The research embedded within this doctoral thesis focused on the intrinsic hamstring injury risk profile in male football players. In spite of the substantial amount of effort that is being invested in the implementation of best practices as concerns (secondary) prevention of hamstring injuries to date, these muscle injuries present the tendency to occur more frequently as the time goes on. Because of the high reoccurrence risk, the detrimental influence on sports performance and mental health, as well as the high financial costs involved, this work attempted to contribute to a better understanding of the complexity of the football related hamstring injury risk, and hence, enable reduction of occurrence.
The first chapter consists of both a retro- and a prospective study, investigating the role of metabolic muscle function and intermuscular interplay between the hamstring muscle bellies in the detection of (1) injury history and (2) future injury vulnerability. Both studies demonstrated that the quantitative cooperation features (spatial muscle fibre recruitment pattern) between the different hamstring muscle bellies, in particular the BF and ST, are of key importance as regards hamstring injury susceptibility. The risk of sustaining an injury demonstrated to be significantly higher, when the medially oriented Semitendinosus muscle (ST) was activated to a lesser extent during prone leg curls, as this most probably obliged the laterally oriented, and most frequently effected, long head of the Biceps Femoris muscle (BFLH) to make an additional (possible excessive) effort during these types of exercise. By analogy with what muscle architecture has us believe, the ST and BFLH demonstrate to have individually differing functional purposes, which appeared to increase the risk of injury when not respected accordingly.
In the second chapter, we adopted a slightly wider scope of interest in assessing the intermuscular interplay characteristics within the posterior sling muscle continuum (back muscle, gluteal muscles and hamstrings). In particular, we investigated whether the order of muscle activation during the Prone Hip Extension test (PHE), could be associated with the risk of sustaining a hamstring injury in conducting a prospective cohort study. We elected this exercise in the hamstring injury risk study
context, as we wanted to examine the influence of neuromuscular coordination properties both within (Chapter I) as well as between the hamstring muscle bellies and their most important synergists in the human body as regards gait and running: the posterior sling muscles (Chapter II). Previous studies suggested that a delayed onset of the gluteal muscles, relative to the activity onset of the back – and hamstring muscles, would be detrimental for lumbo-pelvic function and arthrokinematics of the lower back, which might possibly endanger this region of becoming subject to mechanical overload and associated degenerative changes and complaints. With respect to the hamstring injury risk in football, our study demonstrated that the hamstring muscles of the subjects that got injured during the follow-up period presented a significant delay in activity onset, being solicited after recruitment of the lower back muscles, instead of the other way around as seen in the healthy subjects of our cohort. These findings lead to the conclusion that the quality of intermuscular cooperation and in particular, the relative timing of muscle activity within the posterior sling continuum during respective Prone Hip Extension test, is an important feature to take along in assessing and addressing the hamstring injury risk. Similar to the functional imbalances between the BF and the ST found in Chapter I, the temporal delay in activity onset refers to deficits in neuromuscular guidance of the hamstring muscle unit, making it function less economic and less safe than it ought to do for efficient football performance. Indeed, as the efficacy of eccentric and concentric hamstring strength production during running and kicking activities in football, is highly dependent on (1) the timing of muscle activity and (2) the intermuscular task distribution, the study findings gathered in Chapters 1 and 2 are in accordance with previous findings and should be encountered for in performance enhancement, prevention and rehabilitation.
Both prospective studies embedded in the concluding chapter (III) underline the absolute necessity of adequate neuromuscular coordination and sufficient, optimally timed muscle activity with regards to sound hamstring functioning as well. Concretely, respective study results revealed that a football player runs a higher risk of getting injured in the hamstring area, when the core muscles present lower activity levels and when the pelvis and trunk display more inconsistency (more anterior tilting of the pelvis and side bending of the trunk) throughout the airborne phase of sprinting (front-and backswing phases). The latter indicates that, for the hamstrings to be sufficiently protected against overload and injury, not only the amount and (relative) timing of hamstring (belly-specific) activity is essential (Chapters I and II), but the conditional status and the functional performance capacity of the proximal lumbo-pelvic - or ‘core’ unit is absolutely vital as well.
In the end, this dissertation attempted to enrich the (understanding as regards the) hamstring injury risk profile in male football play by demonstrating that efficient and safe hamstring functioning is most probably dependent on the quality of neuromuscular control and guidance within (1) the hamstring muscle unit, (2) the synergistic entity of the posterior sling and (3) the proximal lumbo-pelvic - or ‘core’ unit. Only in adequately bundling their forces (intramuscularly as well as with their proximal team mates (core)), the hamstring muscles provide themselves with the opportunity to experience a successful season.