HUBBARD, TRICIA J.1,2; CARPENTER, ERICA MULLIS1; CORDOVA, MITCHELL L.1,2
Medial tibial stress syndrome (MTSS) is a condition that plagues many athletes each year. Commonly called shin splints, MTSS was defined by the American Medical Association in 1966 as "pain or discomfort in the leg from repetitive running on hard surfaces or forcible excessive use of foot flexors" (19). Approximately 10-20% of all runners will experience a bout of MTSS during their career; further, MTSS accounts for nearly 60% of all overuse injuries seen in the leg (9). Although runners are typically affected the most by MTSS, athletes who participate in ballistic sports such as basketball, dance, and tennis are also at risk (12,19). Because running and ballistic activity are required for most athletic activity, there is large potential for many athletes to be affected by MTSS.
Although no true singular cause has been identified, MTSS is a condition that has various etiologies with both intrinsic and extrinsic mechanisms. Intrinsic mechanisms include altered biomechanics and/or anatomical alignment, decreased muscle strength, decreased flexibility, low bone mineral density, and hormonal imbalances (2,3,25,27,28). All of these intrinsic mechanisms can either alter the mechanics of the ankle/foot complex or influence the way the talocrural and tarsal bones handle the imposed loads. Nonetheless, over time this excessive stress and cyclic loading can lead to MTSS. There are numerous extrinsic factors that may lead to the development of MTSS as well. The type of surface the activity occurs on can influence the stresses placed on the bone. Running surface composition such as asphalt and running surface grade such as uphill or stairs can elicit bone pain (16). Along with various changes in running surfaces and grade, the quality and the condition of an athlete's footwear are also crucial. Maximizing shock absorption in the soles of running shoes as well as the use of insoles or orthotics construction may be important as well (26). However, no research to date has determined the percentage of shock absorption insoles needed to provide in dissipating applied forces sufficiently to prevent MTSS. Lastly, abrupt changes in training techniques and running mechanics can lead to an onset of symptoms as increases in frequency, duration, or training intensity place greater stresses on the tibia.
Although there are many theories about the origin of MTSS, the exact cause of its etiology to this point remains unknown. There have not been many studies that have examined MTSS prospectively in an athletic population (5,13,25); Most of the prospective studies that have investigated the etiology of MTSS have done so using military personnel (1,17,18,26,28). There are differences in training techniques, previous injury history, body mass index, and type of footwear between military recruits and athletes. Although the military studies do provide valuable information about MTSS, these results may not always be applicable to a traditional athletic population. Athletes afflicted with MTSS are typically unable to compete or remain physically active; thus, there is a need for further research investigating the intrinsic and the extrinsic factors regarding MTSS development. Data from this work will help clinicians and patients better understand the etiology of MTSS, which in turn can lead to improved prevention and treatment interventions for MTSS. Therefore, the purpose of this study was to conduct a prospective investigation into the intrinsic physical factors and the extrinsic historical factors that could lead to MTSS development.
One hundred and forty-six subjects (65 males and 81 females, age = 19.75 ± 1.7 yr, mass = 63.57 ± 13.1 kg, height = 172.52 ± 10.1 cm) healthy collegiate athletes from NCAA Division I and Division II cross-country, tennis, soccer, volleyball, cheerleading, and track and field teams participated in the study after providing written informed consent. Subjects incurred no lower extremity injury 6 wk before enrolling in the study and continued to participate in their regular training protocols. This protocol used in this study was approved by our university's institutional review board.
Classification of MTSS
For the purpose of this study, MTSS was defined using the criteria introduced by Yates and White (28). This criteria are similar to previously reported criteria (1,7,11). Yates and White (28) define MTSS as pain that is experienced along the posteriomedial border of the tibia occurring during exercise and not pain caused by ischemic disorders or stress fractures. For our investigation, subjects were classified as having MTSS if the following were present:
1. Pain must be a product of exercise lasting for hours after exercise.
2. The athlete should not be experiencing any signs of numbness or compression in the lower leg.
3. The pain must be in a general area, spanning an area larger than 5 cm on the posteriomedial border of the tibia.
4. The athlete should experience some diffuse discomfort upon digital palpation of the distal third of the tibia.
5. The posteriomedial surface of the tibia may be uneven during palpation.
To be placed into the symptomatic group, subjects were required to have a positive MTSS evaluation and the absence of other symptoms of exercise-induced leg pain.
Data collection took place during one session in a research laboratory before the start of each of the respective athletic seasons. First, all subjects filled out a questionnaire about their previous history of injury (Table 1) where a series of demographic variables were captured: age, height, weight, previous injury, current footwear, miles ran per week, current tibia pain or leg tightness, dietary supplementation, and for females, information about the menstrual cycle. A physical examination was then conducted to evaluate each subject's ankle strength, range of motion (ROM), tibial varum, and navicular drop. All measurements were conducted by the same experiences certified athletic trainer. The order in which these physical exam measures were taken was counterbalanced according to a Latin square to avoid any potential order effect. All measurements were taken on both limbs of all subjects. After the initial data collection took place, subjects continued to train as usual within their sport and were instructed to report any tibial pain to the certified athletic trainer (ATC) at their university. If the athlete reported tibial pain, they were evaluated by the ATC for the possibility of MTSS and then were directed to the research team for further evaluation. If MTSS was present, the subject was then placed into the symptomatic group.
Ankle and foot ROM assessment
Joint range of motion (ROM) was assessed using a standard clinical goniometer in which the following dependent variables were measured: plantarflexion, dorsiflexion, inversion, and eversion ROM (°). Subjects were measured in a seated position with hips and knees flexed to 90°, with their feet raised from the floor. To measure plantarflexion and dorsiflexion ROM, the fulcrum of the goniometer was centered with the lateral aspect of the lateral malleolus (24). The proximal arm was in line with the lateral midline of the fibula, using the fibular head as a reference point while the distal arm was aligned with the fifth metatarsal (24). Subjects then actively moved their ankle into plantarflexion and dorsiflexion. For inversion and eversion ROM, the fulcrum of the goniometer was aligned with the talus, midway between each malleoli (24). The proximal arm was aligned with the midline of the tibia, with the tibial tuberosity serving as the reference point. The distal arm was then aligned with the midline of the second metatarsal (24). The subject was then instructed to actively move the foot into inversion and eversion.
Isometric ankle and foot strength
Isometric muscle strength was measured using a handheld dynamometer (MicroFet 2, Hoggan Health Industries, Draper, UT) for both limbs of all subjects. Subjects were positioned while seated on a treatment table with the hips and the knees flexed at 90°. The dynamometer was placed in the hand of the investigator, and subjects were tested in middle of the ROM for plantarflexion, dorsiflexion, inversion, and eversion muscle strength. During testing, subjects were instructed to push as hard as they could against the investigator's hand for 5 s. Three maximal repetitions were performed for each joint position, and the mean peak force normalized to body weight for each subject was recorded. Collectively, the dependent variables measured and used for analysis were normalized mean isometric muscle strength for plantarflexion, dorsiflexion, inversion, and eversion joint position.
Tibial varum was measured with methods previously reported (23). The subject stood on a firm surface in a comfortable bilateral stance. The posterior, distal one third of the leg was bisected at the widest point of the gastrocnemius and again between the medial and the lateral malleolus. A line was drawn to connect the midpoints (23). Each leg was measured separately, and a goniometer was used to measure the angle between the bisection of the distal one third of the leg and the vertical axis of the lower leg in degrees.
Navicular drop is commonly measured and used in the clinical assessment of foot pronation. Using the methods previously reported by Buchanan and Davis (8), the midpoint of the navicular tuberosity of the foot was marked while the subject remained in a seated position with their knees and hips positioned at a 90° angle. While in this position the subtalar joint neutral was established, the starting position was measured in millimeters using a Vernier caliper (Mitutoyo America Corporation, Mitutoyo, Japan). Subjects were then asked to stand up and assume a normal weight-bearing position so that the final position of the navicular could be measured. The difference between the starting and the final positions represented the amount of navicular drop present. Each foot for all subjects was measured separately.
Before the data collection began, 10 healthy subjects who did not participate in this in vestigation were used to establish the reliability of the measures used in the study. Intratester reliability for isometric muscle strength, joint ROM, and alignment assessments was estimated by calculating intraclass correlation coefficients (ICC [1,2]). The standard error of measurement (SEM) was calculated as an additional measure of precision as it represents the range of certainty around the observed scores. The ICC and the SEM for each of the continuous variables are displayed in Table 2.
Independent t-tests were first computed to determine whether significant differences existed between MTSS and non-MTSS subjects on each of the continuous dependent variables. Chi-square analyses were performed to establish differences between MTSS and non-MTSS subjects on each of the discrete dependent variables. After the independent t-tests and chi-square analyses, all significant (P ≤ 0.05) dependent variables identified were then used in a series of discriminant function analyses. All significant variables were entered into the equation. Then the variable offering the least relationship to injury status was removed and another discriminant function analysis was run. This was repeated until the variable that explains the most variance in group membership was identified with r2 change scores. The level of significance was set a priori at P ≤ 0.05 for all analyses.
A total of 146 subjects (65 males and 81 females, age = 19.75 ± 1.7 yr, mass = 63.57 ± 13.1 kg, height = 172.52 ± 10.1 cm) participated in the study. The subjects consisted of cross-country athletes (n = 74), track and field athletes (n = 24), soccer athletes (n = 16), cheerleaders (n = 14), tennis athletes (n = 14), and volleyball athletes (n = 4) from NCAA Division I and Division II institutions within the region. Out of the 146 subjects who participated, 29 subjects (63%; 20 males and 9 females, age = 19.0 ± 0.98 yr, mass = 64.4 ± 8.3 kg, height = 173.2 ± 9.0 cm) developed MTSS. The subjects comprising the MTSS group mainly consisted of athletes from cross-country (n = 14) and track and field (n = 9) teams. The remaining athletes competed in volleyball (n = 4) and soccer (n = 2). The healthy group (non-MTSS athletes) consisted of 117 subjects (age = 19.9 ± 1.8 yr, mass = 63.3 ± 13.9 kg, height = 172.4 ± 10.4 cm) across each of the sports previously described above.
There were significant differences (P ≤ 0. 05) between the MTSS and the healthy group for the following continuous variables: plantarflexion ROM (P = 0.004) and how long the subjects have been running (P = 0.002; Table 3). The subjects with MTSS had significantly more plantarflexion ROM than the healthy group. Additionally, those with MTSS have been running significantly less than the healthy group. Surprisingly, there were no significant differences reported in the remainder of physical exam measures. There were also no significant differences for the following questions: the number of miles subjects ran per week (P = 0.931), how often they change their running shoes (P = 0.923), and for females the age of menstruation (P = 0.782).
There were significant relationships (P ≤ 0.05) between the MTSS and the healthy group between the following: previous history of MTSS (P = 0.0001), previous history of stress fracture (P = 0.039), and orthotic use (P = 0.031; Table 4). Subjects with MTSS were more likely to have a history of MTSS, to have stress fractures, and to wear orthotics than the healthy group. There were no significant relationships for vitamin use (P = 0.960) and for female subjects in regard to menstrual cycle (P = 0.974) and birth control use (P = 0.930; Table 3). Additionally, there was no significant relationship between the type of running surface (P = 0.274) and the type of athletic shoe worn (P = 0.477).
Discriminant function analysis
The four variables that were identified as significantly (P ≤ 0.05) different between the MTSS and the healthy group in the chi-square and the independent t-test analysis were then entered in a series of discriminant function analyses (Table 5). All four variables were entered together into the initial discriminant analysis. This identified the variables that best classified group membership. Then, the variable offering the least relationship to injury status was removed and another discriminant analysis was run. This was repeated (four separate analyses) until the variable that explained the most variance in group membership was identified with r2 change scores. Collectively, all four dependent variables that were entered into the discriminant function analysis as a group explained 44.6% of MTSS group membership. Two variables were identified as being statistically significant in the predictive model. These were running history (r2 change = 0.053) and previous history of MTSS (r2 change = 0.393). This model including these two variables correctly predicted group membership for 86.7% of MTSS subjects and 86% of healthy subjects. Table 5 shows the r2 change values for all four variables.
The primary purpose of this study was to conduct a prospective investigation into the risk factors of MTSS among NCAA Division I and Division II athletes. We evaluated this by conducting surveys to determine the athlete's running history and by performing a physical evaluation of the lower extremity where the following measurements: ankle joint ROM, ankle joint strength, tibial varum, and navicular drop were quantified. Overall, the results of this study demonstrated that subjects with MTSS had significantly increased plantarflexion ROM and less years of running. Additionally, athletes who had a previous history of MTSS and stress fracture are significantly more likely to develop MTSS again. Furthermore, athletes who developed MTSS were significantly more likely to wear orthotics than athletes that did not develop MTSS. Collectively, the discriminant analysis revealed that these four factors (plantarflexion range or motion, running years, orthotic wear, and previous history) explained 44.6% of MTSS group membership. Of the four variables used in the model, the previous history of MTSS and the amount of years spent running correctly predicted group membership for 86.7% of the MTSS athletes and 86% of the healthy athletes; previous history of MTSS was the variable that was most predictive for athletes developing MTSS in this study. These results indicate that medical history is important when screening individuals for MTSS.
Previous history of MTSS and stress fracture
A previous history of MTSS was greater in the injured group (87%) than in the healthy group (16%). Furthermore, 46% of the athletes in the injured group had experienced a stress fracture before, whereas 17% of those in the healthy group had experienced a stress fracture. These factors explained 39.3% of the total variance of MTSS group membership. As evidenced by these data, our findings indicate that athletes with MTSS or stress fracture history are more likely to experience these problems again once the training resumes. Therefore, it is crucial that the certified athletic trainer or sports medicine practitioner work with the athletes who experience MTSS to determine whether the athlete has a history of MTSS or stress fracture. On the basis of our data taking a preventative approach by looking at preseason physicals, screening histories in athletes may be necessary. Furthermore, because stress fractures are more prevalent in athletes with MTSS, certified athletic trainers should educate the athletes and the coaches on stress fractures and the implications that can occur if MTSS is not treated properly.
With MTSS, the tibia is chronically inflamed due to injury, and the repetitive force of running further prevents the affected site from healing properly. This may explain why MTSS is a reoccurring injury and why the incidence of stress fracture is significantly higher in subjects (46%) who develop MTSS. Although our athletes did not have bone mineral density measurements taken, our findings could be related to bone mineral density of the athletes. Previous research (3,4) has shown that bone mineral density can influence the development of a tibial stress fracture. In reviewing a synthesis of the literature on stress fractures (3), there is substantial evidence demonstrating that an inverse relationship exists between bone mineral density and development of stress fractures (3). Moreover, in a prospective investigation conducted by Bennell et al. (4), it was established that female track and field athletes who experienced a stress fracture had significantly less bone mineral density in the foot and the lumbar spine than the control group. Magnusson et al. (21) determined that regional bone mineral density is 12% lower in athletes who experience MTSS. After the pain from MTSS subsides, the regional bone mineral density increases by approximately 19% at the MTSS site (21). This indicates that in athletes with MTSS, regional bone mineral density will be lower as long as the MTSS lingers. The inflammation that occurs with MTSS impedes osteoblastic activity, thus making the bone weaker. Most athletes who experience MTSS will not cease activity; thus, predisposing the tibia to greater stresses ultimately develops into microcracks, which then manifests itself into full discrete fractures. With such a significant percentage of subjects having a history of stress fractures and MTSS and the relationship established between bone mineral density, stress fractures, and MTSS, a treatment course combining a significant decrease in activity along with a guided rehabilitation program appears warranted to prevent further injury.
Running is an inexpensive way to exercise, and it is quite popular among nonathletes; moreover, most sports and physical activity involve a running component to it. The findings of our study indicate that when a person has more experience in athletic activity, they are less likely to develop MTSS later in their career. In the present study, athletes who developed MTSS reported to be running significantly less (5.3 yr) compared with athletes who did not develop MTSS (8.8 yr). This difference may indicate that the tibia may adapt and adjust to the forces placed on it from running over extended periods of time. The work conducted by Fredericson et al. (15) does support our finding that an inverse relationship may exist between the years running and the development of MTSS. In their investigation, the authors established that the introduction of activities such as ball sports (basketball, soccer) earlier in one's life influenced the development of a stress fracture in NCAA Division I distance athletes. Specifically, their work showed that subjects who began athletic activity in childhood were significantly less likely to develop a stress fracture later in their cross-country or track career; moreover, it was surmised that exposing these athletes longer to these sports had allowed them to be in better physical condition before them beginning to run competitively (15). According to Wolff's Law, bone adapts to the forces that are applied to it (22), and thus a positive adaptation may have occurred with the athletes who did not sustain MTSS. Because athletes who did not develop MTSS reported running approximately 66% longer than those who did, it is possible that their tibia have adapted well to the applied forces during this time. However, more research is needed to examine if this same relationship conclusively exists in preventing MTSS.
In the present investigation, we demonstrated that 53% of subjects who developed MTSS used orthotics compared with 21% of athletes who did not. It is clear from these data that athletes who developed MTSS are 1.5 times more likely to wear orthotics than healthy individuals. Orthotics are often prescribed to correct any lower extremity malalignments such as excessive pronation. However, we did not examine why subjects in the study were prescribed orthotics. Previous research (6,20,25,27) has revealed that excessive pronation is an important risk factor for developing MTSS. Therefore, excessive pronation may be occurring in the MTSS group, promoting the use of orthotics. Prescription orthotics are thought to be very effective when prescribed for MTSS, and it has been shown that athletes report complete relief of MTSS symptoms when wearing them (13). In the current study, we found that over 50% of the MTSS group regularly wore orthotics for MTSS. If the use of orthotics were a completely effective modality for MTSS, less pain would have been experienced in these subjects.
In the current investigation, plantarflexion ROM was found to be significantly higher in the injured group (46°) compared with the healthy group (40.6°); however, plantarflexion ROM was not a significant predictor of classifying MTSS group membership. Our finding is supported by a previous prospective investigation studying the relationship between the ankle ROM and the incidence of MTSS (25). Conversely, previous research has shown that a decrease in plantarflexion ROM is observed in athletes who experience MTSS (1,7,20). Collectively, it appears more prospective studies are needed in this area to better understand that the relationship between plantarflexion ROM and MTSS in athletes.
In the current investigation, 44.6% of MTSS group membership was accounted for by the following variables: previous history of stress fracture, use of orthotics, running history, and previous history of MTSS. Of these, the most important factors for predicting the development of MTSS were previous history of MTSS and years of running experience. Collectively, 55.4% of MTSS group membership remained unexplained, and it is likely that other intrinsic variables identified in the literature (e.g., rearfoot varus and valgus, forefoot varus and valgus, isokinetic ankle strength, and bone mineral density) that were not assessed in our investigation may account for more variance in predicting MTSS in collegiate athletes. The results of this study support the importance of taking a comprehensive history assessment of an athlete if they present with signs and symptoms of MTSS to establish if this condition has existed before. This knowledge could then be used to establish a rehabilitation program that focuses on allowing the necessary time for the bone to heal, correct any malalignment abnormalities that may exist, and focus on strengthening the ankle and foot musculature. Future investigations should be done to establish if other extrinsic and intrinsic factors significantly predict MTSS in collegiate athletes.
The results of the present study do not constitute endorsement by ACSM.
There was no funding received for this project.
The authors would like to thank J. Timothy Lightfoot, FACSM, for contributions and critical review of this manuscript.
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