Biomechanical predictors of retrospective tibial stress fractures in runners
Introduction
Stress fractures are a common problem among runners and may account for between 6% and 14% of running related injuries (James et al., 1978; McBryde, 1985;Taunton et al., 2002). The tibia is the bone most likely to be affected, accounting for between 35% and 56% of all stress fracture injuries (Matheson et al., 1987; Romani et al., 2002). In addition, stress fractures are reported to be more prevalent in females (Arendt et al., 2003; Taunton et al., 2002). It has been postulated that this may be due to females having a lower percentage lean body mass in the lower limb, a history of menstrual disturbance, a low fat diet and lower bone density compared to males (Bennell et al., 1999).
The etiology of stress fractures is believed to be multifactorial in nature arising from issues related to physiology, training, structure and diet (Bennell et al., 1999; Romani et al., 2002). Nevertheless, there are a number of recent studies linking retrospective tibial stress fractures (TSFs) with running mechanics. In a cross-sectional study, Milner et al. (2006b) found that the occurrence of TSF in female runners was related to greater initial loading of the lower extremity. Instantaneous and average vertical ground reaction force load rates along with vertical peak tibial acceleration were found to be significantly greater in the stress fracture group compared to healthy controls. The tibia is also likely to experience torsional loading during running. Indeed, it has been shown that runners with a history of TSF display both an increased adduction and absolute free moment (FM) in comparison to controls (Milner et al., 2006a).
Kinematic variables may also play a role in the development of stress fractures since they may alter the normal alignment of the lower extremity. For example, compared to controls, TSF runners have been shown to demonstrate increased peak hip adduction (HADD) and knee internal rotation (KIR) along with a reduction in peak knee adduction (KADD) (Milner et al., 2005). In addition, rearfoot eversion (RFEV) has been shown to be greater in athletes with running related lower leg pain (Messier and Pittala, 1988; Willems et al., 2006). Abnormal kinematics during running may alter the normal loading pattern placed on the tibia.
In summary, a number of kinetic and kinematic parameters have been related with retrospective TSF in female runners. However, it is unclear which of these factors are most strongly related to the injury. It is likely that a combination of these variables will better distinguish individuals with a history of TSF from healthy controls. Hence the aim of this study was to determine which kinematic and kinetic parameters were the best predictors of retrospective TSF in female distance runners. It was hypothesised that a subset containing both kinetic and kinematic parameters would significantly predict group membership.
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Subjects
All subjects gave written informed consent prior to the commencement of this study. Approval for all procedures was obtained from the Human Subjects Review Board at the University of Delaware before commencing the study. Sixty female runners who were between the ages of 18 and 45 years and rearfoot strikers participated in this project. Thirty subjects with a history of TSF were compared with 30 control subjects (CON) who had no previous lower extremity bony injuries. All subjects were
Results
High colinearity (r⩾0.65) was found between VILR and both VALR (r=0.936, P<0.001) and PPA (r=0.802, P<0.001). In addition, VALR was highly correlated to PPA (r=0.681, P<0.001). Therefore, it was decided to use VILR as the input variable for the regression calculations since it provided a good representation of the VALR and PPA. None of the other variables demonstrated colinearity.
The mean peak values of VILR, FM, HADD, KIR and RFEV were all greater in the TSF group compared to the controls (
Discussion
The aim of this investigation was to determine which kinematic and kinetic factors were the best predictors of a previous TSF in female runners. We found that increases in peak HADD, peak RFEV and absolute FM increased the likelihood that a subject had sustained a previous stress fracture.
Peak RFEV was found to be 2.7° greater in the TSF group compared to the controls. This value corresponds well with the differences of 2.6° and 1.9° reported between controls and runners with exercise related
Conflict of interest
All the authors confirm that there are no financial or personal relationships with other people or organisations that could inappropriately bias the content of this paper.
Acknowledgement
This study was supported by Department of Defense Grant DAMD17-00-1-0515. This support was financial in nature and no conflicts of interest were encountered.
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