Resumen de Physical performance and hamstring injury risk factors during the off-season period in footballers. Inertial flywheel versus gravity-dependent resistance training methods
The physical demands (high-intensity running and sprinting) of football match play place footballers at increased risk of hamstring strain injuries. Some of the negative effects of a hamstring strain injury such as neuromuscular deficits and a decrease in performance are already known. While previous injury and age are known risk factors for future hamstring injuries, their relationship with the changes in eccentric hamstring strength and performance during the off-season in semi-professional and amateur footballers have not been studied thoroughly. Moreover, a variety of resistance training interventions aiming at improving physical performance and strength are available in the literature, with inertial flywheel resistance training arising lately and providing promising results. However, its superiority against gravity-dependent interventions on physical performance and strength variables is not scientifically available at the moment. The thesis aimed to firstly compare the use of inertial flywheel with gravity-dependent resistance training for improving physical performance and thigh muscle strength and, secondly, to test how previous injury, age and the off-season period affects physical performance and eccentric hamstring strength in semi-professional and amateur footballers.
Five publications (a systematic review with meta-analyses, a letter to the editor, a cross-sectional study, a longitudinal observational study and the design of a randomised controlled trial with an intervention protocol) were developed as the main body of the thesis. The systematic review with meta-analyses (Publication I) included 7 studies (three randomised and four non-randomised controlled trials), that compared inertial flywheel against gravity-dependent training interventions. The two observational studies included semi-professional and amateur footballers. The cross-sectional study (Publication III) included a cohort of 284 footballers from 17 teams (4th-6th tiers) and assessed their eccentric hamstring strength at the beginning of the pre-season. The longitudinal observational study (Publication IV) included 74 semi-professional (3rd-4th tier, n = 28) and amateur (5th-8th tier, n = 46) male footballers which were assessed at the beginning and end of the off-season summer period on selected physical performance measures (sprint, change of direction performance and eccentric hamstring strength). The intervention protocol (Publication V) consists of a randomized pre-mid-post parallel-group trial, including semi-professional footballers, divided into two intervention groups, the inertial flywheel resistance and the gravity-dependent resistance groups. Players will be assessed for sprint, repeated-sprint ability, change of direction and jumping performances and hamstring strength. Participants will perform three supervised sessions a week during the last three weeks of the off-season period, and one supervised session a week during the pre-season (4 weeks). Both groups will perform the same volume, frequency and duration of interventions, including gym-based strength and power sessions, on-field sprinting, COD and plyometrics sessions.
The results from the systematic review and meta-analyses (Publication I) showed no difference between the inertial flywheel or the gravity-dependent interventions in the changes in muscle strength for RCTs (SMD = -0.05; 95%CI -0.51 to 0.40; p = 0.82) or non-RCTs (SMD= 0.02; 95%CI -0.45 to 0.49; p=0.93; I2=0%; and SMD= 0.03; 95%CI -0.43 to 0.50; p=0.88; I2=0%). Meta-analyses on other primary outcomes more related to performance, such as muscle power or rate of force development, could not be performed due to lack of data. The main objections from the Letter to the Editor (Publication II) against the publication were 1) the lack of a protocol registration; 2) the inclusion of six, out of nine, studies that do not compare inertial flywheel against gravity-dependent interventions, but against no training; 3) the inclusion of one non-published study. In the pre-season study cohort (Publication III), age had a negative association with preseason eccentric hamstring strength with a 0.9% reduction per year. Players with a previous hamstring injury duration of more than three weeks (n = 27) had 13% lower preseason eccentric hamstring strength compared to players without previous hamstring injury or a hamstring injury of less than 3 weeks. In the longitudinal off-season study (Publication IV), small to medium increases in sprint times were observed at 5 m (d = 0.26, p = 0.057), 10 m (d = 0.42, p < 0.001) and 30 m (d = 0.64, p < 0.001) at the end of the off-season. Small (d = -0.23, p = 0.033) improvements were observed for change of direction performance (lower time), and no changes in eccentric hamstring strength (d = 0.10, p = 0.317) at the end of the off-season. The changes in the outcomes were not affected by age (p = 0.449 to 0.928), previous hamstring injury (p = 0.109 to 0.995) or off-season length (p = 0.148 to 0.927).
To conclude, given the available literature, inertial flywheel resistance training is not superior to gravity-dependent resistance training on enhancing muscle strength (maximum isometric voluntary contraction). There was not enough evidence to analyse other strength variables (concentric, eccentric or RFD), muscular adaptations or performance outcomes (sprint, jump or COD) at the moment of the meta-analyses. However, recent evidence in team-sport athletes shows interesting results regarding strength and performance adaptations. While an update of the meta-analysis could provide a clearer picture, it could be hypothesised that similar strength but greater performance adaptations (mainly COD and jumping) are present in favour of inertial flywheel interventions. Semi-professional and amateur footballers with a previous hamstring injury of more than three weeks present lower eccentric hamstring strength at the beginning of preseason. Moreover, increasing age is associated with lower eccentric hamstring strength at the beginning of preseason in amateur footballers. Semi-professional and amateur footballers showed impaired sprint performance after the off-season period, independent of age, previous hamstring injury and length of the off-season. Such decrements in sprint performance are not related to the changes in eccentric hamstring strength, which is not altered after the off-season period. Instead, a small improvement during the change of direction test seems to be present after the off-season period.
Ideally, practitioners should monitor the physical performance and strength profiles of the players, trying to personalize part of the off-season training of the players. However, this scenario in a semi-professional or amateur squat of 20-24 players is unlikely. Even though our results showed no decrements in eccentric hamstring strength, the off-season is a great period (without sport-specific demands) to improve such aspect which is related to performance and injury risk reduction. Hence, practitioners are advised to prescribe some sort of strength and conditioning preparation (targeting strength, aerobic and anaerobic conditioning, and also general and sport-specific abilities) before the start of pre-season. Additionally, a greater emphasis should be placed on older players and players with a previous hamstring injury of longer duration, which are more prone to present eccentric hamstring weaknesses. Those weaknesses can be improved through consistent resistance training practices, either using inertial flywheel exercises or gravity-dependent exercises with an emphasis on eccentric actions.
