Hamstring injuries are the most prevalent injuries in football, with an average of 22% of players sustaining at least one hamstring injury during a season. The majority of hamstring injuries (~70%) occur during high-speed sprinting actions. Consequently, it seems logical to expect sprinting to be a key parameter in football from both performance and hamstring injury prevention perspectives.
Sprint acceleration performance has been shown to be associated with the ability to produce and apply high levels of force in the horizontal direction over the entire acceleration. This ability is well described by a macroscopic linear relationship between horizontal force and velocity obtained during sprint acceleration (F-v relationship). FH0 (which is the theoretical maximal force production at zero velocity) represents the forceproduction capacity at extremely low velocity, and V0 (which is the theoretical maximal velocity until which horizontal force can be produced) represents the force production capacity at extremely high velocity.
This study aimed to analyze the association between horizontal force production capacities during sprinting (FH0 and V0) and hamstring injury occurrence in football players.
The authors conducted a one-season prospective cohort study on football players who were assessed for mechanical outputs during sprint accelerations (i.e., horizontal force production during sprinting) in field conditions at the start of the season as well as several times during the season.
At each testing session, the instantaneous sprint velocity was measured during 2 maximal 30 m sprints from a standing start. Running speed time curves were fitted by a mono-exponential function, allowing step-averaged horizontal external anterior-posterior ground reaction force computations. From these data, the maximal power output associated with the antero-posterior component of the ground reaction force (Pmax) and the maximal theoretical force and velocity components of the F-v profile (FH0 and V0) were calculated. FH0 and V0 values from the sprint trial with the highest Pmax were used for analysis.
At the beginning of the season, the history of previous hamstring injuries was collected for each included player. During the season, exposure in hours of football practice (i.e., training and competition) was collected weekly by coaches. All new injuries were prospectively collected by the medical teams.
To analyze the association between horizontal force production capacities during sprinting (FH0 and V0) and hamstring injury occurrence, the authors used a time-to-event approach; information after the occurrence of the hamstring injury was censored. The time to first event was analyzed using cumulative hours of football practice as the time scale. A Cox proportional hazards regression model was used to analyze the association of FH0 and V0 with the occurrence of hamstring injury. A first adjusted Cox regression model (model 1) was conducted using the FH0 and V0 values at the start of the season and new hamstring injury occurring during the season (yes or no) as the outcome, with follow-up until the end of the season with no hamstring injury occurrence. A second adjusted Cox regression model (model 2) was conducted using the FH0 and V0 values at each measurement session within the season and new hamstring injury occurring after the measurement session (yes or no) as the outcome, with follow-up when no hamstring injury occurrence until the next measurement, if any, or at the end of the season.
284 players were included in this study. Among them, a total of 801 player-measurements were performed: 41 players performed one measurement (14%), 106 performed two measurements (37%), 42 performed three measurements (15%), 69 performed four measurements (24%), 10 performed five measurements (4%), and 16 performed six measurements (6%).
A total of 47 new hamstring injuries occurred in 38 players (13%). The mean time between the measurement at the start of the season and the first hamstring injury occurrence was 166 (±158) hours of football practice (model 1), and the mean time between a (subsequent) measurement and the first hamstring injury occurrence was 73 (±48) hours of football practice (model 2).
The data demonstrated that (1) horizontal force production capacity at low (FH0) and high (V0) velocities measured at the start of the season (model 1) was not associated with a new hamstring injury occurring during the season, and (2) lower maximal horizontal force production capacity at low velocities (FH0) measured at more regular intervals of time over the season (model 2) was significantly associated with a higher rate of new hamstring injury (HR = 2.67 (95% CI: 1.51 to 4.73), p < 0.001) occurring within the weeks following the sprint acceleration mechanical output testing, while V0 was not. Every 1 N.kg decrease of FH0 was associated with 2.67 times higher risk of sustaining a new hamstring injury, within a mean timing of 73 h of football practice (corresponding at ~7 weeks with 10 h of football practice per week).
In conclusion, low horizontal force production capacities at low velocity during early sprint acceleration (FH0) may be considered as a potential additional factor associated with hamstring injury risk in a comprehensive, multifactorial, and individualized approach. Sprint acceleration mechanical outputs may change with changes in physical status and training regimen throughout the season and as a result of hamstring injuries. Thus, regular measurements throughout the season are likely more valuable to better inform the player on injury risk.
Edouard P et al. (2021) Low Horizontal Force Production Capacity during Sprinting as a Potential Risk Factor of Hamstring Injury in Football. Int. J. Environ. Res. Public Health 18, 7827.
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