Because a physically active lifestyle is important for personal physiological and psychological well-being (31), the growing proportion of people participating in exercise and sports can be seen as a positive tendency. Unfortunately, this physically active lifestyle leads to an increased risk of exercise-related injuries (3). One of the most common sites of pain and dysfunction due to exercise-related activities seems to be the tibial area (15,25). Therefore, exertional medial tibial pain (EMTP) can be seen as an important injury group of which understanding of the injury–causation relation should be of primary concern especially in female athletes because they suffer from higher rates of EMTP compared with their male counterparts (26,41). EMTP can be described as a subgroup of chronic lower leg injuries and embodies a medial overuse complaint in the part distal to the knee and proximal to the foot, caused by exertion (8,35). The diagnostic entities of medial tibial stress syndrome, tibial stress fracture, chronic exertional compartment syndrome, and muscular and tendon injuries can be categorized under EMTP (8,37).
Injury risk assessment can be complex because there is often more than one risk factor leading to an injury (22). The multifactorial model described by Meeuwisse et al. (28) demonstrates that the combination of several risk factors makes an athlete or a physically active subject predisposed to injury. In the literature, several ipsilateral risk factors for EMTP have already been described. It has been stated that not only local (31) but also distal (29,41) and proximal factors (32,40) farther down and/or up the kinetic chain can contribute to the development of injuries (10). In case of EMTP, local factors include the site of the lower leg and the surrounding tissues, distal factors embody the contribution of the ankle and the foot complex, and proximal factors contain the area of the upper leg, the hip, the pelvis, and the trunk. Concerning the distal risk factors for EMTP, pronation has been investigated in several ways (29,41) and increased foot pronation seems one of the most consistently found risk factors for medial tibial stress syndrome (6,10,29,41,46). Additionally, proximal risk factors such as altered hip range of motion (ROM) (8,29,45), hip muscle weakness (40), and impaired proximal joint stability (39) have also been described as contributing factors for the development of EMTP. This impaired proximal joint stability has been found to be a risk factor for EMTP because this instability has been described to result in transverse plane accessory movements (39). Because a lot of human movements like running and vertical jumping take place in the sagittal plane (23) and because the highest incidence of EMTP occurs in running and jumping sports (15), it was speculated that impaired ability to control accessory movements in the frontal and/or transverse planes might play a role in the development of EMTP.
However, the local, proximal, and distal risk factors described so far concern ipsilateral features contributing to the injury mechanism of EMTP. It can be speculated that not only ipsilateral (4,20,34) but also contralateral factors may be present in subjects developing injuries, on the basis of the described link between contralateral hip muscle weakness and altered kinetic chain function (9,42). The link between factors on the contralateral side of the kinetic chain and the injury mechanism itself is a commonly suggested clinical mindset. However, the acceptance of such clinical approach has come without a firm body of scientific evidence, especially in case of EMTP. It can be concluded that there is a significant lack of studies looking into the possible role of these contralateral factors in the kinetic chain of subjects who develop EMTP.
Therefore, the aim of this study was to prospectively detect contralateral risk factors in subjects who developed EMTP. In this study, contralateral factors were defined as parameters in the lower extremity (LE) that remained uninjured in subjects who developed EMTP. Therefore, we hypothesized that in a subject predisposed to EMTP, differences in the lumbopelvic–hip function of the contralateral leg might be observed when compared with either leg of a subject included in the control group. More specifically, decreased contralateral isokinetic hip muscle strength and impaired contralateral dynamic joint stability (DJS) parameters were hypothesized to be present in subjects who developed EMTP. DJS may operationally be defined as the ability of the joint to maintain position or intended trajectory (5,19,27,47).
MATERIALS AND METHODS
Eighty-one female subjects, who were freshmen in 2010–2011 or 2011–2012 in Physical Education at Ghent University, Ghent University College, and Ghent Artevelde University College in Belgium, were evaluated. The ethical committee of Ghent University Hospital approved the study, and all subjects gave a written informed consent. The exclusion criteria for participating in the study were the following: 1) pain, ache, or soreness in the LE or trunk within the previous year; 2) surgery of the LE or trunk; and 3) neurological problems (resulting in numbness and tingling in the lower extremities) that would affect LE function (7,41).
At the beginning of the academic year, hip muscle strength (11) and full-body kinematic parameters during a single-leg drop jump (SLDJ) were evaluated. During their education, the participants followed a similar sports program under similar environmental conditions for 29 wk per academic year (one introductory course, 24 wk of teaching activities, 2 wk of independent practicing, and 2 wk of sports tests) (17). Freshmen in 2010–2011 were followed throughout two periods of 29 wk of sports education, and freshmen in 2011–2012 were followed throughout one period of 29 wk of sports education. The average weekly sports program is shown in Table 1.
In addition to the basic sports program, the amount of extramural (physical activities beyond sports lessons at school) and unsupervised practice activities were also registered. The athletes were asked to retrospectively (every 3 months) report their average weekly sports participation (basic, extramural, and practice hours). For every subject, this individual amount of sports participation was then used as time at risk during further analysis.
After the follow-up period, students were divided into two groups: a group of subjects who developed EMTP and a control group who did not have any LE overuse injury during this study. Of the 81 subjects, seven developed other LE injuries and were excluded from the comparison. Furthermore, nine of the 21 EMTP subjects had bilateral complaints and were excluded because parameters of the uninjured leg of those in the EMTP group were evaluated in this study. Finally, one subject who developed EMTP had to be excluded because of missing data. In total, 64 subjects were taken into account for statistical analysis, 11 of which were included in the EMTP group and 53 were included in the control group. The uninjured leg of the subjects who developed EMTP was used in the statistical analysis. Those 11 injured legs were matched with the legs of those in the control group. The number of nondominant/dominant legs in the injured group was matched with the number of nondominant/dominant legs in the control group by creating a similar ratio among the 53 participants in the control group; hence, leg dominance would not play a role in the statistical outcome (Fig. 1). Therefore, the uninjured leg of the injured subjects and one leg per participant in the control group were eliminated at random until the number of nondominant/dominant legs in the EMTP group was also present in the control group (Fig. 1).
In this study, both contralateral hip muscle strength and full-body kinematics of SLDJ were evaluated at the beginning of the academic year. For practical convenience and to exclude fatigue effects, the isokinetic test and the SLDJ were assessed with a minimal rest of 1 wk in between. Before the testing procedure, weight, height, and leg dominance were determined. The determination of leg dominance was operationally defined as the leg preferred to kick a ball.
For the isokinetic hip muscle strength protocol, concentric torque of the hip abductors, adductors, internal rotators, and external rotators as well as eccentric torque of the abductors and external rotators were assessed using an isokinetic dynamometer (Biodex System 4) in accordance with the protocol used in Verrelst et al. (40). All tests were performed at 60°·s−1 and contained two series of five repetitions. The order of strength testing series for all subjects was 1) concentric abduction–concentric adduction, 2) concentric abduction–eccentric abduction, 3) concentric external rotation–concentric internal rotation, and 4) concentric external rotation–eccentric external rotation. This order was chosen for experimental convenience and was assumed to have negligible effects on test–retest reliability (11).
Before the actual testing, participants completed a 5-min warm-up on a cycle ergometer and were provided with detailed instructions for the strength procedure. This warm-up and explanation phase was followed by a familiarization procedure for every set of strength testing. Every test contained two series of five repetitions: practice and actual tests. The first of those two series consisted of submaximal practice repetitions; hence, the second and actual maximal testing series could be performed smoothly. Between the practice and the actual testing series, there was a 1-min rest interval. Between the different sets of strength testing, however, there was a 3-min rest interval.
For the abduction–adduction test, the participant assumed a functional standing position on the untested leg and performed the testing as described by Claiborne et al. (11). The abduction–adduction ROM was set from 10° of hip adduction to 30° of hip abduction. The participants were instructed to keep their toes pointed forward and their knee extended to prevent alterations in muscle recruitment and compensation during testing. The hip external–internal rotation test was performed with the participants seated and the hip and knee flexed to 90°. The ROM was set from 10° internal rotation to 10° external rotation (11). Verbal encouragement was provided during all tests. Peak torque–to–body weight (PT/BW) ratio, total work (TW), and average power (AP) were used for statistical procedures in this study. Claiborne et al. (11) found moderate-to-high reliability (intraclass correlation range, 0.62–0.89; SEM range, 7.80–24.11 N·m) for all the movements exhibited in this hip muscle strengthening protocol.
Besides the hip muscle testing, an SLDJ was also included in the screening protocol. This SLDJ was believed to be advantageous as an evaluation tool because 1) numerous physical activities and athletic events require brief but dynamic moments of single-leg stance (13) and 2) a single-leg task is adequate to evaluate LE movement control in the transverse and frontal planes (14,24).
For the SLDJ testing protocol, three-dimensional kinematic data were collected using six Oqus cameras and the Qualisys Track Manager software (Qualisys AB, Göteborg, Sweden). The ground reaction force data were recorded by a 1-m force plate (AMTI©, Watertown, MA) mounted flush in the middle of the wooden running track on which the SLDJ was performed. Ground reaction force and kinematic data were collected synchronously at 1000 and 200 Hz, respectively (14).
Before the actual jump testing, the following three procedures took place: 1) marker placement, 2) standing calibration trial, and 3) warm-up procedure. Marker placement was based on the Liverpool John Moores University lower limb trunk model (38).
After the standing calibration trial, a functional warm-up procedure protocol of 5 min of cycling on a cycle ergometer and 20 submaximal single-leg jumps was performed. Thereafter, the investigator demonstrated the actual SLDJ and the subjects performed two practice trials per side so they would feel comfortable to complete the task. Participants were asked to stand on top of a box 30.5 cm high with both feet in a natural position. To standardize the execution of the SLDJ, all participants got exactly the same instruction; they were instructed, “stand on your right/left leg and drop off the block, land on your left/right leg on the force plate and immediately jump as high as possible.” The SLDJ was executed three times per side. The height of the box was similar to the heights used in recent literature (18,21,30) and to the heights used in common rehabilitation and training exercises. Further analysis of kinematic data was done using the Visual3D software (C-Motion Inc., Germantown, MD). Raw marker positioning was low-pass filtered at 15 Hz with a second-order, bidirectional Butterworth filter with padded end point extrapolation. All joint angles were calculated in reference to the proximal segments, except for the pelvis (43) and thorax segments, which were both referenced to the laboratory coordination system (Table 2).
An X–Y–Z Euler rotation sequence was used (44); rotations around the X-axis can be seen as sagittal plane movements, rotations around the Y-axis, as frontal plane movements, and rotations around the Z-axis, as transverse plane movements.
The SLDJ was divided into two phases: touchdown (TD) until maximal knee flexion (MKF) and then MKF until take-off (TO), representing landing and push-off phases, respectively. The kinematic variables of interest were contralateral DJS parameters of foot, knee, hip, pelvis, and thorax segments during TD-MKF and during MKF-TO. More specifically, ROM values were measured as the difference between maximum and minimum peak values in the frontal and transverse planes during the SLDJ. An increased value of this ROM was described as impaired ability to maintain DJS in this study (33). The kinematic parameters analyzed in this study have already been found to exhibit good-to-excellent reliability (intraclass correlation range, 0.65–0.96) (16,39).
Injury registration and diagnostic criteria.
The injury registration method and diagnostic criteria are very important in the injury recording. Therefore, a multilevel registration method and accurate diagnostic criteria were used.
As the primary registration method, online questionnaires were administered weekly to assess injury follow-up. In this online survey, the students could register their injuries, the localization and other features of which could be specified. When subjects presented with a medial overuse complaint in the part distal to the knee and proximal to the foot caused by exertion (8,35), further diagnosis of EMTP was performed by the medical doctor. More specifically, one or more of the following criteria had to be present in an EMTP subject: 1) an atraumatic occurrence for at least 1 wk of medial lower leg pain exacerbated by running, 2) the presence of focal or diffuse palpation tenderness at the distal two-thirds of the posteromedial tibial border, and 3) pain, ache, or soreness in the medial lower leg with possible functional limitation during physical activity (2,8). Every 3 months, participant interviews were conducted in person in a quiet and private surrounding to check the compliance of the injury registration and to confirm for the occurrence of EMTP if needed.
Cox regression analysis (enter method) was performed to look for significant predictors for the development of EMTP. Variables with P < 0.05 in the Cox regression analysis were seen as significant predictors for EMTP. This approach has been chosen because this method can adjust the fact that the amount of sports participation can vary between subjects and assumes that risk factors affect injury proportionally across time (41). The time of sports exposure was measured from the start of the follow-up period until the injury or the end of the follow-up period for students who were not injured or who dropped out of the education program. In case of dropout, the date of the dropout was taken into account for the individual time of exposure. Statistical analysis for this study was performed using SPSS (version 21.0).
Twenty-one (26%) of the 81 subjects developed EMTP during injury follow-up. Nine were excluded because of bilateral complaints, and one was excluded because of missing data (Fig. 2).
A total of 53 (65%) subjects did not sustain any overuse injury in the LE and were used as a control group. During the study follow-up, two students dropped out of the education program, none of whom presented with EMTP. Anthropometric data on the subjects are listed in Table 3.
Cox regression analysis did not reveal significant contralateral hip muscle strength parameters associated with the development of EMTP. Descriptive statistics and results for the Cox regression analysis are presented in Table 4.
Table 4 presents the kinematic parameters of thorax, pelvis, hip, knee, and ankle. Only an increased dynamic ROM of the contralateral knee in the transverse plane during the landing phase of the SLDJ was found to be a risk factor for EMTP. The Cox regression analysis revealed that the hazard of developing EMTP at any time increases with (1.261 − 1 = 0.261) 26% if the accessory movements of the contralateral lower leg segment in the transverse plane increases with 1° during landing phase. The calculation of Cohen d revealed a large effect size (>0.8) (12) for decreased contralateral transverse ROM of the pelvis during the push-off phase.
The results of this study revealed altered lower leg segment kinematics on the contralateral side of the kinetic chain in the EMTP group. More specifically, increased transverse accessory movement of the contralateral lower leg segment contributed to the development of EMTP, which suggests that this factor may contribute to altered movement patterns within the kinetic chain function of the subjects who developed EMTP. The results of this study did not detect contralateral hip muscle weakness as a risk factor for EMTP, as was hypothesized.
Contralateral kinematic parameters.
The hypothesis concerning the contralateral kinematic parameters was only partially confirmed by the results of this study because only increased transverse accessory movement of the contralateral lower leg segment was found to be a risk factor for the development of EMTP. Therefore, any interpretations about this contralateral instability in relation to EMTP should be made with caution. However, it might be speculated that this factor contributed to altered movement patterns within the kinetic chain function of the subjects who developed EMTP, possibly affecting the degree of loading on the medial lower leg (8).
Calculation of Cohen d coefficient revealed a large effect size (>0.8) (12) for decreased contralateral transverse ROM of the pelvis during the push-off phase. One could say that this decreased contralateral ROM of the pelvis might be a compensation strategy, an excessive attempt to stabilize from the proximal to the distal area to counter the distally increased ROM of the contralateral lower leg. However, this parameter has no predictive value for the development of EMTP, so far-reaching theories concerning this parameter should be made with caution.
Previous research has already described that increased transverse dynamic ROM of the thorax and hip can be seen as risk factors for EMTP (39). The comparison of these results with those of the present study can give interesting insights on the kinetic chain function of the EMTP group. It can generally be stated that the EMTP group is more likely to display instability factors compared with the uninjured group, so it might be suggested that within a subject predisposed to EMTP, you might find instability parameters during the SLDJ. Moreover, you might be able to predict that the side in which proximal accessory movement or dynamic instability is present during the SLDJ will get injured, whereas the side in which accessory movement or dynamic instability of the lower leg segment is present during the SLDJ will not get injured. Proximal accessory movement may lead to altered proximal-to-distal movement patterns and, therefore, increased eccentric traction of the lower leg musculature in an attempt to control the motion (41). Consequently, traction on the deep crural posterior fascia (36) will be increased and EMTP may occur. Because proximal risk factors are found to be distant from the site of injury, the lever arm of an altered proximal-to-distal movement pattern may be more decisive in the aforementioned traction theory than the lever arm of accessory movement of the lower leg segment, as found in the contralateral leg in EMTP-predisposed subjects of this study.
Contralateral hip muscle strength parameters.
The hypothesis concerning the contralateral hip muscle strength was not confirmed through the results of this study because no contralateral hip muscle weakness was found in the subjects who developed EMTP. We hypothesized that contralateral hip muscle weakness could play a role in altered kinetic chain function in subjects who developed EMTP; however, this was not confirmed. These results are in contrast to the described link between contralateral hip muscles and the pelvis, in which it was stated that hip abductor weakness can lead to pelvic drop in the contralateral leg (9), which can affect contralateral LE function. Because altered kinematics has been described to play a role in LE injuries (10), it has been hypothesized that these contralateral hip muscles should be addressed in an attempt to decrease the risk of LE injuries. However, no contributing contralateral hip muscle strength factors were found to be of importance in the development of EMTP, according to the results in this study. These results might reflect that the contralateral hip muscles do not play a role in the development of EMTP or that the hip strength parameters captured in this study are not the most accurate screening parameters to analyze the contribution of the contralateral hip muscles in the development of LE injury. However, it might be interesting for further research to analyze other features besides analytic strength parameters of the contralateral hip muscles as included in this study, e.g., endurance or neuromuscular activity during functional movement tasks, to get better insight in the role of these contralateral hip muscles in the etiopathogenesis of LE injury. Besides evaluating the hip abductors, adductors, and rotator muscles, other muscles of the lumbopelvic–hip muscle complex should also be included in future research.
A limitation in this study was the large amount of independent variables included in the Cox regression analysis, which led to an increased possibility of type I error. Nevertheless, as recommended by Altman et al. (1), unadjusted P values were reported because a Bonferroni correction was not applicable because all the variables evaluated in this study were strongly correlated. Additionally, the rather small number (n = 11) of subjects who developed EMTP and completed the final analysis can also be seen as a limitation. Another important limitation was the exclusion of male students in this study, so these results cannot be extrapolated to a male physical education student population. However, the increased incidence of EMTP in females (26) makes studies concerning risk factors for EMTP in female populations a primary concern.
The clinical acceptance of the role of contralateral factors in the development of LE injuries has come without a firm body of scientific evidence. In this study, impaired DJS or accessory movements were found in the transverse plane of the contralateral lower leg segment of those in the EMTP group. It might be speculated that this factor contributed to altered movement patterns within the kinetic chain function of the subjects who developed EMTP, possibly affecting the degree of loading on the medial lower leg (8). However, any interpretation on this contralateral instability in relation to EMTP should be made with caution because this was the only contralateral risk factor detected. No decreased contralateral isokinetic hip muscle strength was found to be a significant predictor for EMTP in this study.
The authors would like to thank Tanneke Palmans and Fabienne van de Steene for analyzing the data and Dr. Vanden Bossche and Dr. Steyaert for their assistance in data collection regarding the injuries.
This research was funded by BOF—UGent 05V00910.
The authors report no conflicts of interest.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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