The primary purpose of this study was to determine the relationship between select lower extremity kinematics and a history of MSP in the running athlete. Beyond the research regarding athletes with a diagnosis of tibial stress fracture18,19
no previous study has investigated whether or not such a relationship exists. First, it is necessary to recognize that MSP, much like other musculoskeletal dysfunctions, can have multiple and various etiologies. The inclusion/exclusion criteria used for the current study, as well as clinical screening, attempted to limit the variability of etiology. The majority of the subjects had complaints that were consistent with medial tibial stress syndrome but the authors did not use this term because there are no agreed diagnostic criteria for medial tibial stress syndrome.20
Still, it must be recognized that the subjects in the current study most likely were not a completely homogeneous group.
Differences between limbs: In the current study, the authors examined individuals with a history of unilateral shin pain in order to ascertain if there was a limb difference in runners with a history of MSP. These results are displayed in . Within the MSP group, none of the three variables that were measured were statistically different from the involved to the uninvolved limb. No differences were found between limbs in the control group ().
Symmetry of the lower extremity is expected in sports such as running that involves primarily sagittal plane and reciprocal movement between extremities. Shin pain is commonly bilateral and for this population with unilateral pain, limb asymmetry with regards to PT, hip IR, and knee flexion was not statistically different. Presently, to the authors' knowledge, no study has examined the same kinematic variables and differences between limbs in runners with a history of MSP. A study performed by Zifchock et al investigated side-to-side differences of kinetic and kinematic variable in overuse running injuries in twenty runners.21
These researchers found a significant difference in total passive IR motion between groups, with the injured runners having more total IR.21
Although, the measure was passive there is some suggestion that increased passive IR is associated with increased dynamic IR22
while others would state the contrary opinion, that static measures do not correlate with dynamic measures.23
Difference between groups: In comparing the MSP to the control group, statistically significant differences were observed for the variables PT, peak hip IR, and knee flexion (). The retrospective nature of the study design does not allow the authors to ascertain if the differences that were seen are due to the MSP or a result of the MSP. It can only be stated that there was a difference between groups.
In the current study, the subjects ran at an average speed of 3.08 meters/second, with the amount of PT being significantly greater (P = 0.002) for the involved limb of the MSP group (8.56 degrees) as compared to the average of 5.86 degrees for the control group. The IR value was less for the control group (6.25 degrees) and greater in the MSP group (11.48 degrees), a difference that was significantly different between groups (P = .004). The ROM values for IR varies in the literature from 8–10 degrees.24
Differences found in the current study compared to the Schache et al study are probably due to differences in running speed where runners in the current study averaged a slower running speed. Related to knee flexion, the MSP group averaged 37.11 degrees of flexion at foot contact whereas the control group had greater knee flexion at contact (42.12 degrees) and this difference was statistically different (P = 0.02). Differences between groups were relatively small: 2.70 degrees for PT, 5.23 degrees for peak hip IR, and 5.01 degrees for knee flexion. The moderate effect sizes associated with these differences support the results of the statistical analyzes. However, it is not clear if these values are clinically significant.
During running, each foot strikes the ground approximately 600 times per kilometer.25
With each heelstrike forces are transmitted from the foot up the lower extremity to the lumbar spine.26
Even minor malalignments and/or abnormal movement patterns can accumulate, resulting in an overuse injury.10,11,27
A greater amount of motion at the pelvis and hip would suggest a lack of stability in those joints in the selected planes of hypermobility. The authors theorize that excessive pelvic drop and hip rotation results in compensation distally that contributes to the development of MSP. The concept of proximal hip contributing to distal lower extremity pathology is further detailed in a conceptual model that is displayed in . Increased frontal plane PT has been described to create a knee valgus moment at the knee as a result of the body's center of mass shifting medially.28
This increased valgus moment at the knee may result in compensation distally of increased subtalar joint pronation.14
In addition the MSP group had less knee flexion during stance which may affect the load dissipation of the ground reaction forces. Knee flexion during the initial phases of running gait is a key component for shock absorption throughout the lower quarter, without which the shock will be attenuated through the tibia and/or soft tissue.
Conceptual model for proximal (hip) contributions to distal lower extremity pathology.
Although there are not many investigations available for comparison with the current results, previous investigators have theorized regarding the biomechanical variables assessed in the current study. Hip IR, knee abduction and rearfoot eversion has been found to be associated with injuries in runners.8,14,29
Increased levels of hip IR have been previously found in runners with tibial stress fractures.30
Although the subjects in the current study did not have tibial stress fracture, shin pain is a precursor for tibial stress fracture. Hip motion does influence the way the foot hits the ground, stressing either the soft tissue or bone in the lower limb. In the current study, the three kinematic variables were examined independently, however, it is likely that these factors are inter-related and may combine to increase injury risk.
Difference between sexes:
Women suffer disproportionately higher rates of exertional tibial pain in military32
and exercise related leg pain.5,10,31
Also, females demonstrate greater frontal and transverse plane motion than males during running.29,32,33
Specifically, females exhibit greater peak hip IR, hip adduction and greater peak knee valgus.
In this study, secondary analyses were performed between sexes within each group to establish if sex proved to be a distinguishing factor. Within the MSP group on the involved side (), PT was significantly greater in females as compared to males. Within the control group, there were no significantly different variables (in a randomly chosen limb) between sexes (). It is not clear how this variation may have affected the main findings of this study, but it appears that females may be at higher risk for MSP due to the tendency for greater pelvic and hip excursion during running. To further understand these differences, future studies should focus on one gender.
It is worth noting that all runners were able to perform the treadmill running without difficulty or reported symptoms. It could be argued that treadmill running does not simulate over-ground running, but the authors felt that for this study, treadmill running would allow the runners to achieve their normal running speed versus running on the short runway in the experimental lab. Additionally, the authors chose to have the subjects run at their self-selected speed to minimize kinematic variation in them running at a speed that was unnatural to them. Further, subjects wore their own shoes and orthotics to help minimize variation from their normal kinematics. The authors believe that using standardized shoes is artificial for the subject and may change their running pattern.
This study only investigated a few select variables that may distinguish between those with a history of MSP and those without. The authors acknowledge that they may excluded other variables that are important determinants of injury, such as muscle strength and delayed onset of muscle activation. Additionally, the data collected was retrospective; therefore it is impossible to discern if the potential differences between groups occurred prior to or after the onset of MSP.