Bone Stress Injuries Part 2 – Who is at Risk?
Updated: Mar 31
As the running season continues to build towards the ultra’s we will continue to uncover Bone Stress Injuries (BSI). In Part 1 (here) we discussed the clinical diagnosis of BSI from their nebular presentation in the early stages to the more focal presentation of a stress fracture. In part 2 we will discuss who is at risk and attempt to identify the internal and external factors that may play a role in the development of BSI.
In the running community the identification of risk factors is difficult due to methodological research challenges and thus many of the contributing factors remain unproven. The physiology behind BSI remains outside the scope of this article but the proposed microdamage-centric pathophysiology allows us to look for clues of potential risk factors in other cohorts. Microdamage is defined as the load applied to a bone that is below the threshold for fracture development. Bone loading and microdamage is a homeostatic relationship that aims to maintain optimal bone health. When the load applied to the bone outweighs the ability of the bone to resist load without accumulating damage, BSI may occur.
The risk factors can be divided into two broad groups, (A) load applied to the bone and (B) factors affecting the ability of the bone to resist load.
A. Factors affecting the load applied to the bone
Abnormal bone loading can occur when load is increased through a normally aligned lower extremity and when a normal load is applied to an abnormally aligned lower extremity. Although the relationship between static postures and movements patterns remain debateable. Research has demonstrated that high arched individuals had a greater risk of BSI development. Other static variables reported to elevate BSI risk include increased hip external rotation range, and leg length discrepancy. There appears to be a relationship between altered movement patterns and BSI risk as they change the nature of the load on the bone and may load areas of bone that are less-accustomed. These patterns include increased hip internal rotation, peak rear foot eversion in the frontal plane whilst running and decreased knee flexion in the sagittal plane.
Increased training load results in increased bone-loading cycles. As bone-loading cycles increase they need to be matched by bone adaptation otherwise a negative balance is created, and the BSI risk is increased. This often occurs when athletes increase training suddenly. Training factors are closely related to the other risk factors that will be discussed below and should always be viewed as part of the bigger picture. Training history is also important as it has been demonstrated that individuals with a history of impact activity participation have a lower risk when compared to individuals with a sedentary background. This is an important factor for Strength and Conditioning (S&C) coaches working in team environments, especially in the adolescent population, as the group is often filled with a diverse range of participants and exposing them all to the same load will lead to a range of different responses.
Muscles and bones are closely associated in the dissipation of force through the kinetic chain. An increase in muscle strength appears to be protective against BSI. However, it is not only absolute strength that needs to be considered, muscle fatigue and altered activation patterns also play a role in BSI risk. The relationship between muscle fatigue and BSI appears to be directly proportional. Muscle fatigue also affects force distribution through altered lower limb kinematics.
There is a historical association with running surface and BSI risk. BSI risk is proposed to elevate when running on harder surfaces (e.g. asphalt) compared to softer surfaces (e.g. grass, rubber and sand). However, as we are starting to learn with most things in sports medicine, the picture is always more complex. An interesting finding is that athletes appear to alter their leg stiffness depending on the surface they run on in order to maintain normal force exertion on the kinetic chain. Research has failed to show an association between running surface and injury even though scientifically the ground reaction forces appear to be elevated when running on less compliant surfaces. In saying this, a sudden change of surface could play a role in an elevated BSI risk.
For S&C coaches this is an important factor to consider as during off and preseason cardio-vascular training is often done on a different surface (e.g. road running for grass-based sports.
Other factors to consider when planning athletic development programs:
Less compliant surfaces may increase bone strain forces,
Very compliant surfaces (sand) increase energy expenditure and may influence muscle fatigue,
A reduction in shock attenuation during downhill running,
Altered terrain may lead to the loading on skeletal sites unaccustomed to the applied load.
Shoes and inserts
Shoes and inserts remain an ongoing area of debate with most evidence completed in other cohorts; it remains contentious to draw direct comparisons with the running population. There are two possible theoretical frameworks that would support the use of inserts and shoes:
As insoles and shoes are located at the foot-ground interface they theoretically could dissipate the ground reaction forces.
They may influence foot and ankle kinematics and thus influence force distribution up the kinetic chain.
Although these theoretical models offer insight into the possible uses of insoles and shoe selection on managing BSI, further research is required before concrete advice can be given.
B. Factors affecting the ability of the bone to resist load
Many of the factors below remain out of the scope of practice for many practitioners, with the management of these issues requiring appropriate referral. However, increased awareness and the ability to identify when these factors are at play could prove invaluable to the athlete in the primary stages of any injury. As mentioned in part 1, many S&C coaches are on the front line of the sports medicine field and so being aware and referring when appropriate is vital to ensuring optimal athlete well-being. There are three main modifiable factors that affect the ability of bone to resist load in running athletes.
Physical activity history
The greater the activity history of an athlete the less likely they are to develop BSI. As the bone is loaded through years of physical activity it adapts positively and the bone mass and structure are affected to create a resilient area for further loading in the future.
Energy availability is gaining increasing attention in both female and male athletic populations. I would like to direct you to the article in the SSC collection (click here)
Vit D and Calcium Status
Calcium and Vitamin D play an integrated role in the maintenance of bone health. Should any athlete be suspected of developing a BSI, their Vit D and Calcium levels should be assessed. There is evidence to suggest that athletes may need greater than recommended dietary allowances of 1000-1300mg of calcium and 600 IU (individuals between the ages of 14-50 years of age) in order to prevent BSI.
The array of risk factors for BSI developments underlines the complexity of the condition. As discussed, BSI appear on a continuum rather than a defined injury. Many of the risk factors also have the potential to be graded and exposure may not be clear initially. All sports and medical practitioners should maintain a high index of suspicion when dealing with running athletes who may display the clinical signs and symptoms and any of the risk factors mentioned in the article. In Part 3 we will discuss the classification of BSI, until then safe training.
Management and Prevention of Bone Stress Injuries in Long-Distance Runners. Warden, S.J, Davis, I.S & Fredericson, M. 2014, Journal of Orthopaedic & Sports Physical Therapy, pp. 749-765.