Hip Weakness and Shin Splints: A Biomechanical Approach

A variation of this tension theory is that the deep crural fascia (DCF) - the though-connective tissue that surrounds the deep posterior compartment muscles of the leg - pulls excessively on the tibia, again causing trauma to the bone. Researchers at the University of Honolulu examined a single leg from five male and 11 female adult cadavers. They confirmed that in these specimens, the muscles of the posterior compartment originated above the portion of the leg that is typically painful in MTSS, and the DCF indeed attached along the entire length of the medial tibia⁽³⁾.

Doctors at the Swedish Medical Centre in Seattle, Washington, wondered if, given the anatomy, the tension from the posterior calf muscles could produce a related strain on the tibia at the insertion of the DCF and thus be the mechanism of injury.⁽⁴⁾

A descriptive laboratory pilot study of three fresh cadaver specimens found that strain at the insertion site of the DCF along the medial tibia progressed linearly as tension increased in the posterior leg muscles. This confirmed that a mechanism for a tension-induced injury at the medial tibia is plausible. However, studies of bone periosteum in MTSS patients have yet to find inflammatory markers consistently enough to confirm the periostitis theory⁽⁵⁾.

Tibial Bowing

The second causation theory for MTSS is that repetitive or excessive loading causes a bone-stress reaction in the tibia. The tibia, unable to adequately bear the load, bends during weight bearing. The overload results in micro damage within the bone, and not just along the outer layer. When the repetitive loading outpaces the bone’s ability to repair, localized osteopenia can result. Thus, some consider a tibial stress fracture to be the result of a continuum of bone stress reactions that include MTSS⁽¹⁾.

Magnetic resonance imaging (MRI) of the symptomatic leg often shows bone marrow edema, periosteal lifting, and areas of increased bony resorption in patients with MTSS^(1,5). This supports the bone-stress reaction theory. Magnetic resonance imaging of an athlete with a clinical presentation of MTSS can also help rule out other causes of lower leg pain, such as tibial stress fracture, deep posterior compartment syndrome, and popliteal artery entrapment syndrome.

Risk Factors for MTSS

While the etiology of MTSS is still theoretical, the risk factors for athletes developing it are well-determined. A significant navicular drop, as determined by the navicular drop test (NDT), significantly correlates with a diagnosis of MTSS^(2,5). The NDT measures the difference in height position of the navicular bone, from a neutral subtalar joint position in supported non-weight bearing to full weight bearing (see figures 2 and 3). The NDT indicates the degree of arch collapse during weight bearing. An excursion of more than 10 mm is considered excessive and a significant risk factor for developing MTSS⁽⁵⁾. 

Athletes with MTSS are more likely to be female, have a higher BMI, less running experience, and have a previous history of MTSS^(2,5). Running kinematics for females can differ from males and fit a pattern that is known to leave them vulnerable to anterior cruciate ligament tears and patellofemoral pain syndrome⁽⁵⁾. This same biomechanical pattern may also predispose females to MTSS. Hormonal considerations and low bone density are possibly contributing factors in increasing the risk of MTSS in female athletes as well.

A higher BMI in an athlete likely indicates they have more muscle mass rather than being overweight. However, the end result is the same: the legs bear a significantly heavy load. In these instances, the bone growth stimulated by the tibial bowing may not progress rapidly enough, and injury to the bone occurs. Therefore, those with a higher BMI may need to progress their training programs more slowly to allow for adaptation.

Those with less running experience are more likely to make training errors (often identified by the athlete) as the catalyst for MTSS. These include increasing distance too rapidly, changing terrain, overtraining, poor equipment (shoes), etc. Inexperience may also lead the athlete to return to activity too soon, thus accounting for the higher prevalence of MTSS in those who previously suffered MTSS. Full recovery from MTSS can take anywhere from six to ten months, and if the cause of injury is not rectified or the athlete returns to training too soon, the chances are good the pain will return⁽⁵⁾.

Biomechanical Considerations

The NDT is used as a measurable indication of foot pronation. Pronation is a tri-planar movement comprised of eversion at the hindfoot, abduction of the forefoot, and dorsiflexion of the ankle. It is a normal movement and essential in walking and running. When the foot strikes the ground at the initial contact phase of running, it begins to pronate, and the joints of the foot assume a loose-packed position. This flexibility helps the foot absorb ground reaction forces (see figure 4).

During the loading response phase, the foot further pronates, reaching peak pronation by around 40% of the stance phase⁽⁶⁾. In mid-stance, the foot moves out of pronation and back to a neutral position. During terminal stance, the foot supinates, moving the joints into a closed packed position and creating a rigid lever arm from which to generate the forces for toe-off.

Beginning with the loading response phase and throughout the remainder of the single-leg stance phase of running, the hip is stabilized, extended, abducted, and externally rotated by the concentric contraction of the hip muscles of the stance leg (the gluteals, piriformis, obturator internus, superior gemellus, and inferior gemellus). Weakness or fatigue in any of these muscles can result in internal rotation of the femur, adduction of the knee, internal rotation of the tibia, and over-pronation (see figure 5). Overpronation, therefore, can be a result of muscle weakness or fatigue. If this is the case, the athlete may have a relatively normal NDT, yet they can overpronate when the hip muscles don’t function as needed.

In a runner who has significant overpronation, the foot may continue to pronate into mid-stance, resulting in a delayed supination response and, thus less power generation at toe-off. The athlete may attempt two biomechanical fixes here that could contribute to the development of MTSS. Firstly, the tibialis posterior will strain to prevent overpronation. This can add tension to the DCF and strain the medial tibia. Secondly, the gastroc-soleus complex will forcefully contract at toe-off to improve power generation. Again, the increased force within these muscle groups can add tension to the medial tibia through the DCF and irritate the periosteum.

Assessment

Knowing that overpronation is one of the leading risk factors for MTSS, start your evaluation on the ground and work your way up. First, perform the NDT, noting if the difference is more than 10mm. Analyze the athlete’s running gait on a treadmill, preferably when the muscles are fatigued, as at the end of a training run. Even with a routine NDT, you may see evidence of overpronation in running (see figure 6).

Videotaping your runner while on a treadmill enables you to analyze their gait, frame by frame. Typically, suppose the runner is overpronating, and there is a weakness in the hip joint or mid-stance. In that case, you will see knee adduction, foot pronation, abduction of the forefoot, and a drop of the opposite hip of more than four to five degrees. Some runners may have significant pronation but run with hips level (within five degrees) and minimal knee adduction. In this case, weak tibialis posterior, a tight calf muscle, lax ligaments, or structural deviations may cause the pronation, but the hip tests strong and functions well while running.

Next, clinicians must evaluate the knee and identify if there is adduction. They must assess if the hip is level or if either hip is more than 5 degrees from level. These are indications that there is likely weakness at the hip. Traditional muscle testing may not reveal the weakness; therefore, functional muscle testing is required.

Observe the athlete perform a one-legged squat with arms in and arms overhead. Does the hip drop, the knee adduct, and the foot pronate? Test the strength of hip abductors in side lying, with hip in neutral, extended, and flexed, keeping the knee straight (see figure 7). Test all three positions with the hip rotated in neutral and at external and internal rotation end ranges. Test hip extension in prone with the knee straight and bent in all three positions of hip rotation: external, neutral, and internal. The position where you find the weakness is where you should begin strengthening activities.

Treat the Kinetic Chain

If there is weakness in the hip, begin by having the athlete perform isometric exercises in the position of weakness. For instance, if you find a weakness in hip abduction with extension, start isolated isometrics in this position. You should not add movement until the muscles consistently fire isometrically in this position for three to five sets of 10 to 20 seconds. Once the athlete achieves this level, begin concentric contractions in that same position against gravity. Some examples are unilateral bridging and side-lying abduction. Eccentric contractions should follow, followed by sport-specific drills.

Keep in mind if there are other biomechanical compensations, they must also be addressed. If the tibialis posterior is also weak, begin strengthening there. If the calf muscles are tight, initiate a stretching program. Utilize whatever modalities might be helpful. Lastly, consider a stabilizing shoe if the ligaments in the foot are over-stretched. Using a stabilizing shoe for a short time during rehabilitation can be beneficial to cue the athlete to adopt new movement patterns. 

Conclusion

The best way to prevent shin pain from MTSS is to decrease the athlete’s risk factors. Each athlete should have a fundamental running gait analysis and proper shoe fitting. As part of the strengthening program, hip strengthening should be included in functional positions, such as unilateral stance. Pair inexperienced athletes with a more experienced mentor to ensure proper training, use of equipment, and investigation of pain at onset. They may be more likely to tell a teammate they are feeling pain than a coach or trainer. Progress the running schedule of heavier athletes more slowly to allow adaptation of the bone. Ensure that athletes fully rehabilitate before returning to play because the chances of recurrence of MTSS are high.

References

1. Am J Sports Med. 2015 Jun;43(6):1538-47
2. Br J Sports Med. 2015 Mar;49(6):362-9
3. Med Sci Sports Exerc. 2009;41(11):1991-1996
4. J Am Podiatr Med Assoc. 2007 Jan;97(1):31-6
5. J Sports Med. 2013;4:229-41
6. Gait and Posture. 1998;7:77–95