the proximal tendon. In Paper III, maximal eccentric hamstring force was higher when tested with 5 kg (females: +2%, p<0.001, males: +4%, p<0.001) and 10 kg (females: +5%, p<0.001, males: +6%, p<0.001) compared to 0 kg. This was the case for both players who could control the final 20° of the test (5 kg: + 4%, p<0.001, 10 kg: + 7%, p<0.001) and those who could not (5 kg: +3%, p<0.001, 10 kg, +4%, p<0.001). In Paper IV, both groups increased maximal eccentric force (high-volume: 29 N (10%), 95% CI: 19-38 N, p<0.001, low-volume: 37 N (13%), 95% CI: 18-55 N, p=0.001), but there were no between-group differences (p=0.38). Maximal eccentric torque, jump height and sprint performance did not change in either group. Conclusion: The average weekly prevalence of health problems, especially injuries, in the Norwegian women’s premier league was high. Hamstring injuries were the most frequent and third most burdensome injury. Compared to previous findings from men’s football, a higher proportion of hamstring injuries in women’s football had a gradual onset and involved the m. semimembranosus, particularly its proximal tendon. Both players who could and could not control the final 20° of the NordBord test demonstrated higher maximal force when adding weight to testing. Therefore, this should not be used to decide if players should perform NordBord testing with or without weight in the future. Both the high- and low-volume Nordic hamstring programme increased the maximal eccentric hamstring force in female football players, but there was no difference between the programmes.
