With countless different definitions of what constitutes “core stability”, there are many readers who may wonder whether there is truly a scientific evidence base for this (still) fashionable topic.
Fortunately, an interesting paper looking at core stability measures as risk factors for lower extremity injury in athletes, adds some much needed science to the discussion1. Here, the authors, Darin Leetun and colleagues, acknowledge that the closed kinetic chain nature of athletic activity means that it makes sense to consider the joint mechanics both above and below the site of any injury. The effect of foot mechanics on the rest of the body has been extensively studied, say the authors. However, “the influence of proximal stability on lower extremity structure and pathology remains largely unknown”.
Reviewing the literature briefly, the authors note that (arguably) Bouisset2 initially conceptualised the idea of stabilisation of the pelvis and trunk being necessary for all movement of the extremities, while Hodges and Richardson3 observed trunk muscle activity before lower extremity action. (NB: note also that in the 1980s, Anders Bergmark described a spinal model with joint stiffness involving 40 muscles.4 Yet, while the fitness industry seems to be obsessed with the abdominal muscles (in terms of what constitutes the “core”) it is noteworthy that Leetun et al comment here that “recent research demonstrates that the contribution of different muscle groups to lumbar spine stability depends on the direction and magnitude of trunk loading”. So, the abdominal muscles are important (in conjunction with the lumbar extensors) in spinal stability, but also in helping to control excessive anterior pelvic tilt. However, excessive pelvic tilt is believed to be coupled with femoral internal rotation and adduction, hence the role of muscles associated with these movements (and the contrary movements of hip abduction and external rotation) must be considered. Further, as has been noted by previous researchers, the quadratus lumborum cannot be ignored.
In terms of athletic performance, Leetun et al also observe that “hip muscle activation significantly affects the ability of the quadriceps and hamstrings to generate or resist forces experienced by the entire leg during jumping (and landing)”. This fact, and the observations of many clinicians, has led to the concept of “the position of no return” as far as knee injury (and in particular, injury to the anterior cruciate ligament) is concerned. This is hip adduction and internal rotation, leading to knee valgus and tibial external rotation and potential damage.
Research method
Bearing the above in mind and noting the commonly held belief that females are more at risk of lower extremity injury, the authors decided to undertake a prospective study over a period of two years, involving 139 initially injury-free athletes (basketball and cross-country running), both male and female.
Tests chosen to assess regularly each athlete’s core stability were: hip abduction isometric strength; hip external rotation isometric strength; a modified Biering-Sorensen test (posterior core strength); the side bridge test (for lateral core muscle capacity); and either a straight leg lowering test (year 1 of testing) or McGill’s flexor endurance test (year 2).
Throughout the study period, head athletic trainers were responsible for recording all back and lower extremity injuries. An injury was defined as an event that occurred during athletic participation and required professional treatment. Furthermore, it needed to result in at least one full day of missed practice or competition.
Results
Of the 139 subjects who completed the two years of this investigation, 41 (28 females and 13 males) sustained 48 back or lower extremity injuries during a single competitive season. 35% of the females sustained an injury compared with 22% of the males. There were two season-ending injuries (ACL tear, female; metatarsal stress fracture, male).
In terms of the core stability measures, the males demonstrated greater core stability measures than the females, regardless of sport. Significant differences were observed between males and females for hip abduction, external rotation and side bridge values. Average abdominal muscle performance was also slightly better for males than females.
With respect to injury, those athletes who experienced an injury over the course of the season generally demonstrated lower core stability measures than those who did not. Specifically, statistically significant strength differences were observed for hip abduction and hip external rotation and mean abdominal performance was lower. The authors note additionally that the one female athlete who experienced the ACL tear “demonstrated pre-season deficiencies in each core stability test”. However, when all the data were analysed, it would appear that the only true significant risk factor for injury was hip external rotation strength.
Discussion
This study, say the authors, “set out to examine prospectively differences in core stability measures between males and females and between those athletes who became injured and those who did not”.
In common with several other studies, females in this study had reduced side bridge endurance and reduced hip abduction and external rotation isometric strength. This finding, according to the authors, reduces the ability of females to stabilise the hip and trunk, making them more vulnerable to the forces generated during athletics.
Athletes who became injured had significantly less hip abduction and external rotation strength than those who did not. This finding is consistent with indications from previously published retrospective and cross-sectional studies. In addition, hip external rotation strength weakness most closely predicts injury status (over the course of one athletic season). This finding should perhaps be treated with caution. As the authors themselves comment, “hip external rotation strength is only one element of core stability and other elements not included in this study may add to the predictive value of the regression equation”.
In contrast to the findings of McGill – who studied trunk extension endurance and the prediction of back pain in the general population – trunk endurance measures appear not to be accurate predictors of injury among the athletes studied here. However, the endurance times of injured and uninjured subjects in this study are similar and this may indicate that issues of motor control and recruitment would need to be determined during high speed athletic activity to gain a more complete understanding of who gets injured, how and why.
Study limitations
The authors duly acknowledge the limitations of this study. Of particular concern is the lack of a dynamic assessment of muscle function during athletic activity. In turn, the relationship between static (clinical) tests of muscle force production and capacity and dynamic activity needs to be fully determined. Finally, the authors conclude that “the relationship between core strength and lower extremity mechanics needs to be examined”.
References
- Leetun DT et al, Core Stability Measures as Risk Factors for Lower Extremity Injury in Athletes, Med Sci Sports Exerc, Vol 30, No 6, pp926-934, 2004.
- Bouisset S, Relationship between postural support and intentional movement: biomechanical approach, Arch Int Physiol Biochem Biophys 99: pp79-92, 1999.
- Hodges PW and Richardson CA, Contraction of the abdominal muscles associated with movement of the lower limb, Phys Ther 77: pp132-144, 1997.
- Bergmark A, Mechanical stability of the human lumbar spine, Sweden: Lund University, Department of Solid Mechanics, 1987, doctoral dissertation.
