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Factors Contributing to Medial Tibial Stress Syndrome in Runners: A Prospective Study

BECKER, JAMES1,2; NAKAJIMA, MIMI1; WU, WILL F. W.1

Author Information
Medicine & Science in Sports & Exercise: October 2018 - Volume 50 - Issue 10 - p 2092-2100
doi: 10.1249/MSS.0000000000001674
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Abstract

It is estimated that there are over 29 million individuals in the United States who run 50 or more days per year (1). Unfortunately, epidemiologic data suggest that between 19% and 79% of these individuals experience an overuse injury in any given year (2). Of these injuries, medial tibial stress syndrome (MTSS) is one of the most common, accounting for up to 30% of all running related injuries (3). However, despite the prevalence of the injury, biomechanical risk factors for developing MTSS are not well understood.

Some authors have reported reduced passive hip internal rotation range of motion (4) or limited passive ankle dorsiflexion range of motion (5) in individuals with MTSS. Yet, other authors have reported no differences in hip (6,7) or ankle (6,8,9) range of motion, or even increased ankle range of motion in injured individuals (10). Similarly, some authors have reported weaker hip abductor muscles in individuals with MTSS compared with controls (11), while others have reported no differences in hip muscle strength (6,12). Plantar pressure distributions also present conflicting evidence. Some authors have reported higher pressures under the medial aspect of the foot than the lateral while walking or running in individuals who developed MTSS (13,14). However, others have found that mediolateral pressure ratios did not predict development of MTSS (15). In terms of kinematic factors, several studies have reported increased rearfoot eversion while running in individuals with MTSS compared with healthy controls (14,16,17), whereas others have reported no differences in the amount or velocity of rearfoot eversion (18,19). Finally, while proximal kinematics at the hip and pelvis are important for other running related injuries (20,21), to date studies on MTSS have focused primarily on the knee or ankle. Thus, it is unknown whether runners who develop MTSS display altered kinematics at the hip or pelvis level.

In relation to foot kinematics in particular, excessive amounts or velocities of rearfoot eversion have been suggested as factors contributing to MTSS (14,16). However, an alternative hypothesis suggests that it is not the amount or velocity of eversion which is the issue, rather the duration during stance phase over which the foot remains in an everted position (22). During the push off portion of stance, the function of the foot is to be a rigid lever thereby efficiently transmitting force (23). However, if the foot is still in an everted position during push off then the bony anatomy remains in a flexible configuration, thus requiring higher forces from the intrinsic and extrinsic foot musculature to both stabilize the foot and generate a sufficient push off (22). When repeated over and over again on each step these higher forces may contribute to the development of MTSS. In support of this hypothesis, a recent retrospective study reported that runners currently symptomatic with MTSS demonstrated longer durations of eversion but not increased amounts or velocities of eversion (18). However, apart from this single study, as it relates to MTSS, the hypothesis of prolonged eversion has been minimally investigated.

Ideally, studies examining factors related to the development of common running injuries should be prospective in nature. While many of the studies examining passive range of motion (6–9,14,19), strength (6,11,24), plantar pressures (13–15), or running kinematics (14) in individuals with MTSS were prospective studies, they typically focused on only one (i.e., passive range of motion or plantar pressures) or two (i.e., passive range of motion and plantar pressure or passive range of motion and kinematics) factors. However, the etiology of MTSS is multifactorial and may involve passive range of motion, strength, plantar pressure distributions, and kinematic factors. As such, studies on MTSS should quantify metrics in all of these areas. Therefore, the purpose of this study was to prospectively compare passive range of motion, strength, mediolateral plantar pressure balances, and kinematics between runners who developed MTSS and those who did not. Second, we sought to determine whether any range of motion, strength, mediolateral plantar pressure balance, or kinematic measures predicted which runners developed MTSS. We hypothesized that compared with controls, runners who developed MTSS would have limited passive range of motion at the hip and ankle during a clinical examination, would have weaker hip muscles, would have more pressure under the medial aspect of their feet than the lateral, and would have altered hip and rearfoot kinematics while running. Specifically, we hypothesized that runners who developed MTSS would demonstrate greater amounts of hip adduction and internal rotation, and similar magnitudes but longer durations of rearfoot eversion during stance. Finally, we hypothesized that duration of rearfoot eversion would predict which runners developed MTSS.

METHODS

Participants

Twenty-four runners participated in this prospective study. All participants were members of an National Collegiate Athletic Association Division 1 cross country team and testing was performed as part of their regular preseason preparticipation screening. All participants were currently free of musculoskeletal injuries at the time of testing and, given that all participants were members of the same team, their weekly training volumes and intensities were relatively similar throughout the year. Participants were followed up for 2 yr during which time any injuries were evaluated and treated by the certified athletic trainer assigned to the team. At the end of 2 yr, the team’s athletic trainer identified athletes who developed MTSS during the 2-yr period. This resulted in the formation of two groups: those who developed MTSS (INJ) and those who did not develop MTSS (CON). The occurrence of any injuries aside from MTSS was also noted. Prior to participation all participants provided written informed consent and the procedures of this study were approved by the university institutional review board.

Experimental protocol

Passive range of motion for the hamstrings, iliotibial band, hip flexors, hip rotators, and ankle plantar flexors were measured using established protocols (25). All measurements were performed by either one of the authors (MN) or a member of the sports medicine staff assisting with the evaluations, both of whom are certified athletic trainers with extensive experience performing these measurements. Hamstring flexibility was evaluated using a straight leg raise test. Lying supine participants performed a straight leg raise until end range of motion was reached. A goniometer recorded the angle between the thigh and horizontal. Reported interrater and intrarater reliability measures for this test are 0.93 and 0.91, respectively (26). Flexibility of the iliotibial band and hip flexors were evaluated using Ober and modified Thomas tests, respectively. A bubble inclinometer was zeroed at the starting position of each test and then used to quantify the range of motion of the thigh segment during each test. Interrater and intrarater reliability of the modified Thomas test have been reported as 0.89 and 0.92, respectively (27). Interrater reliability of the Ober test ranges from 0.59 to 0.97 while intrarater reliability has been reported as 0.91 (25). Hip internal and external rotation range of motion was measured with the patient lying prone with the legs together and knees flexed ninety degrees. Participants either internally or externally rotated their hip and range of motion relative to the vertical starting position was recorded. During these measurements, a second examiner placed pressure on the pelvis to ensure it remained fixed on the examination table. These measurements have interrater and intrarater reliability values of 0.94 and 0.9, and 0.88 and 0.9 for internal and external rotation, respectively (25,26). Ankle dorsiflexion was measured while participants lay prone with feet off the edge of the examination table and actively dorsiflexing the ankle. A goniometer was used to measure the angle between the tibia and foot. The goniometer was centered over the lateral malleolus with the stationary arm aligned with the long axis of the shank and the moving arm aligned parallel to the long axis of the fifth metatarsal (25). This measurement has been reported to have interrater and intrarater reliabilities ranging from 0.5 to 0.7 and 0.89, respectively (25). For all tests, three measurements were made, and the average of the three was used in the data analysis.

Isometric strength for the hip abductors, extensors, and external and internal rotators was evaluated using a hand held dynamometer (Hoggan Scientific, Salt Lake City, UT). Previous authors have shown that handheld dynamometry can have high interrater and intrarater reliabilities with experienced examiners (28). Thus, all dynamometer tests were performed by either by one of the authors (M.N.) or by a member of the sports medicine staff assisting with the evaluations, both of whom are certified athletic trainers with extensive experience performing these evaluations. All muscle strength tests were performed following methodologies described by Starkey et al. (25). Hip abduction strength was measured with the patient lying on their side with the hip abducted in the middle of the range of motion and dynamometer placed immediately proximal to the lateral femoral epicondyle. Hip extensor strength was measured with the patient lying prone, with the dynamometer placed on the posterior aspect of the femur immediately proximal to the popliteal fossa. Internal and external hip rotator strengths were measured with the participant seated with knees flexed ninety degrees over the edge of the table. The dynamometer was placed immediately proximal to the medial and lateral malleoli for external and internal rotators, respectively. For each test, participants completed three repetitions of maximal effort, with each repetition held for 3 s and followed by 5 s of rest. This work to rest ratio was specifically taken from previous studies on associations between hip strength and running mechanics (29). However, the short rest may not have allowed for complete recovery between repetitions. Peak force produced on each repetition was recorded by the dynamometer, forces were expressed as newtons per kilogram body mass, and the average of the three trials was used for subsequent data analysis.

Next, to quantify mediolateral pressure distribution ratios under the foot, plantar pressure measurements were recorded while participants walked across a plantar pressure mat (Tekscan, Inc., Boston, MA) at self-selected speeds. The pressure mat was 43.6 by 36.9 cm with a sensor resolution of 1.4 sensors per square centimeter. Pressure data were sampled at 400 Hz. Participants completed three trials with both left and right feet. A trial was deemed acceptable when the participant’s foot landed in the middle of the mat with no visible evidence the participant altered their gait to target the mat.

Finally, participants completed a three-dimensional running gait analysis. Lower-extremity kinematics were recorded using a 12-camera motion capture system (Qualisys AB, Goteborg, Sweden) sampling at 200 Hz while participants ran on a treadmill (model S-Trc, Star Trac, Vancouver, WA). Reflective markers were placed bilaterally on the following bony landmarks: anterior superior iliac spine, posterior superior iliac spine, medial and lateral femoral epicondyles, medial and lateral malleoli, and head of the second metatarsal. Additional tracking markers were placed on the lateral aspect of the thigh, medial aspect of the shank, two along the vertical bisection of the posterior shoe heel counter, and one along the lateral aspect of the shoe heel counter. A ruler was used to ensure the two vertical markers on the heel counter were oriented vertically while the lower heel counter marker and the lateral heel counter marker were collinear. A static calibration trial was collected after which the markers on the medial femoral epicondyles and malleoli were removed. Participants were allowed 5 min to warm-up on the treadmill after which they ran for 10 min at self-selected speeds approximating their easy training run pace. Kinematic data were recorded during the last minute of the 10-min trial.

Data analysis

Matscan Research software (Tekscan, Inc., Boston, MA) was used to analyze plantar pressure data. For each trial twelve regions of the foot were defined: medial heel (MH), lateral heel (LH), midfoot (MF), each of the metatarsals (M1–M5), hallux (T1), second toe (T2), third toe (T3), and fourth/fifth toe (T45). The software defines each region using specific percentages of foot length and width. A mediolateral pressure ratio was then calculated (14,19):

Using this notation a negative value represents more lateral pressure while a positive value represents more medial pressure. The mediolateral pressure ratio was specifically calculated at five distinct time points during stance phase as described by Willems et al. (14,19): first foot contact (the first frame in which the foot made contact with the pressure mat), first metatarsal contact (the first frame one of the metatarsals made contact with the pressure mat), forefoot flat (the first frame where pressure was registered under all five metatarsal areas), heel off (the first frame where there was no longer pressure registered in either heel region), and last foot contact (the last frame where pressure was registered under the metatarsals). Pressure ratios were calculated on each trial, and average values across the three trials were used for statistical analysis.

Raw marker trajectories from ten consecutive foot strikes were tracked using Qualisys Track Manager (Qualisys AB, Goteborg, Sweden) and exported to Visual3D (C-Motion, Inc. Rockville, MD) where they were filtered using a fourth order, zero lag, low pass Butterworth filter with a cutoff frequency of 8 Hz. Joint angles at the hip, knee, and ankle were computed using a flexion/extension, ab/adduction, internal/external rotation Cardan rotation sequence describing the orientation of the distal segment relative to the proximal segment. Segment angles for the pelvis were computed using a similar Cardan rotation sequence describing the orientation of the segment relative to the fixed laboratory coordinate system. Gait events were determined using the vertical velocity of the pelvis center of mass to identify foot contact (30) and the subsequent minimum knee flexion angle to determine toe off (31). Each stance phase was normalized to 100% using cubic spline interpolation. Dependent variables of interest were then calculated, including angle at foot contact and peak angle during stance for contralateral pelvic drop, hip adduction, hip internal rotation, and rearfoot eversion. Additional dependent variables included the percent of stance spent in rearfoot eversion, and peak rearfoot eversion velocity.

Statistical analysis

All range of motion, strength, plantar pressure, and kinematic measurements were recorded bilaterally. However, only values for the right legs were analyzed from the CON group while values from the leg which developed MTSS were analyzed for the INJ group. Independent observations t-tests were used to assess differences in passive range of motion, muscle strength, mediolateral pressure ratios, and kinematics between INJ and CON groups. Effect sizes (ES) based on mean differences and average standard deviations were calculated to aid in interpretation of results, with small, medium, and large effects being defined as 0.2 or under, 0.63, and 1.15 or greater, respectively (32). An alpha level of P < 0.05 was used to indicate statistically significant differences between groups.

All variables which resulted in a statistically significant difference between groups (P < 0.05) were considered for entry into a stepwise forward binary logistic regression to evaluate which variables were significant predictors of group membership. However, before performing the regression a bivariate correlation was conducted among all combinations of these variables. To avoid multicollinearity, when a variable demonstrated high (r > 0.6) correlations with several other variables, only the variable with the least number of strong correlations was retained for input into the regression model. At each step, an alpha of 0.05 and 0.1 were used for entry or removal from the regression model, respectively. All statistical analyses were conducted using Statistical Package for the Social Sciences version 23 (IBM Corp., Armonk NY).

RESULTS

Of the initial twenty four participants, seven (29%) developed MTSS (INJ group) during the 2-yr follow-up (Table 1). There were two participants in the INJ group who developed additional injuries besides MTSS, however these occurred almost five months after the MTSS diagnosis. In the remaining group of seventeen athletes, none developed MTSS. However, eleven of these runners sustained injuries other than MTSS. Some of these injuries were common overuse running injuries such as plantar fasciitis and Achilles tendinopathy while others were sudden onset acute injuries such as lateral ankle sprains or hamstring strains (Table 1). Since the acute injuries were likely not related to an individual’s running mechanics in the same manner as the overuse injuries we therefore included the six athletes who did not sustain any injuries and the five athletes who only sustained acute muscle strains or ankle sprains in the control group (CON).

TABLE 1
TABLE 1:
Total number of injuries recorded during the study and injury distribution by group.

There were no differences in age, height, mass, weekly mileage, or running speed during the gait analysis between INJ and CON groups (Table 2). The INJ group had tighter IT bands than the CON group but there were no differences between groups for hamstring, hip flexor, hip internal or external rotator, or ankle dorsiflexion range of motion (Table 3). Besides the IT band, all passive range of motion variables demonstrated small ES (Table 3). The INJ runners had weaker hip abductors than the CON runners (INJ: 16.03 ± 3.61 N·kg−1, CON: 23.04 ± 5.46 N·kg−1, P < 0.001, ES = 1.51; Fig. 1). However, there were no differences in strength for the hip external rotators (INJ, 15.40 ± 5.46 N·kg−1; CON, 14.93 ± 3.61 N·kg−1; P = 0.854; ES, 0.102), hip internal rotators (INJ, 15.53 ± 5.06 N·kg−1; CON, 14.17 ± 2.69 N·kg−1; P = 0.562; ES, 0.336), or hip extensors (INJ, 24.46 ± 9.44 N·kg−1; CON, 20.02 ± 7.07 N·kg−1; P = 0.345; ES, 0.533; Fig. 1).

TABLE 2
TABLE 2:
Descriptive statistics for the participants who did (INJ) and did not (CON) develop MTSS.
TABLE 3
TABLE 3:
Mean and standard deviations for clinical examination variables for the participants who did (INJ) and did not (CON) develop MTSS.
FIGURE 1
FIGURE 1:
Mean hip muscle strength values for the CON and INJ groups. *A statistically significant difference at the P < 0.05 level. Error bars represent standard error of the measurement.

Mediolateral pressure ratios were higher in the INJ group than the CON group at first foot contact, fore foot flat, and heel off, indicating the INJ group had more pressure under medial aspects of their foot at these time points (Table 4). There were no differences between groups in mediolateral pressure ratios at first metatarsal contact or last foot contact (Table 4).

TABLE 4
TABLE 4:
Mean and standard deviations for the mediolateral pressure ratios, joint angles at initial foot contact, and peak joint angles during stance for the participants who did (INJ) and did not (CON) develop MTSS.

Ensemble average curves for pelvis, hip, and rearfoot kinematics are shown in Figure 2. At initial foot contact there were no differences between CON and INJ groups for any of the kinematic variables (Table 4). Peak contralateral pelvic drop and peak rearfoot eversion during stance phase were greater in the INJ group than the CON group (Table 4). Duration of rearfoot eversion was also higher in the INJ group than the CON group. None of the other peak values were different between groups (Table 4).

FIGURE 2
FIGURE 2:
Ensemble average curves for frontal plane pelvis (A), frontal plane hip (B), transverse plane hip (C), and frontal plane rearfoot (D) kinematics. Gray line represents CON group while black line represents INJ group. Shaded error represents ± 1 SD from the CON group.

Bivariate correlations revealed that peak rearfoot eversion was highly correlated with duration of rearfoot eversion (r = −0.681, P < 0.001). Mediolateral pressure ratio at first foot contact was highly correlated with duration of rearfoot eversion (r = 0.737, P < 0.001), mediolateral pressure ratio at forefoot flat (r = 0.672, P = 0.013), and mediolateral pressure ratio at heel off (r = 0.632, P = 0.004). Therefore, only values from Ober’s test, hip abductor strength, mediolateral pressure ratio at heel off, peak contralateral pelvic drop, and duration of rearfoot eversion were considered for entry into the logistic regression. Correlation coefficients among all possible combinations of variables input into the logistic regression ranged from 0.046 to 0.528, with an average of 0.109. The overall model was significant (χ2 = 21.31, P < 0.001) and was able to correctly classify 91% of participants into CON or INJ groups. The model indicated that duration of rearfoot eversion was a significant predictor of group membership, and that every 1% stance increase in eversion duration increased the odds of being in the INJ group by 1.38 (P = 0.015, 95% confidence interval, 0.89–2.14). Values from Ober test (P = 0.153), hip abductor strength (P = 0.128), mediolateral pressure ratio at heel off (P = 0.143) and peak contralateral pelvic drop (P = 0.206) were not significant predictors of group membership.

DISCUSSION

The purpose of this study was to prospectively compare passive range of motion, strength, mediolateral plantar pressure balances, and kinematics between runners who develop MTSS and those who do not. Our main findings indicate that the development of MTSS was multifaceted involving range of motion, strength, mediolateral pressure balance, and running kinematics. Additionally, deficits related to MTSS development were observed both proximally (i.e., at pelvis and hip) and distally (i.e., foot and ankle). In partial support of our hypothesis, compared to the uninjured runners, those who developed MTSS demonstrated weaker hip abductor muscles, tighter iliotibial bands, and longer durations of rearfoot eversion during stance. However, contrary to our hypothesis, they also demonstrated higher peak rearfoot eversion and higher peak contralateral pelvic drop with no differences in hip adduction or internal rotation. The results also support our hypothesis that duration of rearfoot eversion would predict which individuals develop MTSS.

Although runners who developed MTSS demonstrated both greater amounts and durations of eversion, the results of the logistic regression showed that only the duration of eversion was a significant predictor of group membership. Similar results were observed in a retrospective study which found that duration of rearfoot eversion was the best predictor of individuals who were currently symptomatic with either Achilles tendinopathy or MTSS (18). However, that study also reported no differences in peak eversion between injured and noninjured runners. In contrast, previous retrospective (16,17) and prospective (14) studies have reported higher peak eversion in runners with MTSS compared to healthy controls. While these studies did not specifically quantify eversion duration, several did discuss how their observed differences in timing of peak eversion or inversion velocity could be indicative of prolonged eversion during stance phase (14,17,19). To date, only one previous prospective study has quantified eversion duration in runners who subsequently developed any running related injury (33). Although the authors did not find statistical differences in eversion duration between subsequently injured and uninjured runners, they did observe moderate ES. Based on these discrepancies, the authors suggest that additional prospective studies examining the role of eversion duration in development of running injuries, and MTSS in particular, are needed.

There, currently, is a disagreement in the literature regarding the etiology of MTSS, with two prevailing hypothesis. First, the injury may result from a traction induced periostitis due to muscle forces or forces transmitted through the deep crural fascia (34). Second, MTSS may represent a failure of bone remodeling similar to stress reactions or stress fractures (35). The higher amounts and longer durations of rearfoot eversion observed in the current study could support both hypotheses. Higher peak rearfoot eversion during stance phase, and corresponding lowering of the medial longitudinal arch, would increase strain in the flexor digitorum (34,36), tibialis posterior (37), and soleus muscles (34,38), the muscles most commonly cited as being involved in the development of MTSS. Since strain in the tibial fascia increases in a linear manner with strain in these muscles (34), greater peak rearfoot eversion could result in greater tensile forces transmitted to the tibial periosteum. Alternatively, the prolonged durations of rearfoot eversion mean that individuals are starting push off with the foot still being a relatively flexible structure instead of a rigid lever (22,23). It has been suggested that in this configuration, greater forces are required from both the intrinsic and extrinsic foot muscles to stabilize the foot during push off (22). Muscle forces are the main contributor to skeletal loading, especially when directed nonaxially and applied to the periphery of bones (39). Thus, higher muscle forces applied to the tibia would likely increase the bending stress on each step. When repeated over and over again, and without proper recovery, this may ultimately lead to a bony stress injury. However, at the current time these relationships are speculative. Future studies not only should seek to clarify the exact etiology of MTSS, but also independently evaluate the impact of both increased amounts and durations of rearfoot eversion.

In support of our hypothesis, the runners who developed MTSS demonstrated higher mediolateral pressure ratios than the uninjured runners at first foot contact, forefoot flat, and heel off. These higher pressure ratios indicate the injured runners had more pressure on the medial side of their feet than the lateral. However, interpretation of our mediolateral pressure ratios must be made with caution as they were recorded during walking not running. Although walking measurements have been used in previous studies on MTSS (13), it is unclear whether values obtained during walking reflect those which would be observed during running. That said, the mediolateral pressure ratio values observed in the current study are quite similar to those reported by Willems et al. (14) who, using a running analysis, also observed higher mediolateral pressure ratios at forefoot flat and heel off in individuals who subsequently developed exercise related lower leg pain. In agreement with the current study, Sharma et al. (13) also reported more pressure under the medial side of the foot during the first twenty percent of stance phase in military recruits who subsequently developed MTSS. Combined, these pressure ratios provide additional support for the kinematics observed in the current study. The higher mediolateral pressure ratios at foot contact may reflect a foot strike located more on the central aspect of the heel rather than the lateral aspect, a difference which may be present even in the absence of differences in rearfoot eversion angle at foot contact (14). As defined in the current study, forefoot flat occurs during midstance at approximately the same time as peak rearfoot eversion. Increased eversion would increase pressure under the medial aspect of the foot. Finally, it has been shown that runners with MTSS display a more everted position at heel off (18). A more everted position at heel off would result in more pressure under the medial aspect of the foot at this instant, as observed in the current study.

In addition to distal issues at the foot, runners who developed MTSS also showed proximal deficits at the hip. Compared with plantar pressures or foot kinematics, the role of muscle strength in the development of MTSS has received relatively little attention. Using an isokinetic dynamometer Verrelst and colleagues (11) observed that individuals who prospectively developed exercise related medial leg pain demonstrated weaker hip abductors than those who did not develop the injury. The hip abductor muscles play a critical role in stabilizing the hip and pelvis during stance. As such, weakness in these muscles can negatively influence kinematics throughout the entire kinetic chain. Although it has previously been suggested that weak hip abductors play a role in the development of knee injuries in runners (21), our results when combined with those of Verrelst et al. (11) suggest weak hip abductors may also play a role in the development of injuries distal to the knee. However, this cannot be concluded definitively as two other prospective studies examining the relationship between strength and development of MTSS reported no differences between injured and noninjured participants (6,24). Given the relatively small number of studies in this area it is suggested that future studies on MTSS consider incorporating measures of hip muscle strength.

Interestingly, the runners who developed MTSS demonstrated increased peak contralateral pelvic drop compared to controls while not showing differences in hip adduction. This suggests that while the pelvis segment dropped, the thigh segment actually abducted slightly so the hip adduction angle remained constant. This could partially be explained by the tighter iliotibial bands in the MTSS group as a tighter iliotibial band might result in a wider step width. It has been previously shown that increasing step width reduces peak hip adduction angles during stance (40). Therefore, if the participants who developed MTSS utilized an increased step width this would allow the hip angle to remain the same while also allowing increased contralateral pelvic drop. However, utilizing a wider step width also results in reduced peak rearfoot eversion during stance (40). However, in the current study, the MTSS group demonstrated greater peak rearfoot eversion, not less. Additionally, it has been reported that there is no correlation between iliotibial band tightness and peak hip adduction, at least in female runners currently or previously diagnosed with iliotibial band syndrome (20). Whether such relationships exist in runners who develop MTSS requires further investigation.

A final proximal factor which needs consideration is the lack of differences in hip rotation between the injured and control participants. Several studies have examined passive hip range of motion in individuals with MTSS, however the results are mixed. Moen et al. (4) reported that individuals who subsequently developed MTSS demonstrated reduced passive hip internal rotation range of motion compared to controls. In contrast, Yagi and colleagues (6) reported that runners who subsequently developed MTSS demonstrated increased passive hip internal rotation range of motion compared to controls. However, they also reported greater passive hip external rotation range of motion in individuals who subsequently developed MTSS, a difference not observed by Moen et al. (4). In context of these studies, the findings of the current study support the hypothesis that there are no differences in passive hip rotation range of motion; however, additional prospective studies using larger sample sizes are required to confirm this hypothesis.

There are a few limitations to the current study which must be considered when interpreting the results. First, this study had a small sample size consisting of competitive collegiate runners. Thus, these results may not generalize to the broader running community. Second, beyond recording weekly mileage, we did not track the training programs of the individuals participating in this study. As such we cannot separate out the role of possible training related errors in the development of MTSS. Given that the participants were all on the same team, it is likely their training was relatively homogenous. This might explain why, despite a small sample size, a relatively high number of participants developed MTSS (29%) in this study compared with previous studies (9,19), and also why a relatively high percentage of the participants (75%) developed any running related injury in the 2-yr follow-up. It is possible that some of the biomechanical differences observed between injured runners and controls are not problematic in and of themselves and only lead to injury when compounded with training related errors. Third, we did not analyze men and women separately. Given the small sample size of this study, splitting sexes in the analysis was not possible, but should certainly be considered in future studies on this topic. Fourth, we did not control the footwear the participants used, either in testing, or during the follow-up period. Although they all wore shoes from the same manufacturer, it is possible that differences in footwear may have been responsible for some of the observed biomechanical differences or for determining which runners developed injuries. Finally, given the small sample size, we did not exclude runners from our CON group if they developed acute injuries such as ankle sprains or muscle strains. This was done because the mechanisms of these injuries are likely different than those involved in common overuse running injuries. As a result, even though our INJ and CON groups differed on numerous variables, we cannot definitively say these variables are unique to MTSS and did not play a role in the development of other injuries as well.

In summary, the findings of this small prospective study on MTSS suggest that runners who develop MTSS demonstrate deficits in numerous areas including flexibility, muscle strength, foot function, and both proximal and distal kinematics. As such, it is recommended that coaches or sports medicine professionals consider broad preseason evaluations which assess these multiple areas. The exact role of each one area, how they interrelate, and whether any one is more important than others for injury development remains to be seen.

The authors would like to thank students enrolled in KIN 489G for their assistance in data collection.

No funding was received for this work. The authors have no conflicts of interest to declare. The results of this study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of the present study do not constitute endorsement by the American College of Sports Medicine.

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Keywords:

RUNNING INJURIES; PROLONGED PRONATION; HIP MUSCLE WEAKNESS; PLANTAR PRESSURE

Copyright © 2018 by the American College of Sports Medicine