Bone stress injuries: RISK FACTORS (part 2)

Training Factors

Chronic introduction of high absolute load magnitudes, rates and accelerations may reduce bone fatigue life, particularly when the number of cycles is high (i.e. running long distances). However, the influence of these variables may be most prominent when an athlete attempts to progress their training. Increases in running speed increase GRFs and their rates of introduction. Increases in the duration and/or frequency of training sessions increase the total number of bone-loading cycles. In the absence of a change in the load-bearing capacity of the bone, altered loading associated with large changes in training may contribute to microdamage accumulation and the generation of a BSI.

Evidence from military studies confirms that those exposed to large changes in physical activity have a heightened BSI risk. For instance, recruits with a lesser history of regular physical activity prior to the commencement of standardised basic training are at a greater risk of developing a BSI. While most athletes do not introduce changes in their bone-loading environment to the same extremes as many military recruits, change is a frequent and required means of inducing adaptation. Incrementing training too rapidly or frequently relative to an athlete's usual activities is thought to be central to disrupting the balance between bone microdamage formation and removal.

Muscle Factors

Training changes may independently contribute to BSI development, but the relative risk associated with the change may be compounded by muscle factors. An intimate mechanical relationship exists between muscle and bone and the general hypothesis is that muscle generally protective of BSIs, particularly those occurring in the lower extremities. During impact loading, muscle acts as an active shock absorber, helping to reduce loads as they are transmitted proximally.

When muscles are dysfunctional (weakened, fatigued or altered in their activation patterns) their ability to attenuate loads becomes compromised, leading to increased loading of the skeleton. For example, fatigue results in a decrease in shock attenuation, increase in GRF loading rates and peak accelerations, and increase in bone strain rate and magnitude. In addition, fatigue can lead to altered kinematics, which may alter the direction a bone is loaded and result in increased bone strain at less accustomed bone sites. Further support for muscle's protective role in stress fracture development comes from prospective clinical studies which demonstrated that BSI susceptibility was directly related to muscle size and strength.

REFERENCES

Brukner, P., 2012. Brukner & Khan's clinical sports medicine. North Ryde: McGraw-Hill