The role of acceleration related variables for hamstring muscle (re-) injury prevention in elite association football

Hamstring muscle injuries constitute a major concern in football and a major challenge for physiotherapists working in this sport, being an injury with long absence from playing and training. Although clinical strategies to rehabilitate these injuries and clinical injury risk assessments have been explored over the years, a broader comprehension of how variables regarding running performance may in some form relate to hamstring injury risk has been missing for clinical professionals. Together with this, its incidence has been increasing despite many preventive efforts, which reveals a necessity for developing risk assessment methods to better inform preventive strategies. The key involvement of hamstring muscles during accelerations and decelerations during football running actions justifies research into acceleration related observations. Therefore, the aims of the current programme of research were to develop new laboratorial and load monitoring strategies related to acceleration actions, by exploring biomechanical factors from a physiotherapist perspective. Additionally, implementing assessments and exposing some key limitations of these assessments in professional clubs is also described throughout the experimental studies of this thesis (chapters 3 and 5).

For the purposes of this thesis, force development variables were analysed in chapter 3, during maximal accelerations on a non-motorised treadmill, and comparisons between professional players with and without previous injuries were performed. Results from this study revealed no differences between groups during both maximal acceleration and steady state of a maximal sprint effort.

A second approach regarding risk analysis and acceleration variables considered the mechanical load based on trunk-mounted accelerometry used in outfield training, as detailed in chapter 5 of the present thesis. In this study mechanical load expressed by PlayerLoad^TM, an accelerometer-derived variable aimed to express the rate of change in acceleration, was collected for the training sessions during three weeks previous to a hamstring injury event, in English Premier League clubs, using matched healthy controls. Although the results did not show significant differences between experimental and control group, this exploratory method may constitute a promising method to assess hamstring injury risk.

Reliability and validity of the acceleration related variables were addressed first for each of the two experimental studies detailed in chapter 3 and 5. For this purpose, a pilot study on reliability of force collection using a non-motorised treadmill was performed to test the experimental protocol with results showing good overall reliability. For the PlayerLoad^TM, a laboratorial study detailed in chapter 4 using a laboratorial overground soccer simulation protocol was adopted and convergent validity with subjects´ anthropometrics together with reliability analysis of four isolated football actions (jogging, side cut, stride and sprint) was performed. Results of this study revealed no association between PlayerLoad^TM and the subjects height or body mass and also an overall good reliability for the four actions analysed.

In summary, the research presented in this thesis helped better understand the current value and limitations of screening and monitoring acceleration related variables in the context of hamstring (re-)injury prevention in professional football, introducing to the clinical field a different perspective of addressing hamstring behaviour during acceleration actions, and its hypothetical relation with hamstring injury.