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Running Injuries - Changing Trends and Demographics

Fields, Karl B. MD

doi: 10.1249/JSR.0b013e31822d403f
Sport-Specific Illness and Injury
Free

Running injuries are common. Recently the demographic has changed, in that most runners in road races are older and injuries now include those more common in master runners. In particular, Achilles/calf injuries, iliotibial band injury, meniscus injury, and muscle injuries to the hamstrings and quadriceps represent higher percentages of the overall injury mix in recent epidemiologic studies compared with earlier ones. Evidence suggests that running mileage and previous injury are important predictors of running injury. Evidence-based research now helps guide the treatment of iliotibial band, patellofemoral syndrome, and Achilles tendinopathy. The use of topical nitroglycerin in tendinopathy and orthotics for the treatment of patellofemoral syndrome has moderate to strong evidence. Thus, more current knowledge about the changing demographics of runners and the application of research to guide treatment and, eventually, prevent running injury offers hope that clinicians can help reduce the high morbidity associated with long-distance running.

Cone Health Sports Medicine Fellowship, Greensboro, NC; and Department of Family Medicine, The University of North Carolina at Chapel Hill School of Medicine, Chapel Hill, NC

Address for correspondence: Karl B. Fields, MD, Moses Cone Sports Medicine Fellowship, Greensboro, NC (E-mail: bert.fields@mosescone.com).

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Introduction

Long-distance running and recreational road races are a major part of sports activities in communities throughout the United States. Their linkage to charitable fundraising memorial events, weight loss programs, and community celebrations has led to an explosion in the number of road races that occur in towns of all sizes. In 2010, a record 507,000 Americans finished a marathon, with most of them being inexperienced runners. An estimated 20% of Americans run regularly for fitness, but as many as 30% may participate at times in one of these events. The numbers of individuals who take part in recreational running far exceed those who take part in competitive track-and-field competitions (30).

The trend for wider community participation has changed the demographic of the running community and the pattern of injuries that affect runners. In particular, the average age of runners in road races is 40 yr, including 46% of marathoners. This means that injuries more common to master runners have become prevalent. In addition, individuals who do not have the low body mass index and body habitus of a typical competitive marathon runner have chosen to participate in longer race events like half-marathons and marathons. These runners typically follow training schedules that have a relatively low total mileage and emphasize building up to one long run every other week. Some encourage the participants to walk regularly within the race to complete the event. Not surprisingly, impact injuries that occur in elite runners during high-mileage training now affect heavier runners at a lower mileage.

One consistent feature of running injury research is that injury rates remain high and, in published reports, range from a low rate of 19% to a high rate of 79% of runners injured yearly (35). Typically, 25% of runners are injured at any given time, and about half experience an injury that takes them out of running for a period during any year (9). The recovery and rehabilitation of these injuries clearly play a role in the subsequent risk of new running injuries. Only two factors have shown a strong statistical correlation with the prediction of running injuries, and these are previous injury and total running mileage. In the only true prospective study of running injuries, previous injury showed a relative risk of 1.51 in predicting future injury. The other significant risk factor was running mileage, with 40 miles or more per week showing an RR of 2.88 for subsequent injury (3).

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Epidemiology of Running Injury

Older surveys of running injury in general have revealed similar anatomic patterns. The most common injuries are those on the knee, followed by lower leg injuries, Achilles tendon and calf injuries, stress fractures, and foot injuries. Considerable variation exists within these studies related to the demographic of participants, as well as the type of running - competitive, recreational, track racing, and shorter distances or longer distances. Compilation of these studies reveals a pattern that suggests approximate injury frequencies per anatomic site as follows (8):

  • Knee = 25%
  • Lower leg = 20%
  • Foot = 16%
  • Ankle = 15%
  • Upper leg = 10%
  • Hip and pelvis = 7%
  • Lower back = 7%

More recent epidemiologic studies show a changing pattern of injury, consistent with the findings that older runners make up a higher percentage of participants in road races. A 2005 survey study of 2,886 runners of whom 34% were master runners also showed differing patterns between younger and older runners. Knee and shin problems affected more of the younger runners (P < 0.005), whereas calf, Achilles, and hamstring injuries were more common in older runners (P < 0.001). Speculations about the potential causes of greater muscle injury in older runners include age-related changes in the muscle tendon unit such as decreased strength and increased stiffness. A second possible explanation is that the normal muscle injury that occurs with training seems to take greater time to repair with aging, and older runners continue running at a frequency similar to that of younger runners (24).

This review will focus on the most common injuries in the above areas, with the exclusion of foot problems that are covered in the companion article by Simons in this issue (32). Diagnostic evaluation and specific treatment recommendations are offered. Included are some musculoskeletal images of running injuries because these diagnostic tools have become a valuable adjunct to the diagnosis and subsequent treatment monitoring of running injuries.

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Knee Injuries

ITB

ITB syndrome has surpassed PFS as the most common knee problem in runners in recent reports (24). Although lateral knee pain also arises from popliteus tendinopathy, vastus lateralis strain, and lateral meniscal pathologic disease, the typical runner with lateral knee pain has ITB syndrome. Even in older epidemiologic studies, ITB syndrome occurred more commonly in more competitive runners than did PFS. One possible reason for an increase in this injury among runners may be the explosion in the number of individuals competing in road races.

Classic treatments for ITB syndrome include many stretches, friction massage to the ITB, correction of the inequality in leg length, training changes including variable paced running, avoidance of running on cambered surfaces, and correction of biomechanical issues such as excess pronation. None of these interventions have evidence stronger than expert opinion (SOR C in the Strength of Recommendation Taxonomy (SORT)). The strongest evidence for an effective treatment comes from a prospective trial that emphasized correction of weakness in hip abductor strength. In this trial, of the 24 affected runners, 22 resolved symptoms by 3 months and a 23rd by 6 months, using a regimen of lateral leg lifts. (SOR B, moderate evidence) (11). This finding was supported by subsequent studies showing the association of hip abduction and hip flexion weakness with overall risk of injuries to the lower extremity in runners. The rationale behind this observation is that the gluteus medius is the key stabilizer of the ITB and that it functions in the stance phase of running gait to stabilize foot strike and block dynamic genu valgum (12,27).

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PFS

PFS traditionally ranks as the most common running injury. This probably remains true for younger runners and beginners. Moderate evidence links vastus medialis weakness, hip abduction weakness, and cavus feet with PFS (level of evidence (LOE) 2, or moderate evidence, in the SORT system) (4,7,16). Other specific associations such as hamstring tightness, pronation, and Q angles lack strong evidence, and whether to address these in treatment protocols remains controversial (SOR C). Clinical conditions that cause anterior knee pain often are lumped into the PFS category, although the advent of musculoskeletal ultrasound allows clinicians to identify more specific injuries that cause anterior knee pain. These include quadriceps and patellar tendinopathy and partial tears, plica syndrome, and supra- and infrapatellar bursitis. Additional conditions that cause anterior knee pain, such as osteochondral lesions, patellar subluxation or dislocation, patellofemoral arthritis, and systemic medical conditions, require a more extensive workup to make a definitive diagnosis.

Treatment of PFS with orthotics now has relatively strong evidence (SOR A). This comes from recent prospective controlled trials, as well as several case series. One systematic review rated orthotics as likely to provide superior relief in PFS to flat insoles (2). To date, the highest-quality study on PFS evaluated 147 patients randomized to semicustom heat-molded orthotics, physical therapy (PT), both interventions, and flat insoles (the null intervention). Patients rated their functional assessment and pain at the end of the trial. The study showed a statistically significant improvement in pain in all three intervention groups over the control group (i.e., orthotics and formal PT both led to reductions in pain and to improved function). Results were similar, but the two interventions combined were not better than either alone. The number of patients needed to treat with foot orthoses versus flat inserts was 4 at 6 wk and 9 at 52 wk (based on patient-perceived global success outcomes) (5).

This randomized trial also provided strong evidence that PT can benefit patients with PFS. Presumably, PT addresses both of the associated weakness findings in the vastus medialis and the gluteus medius muscle groups. Although only a small number of strong prospective trials looking at the benefit of PT for PFS exist, of three of these Cochrane reviews, pain reduction was noted in two trials and functional improvements were noted in one trial. Strong evidence that open- and closed-chain rehabilitation programs are equivalent does exist (SOR A) (36). One similar finding in both orthotic and PT intervention trials for PFS is that virtually all trials regardless of type showed some level of benefit from the intervention.

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Meniscus Injuries

Meniscus injuries did not seem significant in earlier epidemiologic studies but now comprise a significant percent of the knee problems noted in recent running injury studies. This parallels the older age of competitors in road races. One study of injured runners in a sports medicine clinic noted a higher risk in male runners and found meniscus injuries to be the fourth most common problem (33). No long-term studies guide the outcome of those individuals who continue to run with a meniscal pathologic abnormality. Orthopedic research demonstrates the poorer ability of the knee to dissipate impact after meniscectomy or significant meniscal cartilage loss. However, patient outcome evidence about whether running after meniscectomy or meniscus injury treated nonoperatively accelerates osteoarthritis of the knee is lacking. Most physicians would caution patients against this type of weight bearing stress, but injury data from a recent race indicate that runners are participating regardless of this advice.

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Calf/Achilles Tendinopathy

Calf injuries affect older runners more commonly, and Marti et al. (22) noted these as the most common injury of runners older than 40 yr who participated in a major Swiss road race. Achilles tendinopathy was the most common injury in a recent study of master runners (18). Achilles tendinopathy also occurs more commonly in elite runners - approximately 10% of elite runners annually - and runners with more than 10 years of experience (19). The lifetime risk in former elite male distance runners may be as high as 52% (36). Runners typically attribute this injury to excess speed work, hill running, or overtraining. Cavus feet and inflexible Achilles-calf complexes are suspected of increasing risk. However, these observations about etiology lack any rigorous evidence.

A variety of injuries occur in the calf including the "classic tennis leg," which represents a tear at the musculotendinous insertion of the medial head of the gastrocnemius muscle. The lateral head of the gastrocnemius experiences less injury perhaps because the toe-off phase of gait occurs with most of the force directed along the medial aspect of the leg. Other soft tissue structures including the intersection of the two heads of the gastrocnemius and the plantaris tendon can tear with excess training.

Pain deep to the gastrocnemius and palpable on squeezing of calf from the medial and lateral aspects suggest soleus syndrome. The soleus is a slow-twitch muscle, and chronic soleus pain may indicate a runner who trains at a mileage too great for his or her fitness.

Achilles tendon injuries occur primarily in two areas. These most commonly occur in what is called the avascular zone, from 2 to 5 cm above the insertion. This is the area in which the fibers of the gastrocnemius and the soleus weave together to form the substance of the tendon. The second area is at the insertion of the Achilles tendon into the heel, representing an enthesopathy - a separation of tendon fibers from their insertion in the bone. These injuries act more like partial tears and pose greater challenges for the runner to heal. Retrocalcaneal bursitis and frank Achilles ruptures occur less commonly in runners. The exceptions to this would be runners wearing shoes with very firm heel counters that may lead to a contusion at the back of the heel and a secondary bursitis. Highly competitive sprinters also have experienced several complete Achilles ruptures presumably from the speed and high tensile stress required in maximal sprinting.

The best evidence for effective treatment of Achilles tendinopathy comes from eccentric exercise protocols studied by Alfredson et al. (1). This type of rehabilitation led to the resolution of tendinopathy in most individuals with noninsertional injuries. Imaging studies with both musculoskeletal ultrasound and magnetic resonance imaging (MRI) that assessed healing of the Achilles tendon of patients using these protocols demonstrated resolution of the pathologic changes of tendinopathy (SOR A) (15,31). Subsequent studies looking at insertional tendinopathy modify the eccentric protocols to make these less stressful and have shown benefit but slower healing times and more treatment failures. Although these studies did not specifically examine the treatment of other calf injuries, most clinicians use this approach for gastrocnemius and soleus problems as well because they are all part of the same musculotendinous complex (18).

Adjunctive treatments using ice massage, heel lifts, and stretching arise from clinical experience and physician opinion (SOR C). Other studies with some evidence include a study of 31 male runners who ran more than 30 miles·wk−1. In this trial, 4 wk of PT or wearing custom orthotics reduced pain by 50% compared with an untreated control group (23). Paoloni et al. (28) studied the use of low-dose topical nitroglycerine patches for the treatment of noninsertional Achilles tendinopathy and noted 78% with excellent results when combined with the eccentric rehab protocol of Alfredson et al. (1) versus only 49% on the PT alone. A recent meta-analysis including seven randomized controlled trials found strong evidence that topical nitroglycerin therapy helped relieve pain in chronic tendinopathies (SOR A) (13). To date, studies of platelet-rich plasma injection, prolotherapy, and other modalities yielded mixed results.

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Medial Tibial Stress Syndrome/Compartment Syndrome/Tibial Stress Fracture

Medial shin pain affects young runners as commonly as any type of injury. Particularly, when this is bilateral, this represents the classic "shin splints," which is a nonspecific type of injury delineated by pain along the posterior medial border of the tibia. Diagnosis primarily comes from physical examination, and advanced imaging, at most, shows linear uptake on a bone scan or nonspecific edema of the periosteum on an MR image. Some evidence supports the use of shock-absorbing insoles because Cochrane cites four medium-quality studies that show some reduction of risk of medial tibial stress syndrome (shin splints)/tibial stress fracture with the use of shock-absorbing insoles (29). A more recent review of newer literature found inconsistent results, with two studies that moderately supported prevention and one that supported shock-absorbing insoles/custom orthotics for treatment (SOR B) (21,26,34).

Compartment syndrome should be considered in runners who have persistent shin or lower leg pain that comes with exertion and has no direct association with a defined injury. A higher index of suspicion is merited in fit runners with prior experience who continue to have these symptoms. Weakness in the affected leg and numbness in the foot are symptoms that, if frequent or persistent, lead the clinician to diagnostic compartment testing. Orthotics, PT, and rest can be initial treatments, but those who fail to have resolution of their symptoms may require surgical compartment release.

Tibial stress fractures present like other types of shin pain, but typically, a focal area along the tibia becomes tender to pressure, percussion, tuning fork vibration, or dynamic testing such as hopping.

In male runners, tibial stress fractures are the most common, with the juncture of the distal and medial third of the medial tibia as the most common site. The tibial metaphysis, anterior tibia, tibial plateau, and medial malleolus all heal poorly and require careful follow-up and conservative treatment to allow resolution. EBM supports use of long air splints to speed resolution of tibial stress fractures with return to training accelerated by 42 d in a compilation of studies collected by the Cochrane Database (SOR B) (14,29).

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Stress Fractures

In recent studies of running injuries, approximately 10% of those documented are stress fractures. For those above the foot, the tibia (mentioned above), femoral neck, femoral shaft, fibula, and pelvis and sacrum are locations of concern in distance runners. The highest risk of these fractures is the femoral neck. Older surveys on running injury noted these as affecting marathon runners at high mileage. However, more recent case reports note inexperienced runners developing this injury at modest amounts of running mileage. Groin pain in a runner merits aggressive pursuit of the diagnosis so as not to miss this injury. When doubt exists, MRI is considered the diagnostic test of choice for diagnosis.

Most evidence points to rates in women runners at least twice those in males and higher in some studies. Caucasians are at greater risk than African Americans (25). Key factors in predicting stress fracture in female cross-country runners aged 18 to 26 yr included prior stress fracture (RR = 5) and low bone mineral density, with a drop of one standard deviation doubling the risk (17). Some evidence for prevention in women comes from a study of women in basic training in the U.S. Navy in which supplementation of 2,000 mg of calcium and 800 IU of vitamin D decreased risk by 27% (20).

In all stress fractures, reduction in training period to lessen bone impact is the mainstay of treatment. A Cochrane review also notes that shock-absorbing insoles may play a role in the prevention of stress fractures (29). In stress fractures with the lowest risk - fibula and distal third of tibia - resumption of low levels of running can occur within 10 d to 2 wk. Other stress fractures require cross-training that allows rest of the injured bone. In higher-risk injuries such as femoral neck stress fractures, non-weight bearing use of crutches for a minimum of 6 wk usually promotes healing. The superior side of the femoral neck stress fracture may require surgical consultation for pinning and poses a greater challenge for healing. Sacral stress fractures also heal slowly, and the majority require at least 12 wk before training resumption. Other stress fractures, such as those on the pelvic bones and the femoral shaft, typically respond to 6 wk of rest.

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Muscular Injuries in Runners

Hamstring injuries frequently affect competitive athletes (36). These have recently turned up in more studies that include older runners in which the mechanism may relate to fatigue and more muscle flexibility associated with aging. Quadriceps injuries, hip flexor injuries, and piriformis injuries all may relate to training that includes a fair amount of hill training. The greater stress on quads and hip flexors in uphill running is logical from the biomechanics required. Downhill running surprisingly contributes a lot of injuries because the bigger muscles have to function as shock absorbers and the impact of running may increase to four to six times the body weight on faster downhill runs.

The hip abductors and rotators play a much bigger role in stabilizing running gait than previously thought. Recent studies showed a ubiquitous association of hip flexor and abduction weakness with lower extremity running injuries. The hip rotators also assist in this process, and perhaps, the piriformis and other rotators have greater stress when there is relative gluteus medius weakness. Treatment of muscular injury requires a good rehabilitation program that incorporates the eccentric strength required to run effectively.

Other less common running injuries include osteitis pubis, which presents often as groin pain and direct tenderness over the symphysis pubis. This same type of vague groin pain, in some individuals, represents a conjoint tendonitis or a true sports hernia. Whereas osteitis pubis has classic findings on plain radiography, the other conditions can be difficult to diagnose confidently. Lumbar disk disease and degenerative arthritis of the lumbar spine may refer symptoms to the leg that mimic a running injury. Nerve entrapments, popliteal artery entrapment, and other obscure entities must enter the differential diagnosis when standard examination and diagnostic workup fail to find an etiology for a running injury.

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Summary

Running injuries are common, affecting up to 50% of frequent long-distance runners yearly. Recently, the demographic has changed, that most runners in road races are 40 yr or older. With this increased participation of master runners, the most common injuries have shifted to include those more common in this age group. In particular, Achilles tendon and calf injuries, ITB syndrome, meniscus injury, and muscle injuries to the hamstrings and quadriceps represent higher percentages of the overall injury mix in more recent epidemiologic studies than in prior years.

Evidence suggests that running mileage and previous injury are the strongest predictors of subsequent running injury. Evidence-based research now helps guide the treatment of some of the most common running injuries. Greater focus on hip abduction strength for ITB syndrome and possibly PFS, eccentric strengthening protocols for Achilles tendinopathy, use of topical nitroglycerin in tendinopathy, and orthotics for the treatment of PFS and possibly MTSS all have moderate to strong evidence. A recent study raised the intriguing possibility that, for heavy physical training, an orthotic intervention also might reduce the risk of overall lower extremity injury (10). Thus, more current knowledge about the changing demographics of runners and the application of evidence-based medical research to guide treatment and eventually prevent running injury offers hope that clinicians can help reduce the high morbidity associated with long-distance running.

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Acknowledgments

The author has no funding disclosures.

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References

1. Alfredson H, Pietilä T, Jonsson P, Lorentzon R. Heavy-load eccentric calf muscle training for the treatment of chronic Achilles tendinosis. Am. J. Sports Med. 1998; 26:360-6.
2. Barton CJ, Munteanu SE, Menz HB, Crossley KM. The efficacy of foot orthoses in the treatment of individuals with patellofemoral pain syndrome: a systematic review. Sports Med. 2010; 40:377-95.
3. Boven AMP, Janssen GME, Vermeer HGW, et al. Occurrence of running injuries in adults following a supervised training program. Int. J. Sports Med. 1989; 10(Suppl. 3):S186-90.
4. Cichanowski HR, Schmitt JS, Johnson RJ, Niemuth PE. Hip strength in collegiate female athletes with patellofemoral pain. Med. Sci. Sports Exerc. 2007; 39:1227-32.
5. Collins N, Crossley K, Beller E, et al. Foot orthoses and physiotherapy in the treatment of patellofemoral pain syndrome: randomised clinical trial. Br. J. Sports Med. 2009; 43:169-71.
6. Copland ST, Tipton JS, Fields KB. Evidence-based treatment of hamstring tears. Curr. Sports Med. Rep. 2009; 8:308-14.
    7. Duffey MJ, Martin DF, Cannon DW, et al. Etiologic factors associated with anterior knee pain in distance runners. Med. Sci. Sports Exerc. 2000; 32:1825-32.
    8. Epperly T, Fields KB. Running epidemiology. In: Wilder, RP and O'Connor, FG editors. Textbook of Running Medicine. New York: McGraw Hill (in press).
    9. Fields KB, Sykes JC, Walker KM, Jackson JC. Prevention of running injuries. Curr. Sports Med. Rep. 2010; 9:176-82.
    10. Franklyn-Miller A, Wilson C, Bilzon J, McCrory P. Foot orthoses in the prevention of injury in initial military training: a randomized controlled trial. Am. J. Sports Med. 2011.
    11. Fredericson M, Cookingham CL, Chaudhari AM, et al. Hip abductor weakness in distance runners with iliotibial band syndrome. Clin. J. Sport Med. 2000; 10:169-75.
    12. Fredericson M, Weir A. Practical management of iliotibial band friction syndrome in runners. Clin. J. Sport Med. 2006; 16:261-8.
    13. Gambito ED, Gonzalez-Suarez CB, Oquiñena TI, et al. Evidence on the effectiveness of topical nitroglycerin in the treatment of tendinopathies: a systematic review and meta-analysis. Arch. Phys. Med. Rehabil. 2010; 91:1291-305.
    14. Gillespie WJ, Grant I. Interventions for preventing and treating stress fractures and stress reactions of bone of the lower limbs in young adults. Cochrane Database Syst. Rev. 2000; CD000450.
    15. Grigg NL, Wearing SC, Smeathers JE. Eccentric calf muscle exercise produces a greater acute reduction in Achilles tendon thickness than concentric exercise. Br. J. Sports Med. 2009; 43:280-3.
    16. Ireland ML, Willson JD, Ballantyne BT, Davis IM. Hip strength in females with and without patellofemoral pain. J. Orthop. Sports Phys. Ther. 2003; 33:671-6.
    17. Kelsey JL, Bachrach LK, et al. Risk factors for stress fracture among young female cross-country runners. Med. Sci. Sports Exerc. 2007; 39:1457-63.
    18. Kingma JJ, de Knikker R, Wittink HM, Takken T. Eccentric overload training in patients with chronic Achilles tendinopathy: a systemic review. Br. J. Sports Med. 2007; 41:e3.
    19. Knobloch K, Yoon U, Vogt PM. Acute and overuse injuries correlated to hours of training in master running athletes. Foot Ankle Int. 2008; 29:671-6.
    20. Lappe J, Cullen D, et al. Calcium and vitamin D supplementation decreases incidence of stress fractures in female navy recruits. J. Bone Miner. Res. 2008; 23:741-9.
    21. Loudon JK, Dolphino MR. Use of foot orthoses and calf stretching for individuals with medial tibial stress syndrome. Foot Ankle Spec. 2010; 3:15-20.
    22. Marti B, Vader JP, Minder CE, et al. On the epidemiology of running injuries. The 1984 Bern Grand-Prix Study. Am. J. Sports Med. 1988; 16:285-94.
    23. Mayer F, Hirschmüller A, Müller S, et al. Effects of short-term strategies over 4 weeks in Achilles tendinopathy. Br. J. Sports Med. 2007; 41:e6.
    24. McKean KA, et al. Musculoskeletal injury in the masters runners. Clin. J. Sport Med. 2006; 16:149-54.
    25. Milner CE, Ferber R, Pollard CD, et al. Biomechanical factors associated with tibial stress fracture in female runners. Med. Sci. Sports Exerc. 2006; 38:323-8.
    26. Moen MH, Tol JL, Weir A, et al. Medial tibial stress syndrome: a critical review. Sports Med. 2009; 39:523-46.
    27. Niemuth PE, Johnson RJ, Myers MJ, Thieman TJ. Hip muscle weakness and overuse injuries in recreational runners. Clin. J. Sport Med. 2005; 15:14-21.
    28. Paoloni JA, Appleyard RC, Nelson J, Murrel GAC. Topical glyceryl trinitrate treatment of chronic noninsertional Achilles tendinopathy. A randomized, double-blind, placebo-controlled trial. J. Bone Joint Surg. Am. 2004; 86-A:916-22.
    29. Rome K, Handoll HHG, Ashford RL. Interventions for preventing and treating stress fractures and stress reactions of bone of the lower limbs in young adults. Cochrane Database Syst. Rev. 2005; CD000450.
    30. Running USA. Running USA's Annual Marathon Report [Internet]. 2011. Available from: http://www.runningusa.org/node/76115#76116.
    31. Shalabi A, et al. Am. J. Sports Med. 2004; 32:1286-96.
    32. Simons SM, Kindred J, Trubey C. Foot injuries in runners. Curr. Sports Med. Rep. 2011; 10:249-54.
    33. Taunton JE, Ryan MB, Clement DB, et al. A retrospective case-control analysis of 2002 running injuries. Br. J. Sports Med. 2002; 36:95-101.
    34. Thacker SB, Gilchrist J, Stroup DF, Kimsey CD. The prevention of shin splints in sports: a systematic review of literature. Med. Sci. Sports Exerc. 2002; 34:32-40.
    35. van Gent RN, Siem D, van Middelkoop M, et al. Incidence and determinants of lower extremity running injuries in long distance runners: a systematic review. Br. J. Sports Med. 2007; 41:469-80.
    36. Witvrouw E, Danneels L, Van Tiggelen D, et al. Open versus closed kinetic chain exercises in patellofemoral pain: a 5-year prospective randomized study. Am. J. Sports Med. 2004; 32:1122-30.
    37. Zafar MS, Mahmood A, Maffulli N. Basic science and clinical aspects of Achilles tendinopathy. Sports Med. Arthrosc. 2009; 17:190-7.
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