What is bone stress?
Like most of the body, bones are living tissue that break down and regenerate themselves in response to the stress and activity they are exposed to. Bone stress occurs as a response to an external force, triggering the bone to adapt and remodel in preparation for similar activity. If there is insufficient recovery time or this remodeling occurs too slowly, an imbalance in created between bone formation and the breakdown of damaged bone. This may lead to the presentation of a bone stress injury.
Stress reactions to fractures
Bone stress occurs along a continuum – from normal levels of bone stress (that remodel appropriately before further activity) towards a stress reaction (where bone microdamage is present) and furthermore a stress fracture. If identified early, injuries that are classified as a stress ‘reaction’ may be managed to prevent the progression towards a fracture.
What causes bone stress?
Bone stress injuries are often multifactorial, with these factors able to be placed into two groups:
- Factors modifying load placed on bone
- Factors influencing the ability of bone to resist load and recover from damage
Athletes may have one or two clearly identifiable factors, or a combination of multiple that have contributed to the development of bone stress. The following risk factors may be worth considering in the endurance athlete:
Training loads (volume, intensity, frequency)
Training load is often one of the most significant factors and contributors to the development of bone stress. Rapid spikes in volume (amount) of training, or increases in training frequency that lessen recovery times between sessions, may overload bony structures and shift the balance between bone breakdown and remodeling.
The volume of training an athlete is able to withstand is largely dependent on the training load they have been exposed to previously (chronic training load). If they are completing an acute training load much higher than this, they risk overload-related injuries such as the presence of bone stress.
Biomechanical factors such as limb alignment, muscle strength and movement patterns may alter the amount of load placed upon bone when completing certain activities. Bones tend to better withstand vertical (compressive) forces over horizontal (shearing) forces, meaning the loading response to an activity may be influenced by changes in body positioning or alignment. Surrounding muscles also work to withstand these forces, resulting in potential for higher bone loads in the presence of muscle weakness or fatigue.
Energy availability is the measure of difference between an individual’s energy intake and their exercise energy expenditure (above that of usual daily living). Low energy availability presents as a mismatch between intake and expenditure, leaving the athlete with insufficient energy to support healthy physiological function and recovery following exercise. This can contribute to hormonal & menstrual dysfunction, poor bone health and compromised immune function. The presence of low energy availability places athletes at considerable risk of a bone stress injury, due to insufficient energy for recovery and bone remodeling post exercise.
Bone density can be influenced by a number of factors including genetics, medication use, hormone function and diet, particularly during key periods such as adolescence and menopause in women. Children and adolescents completing high impact, multi-directional sports (such as basketball or soccer) tend to develop high bone density as a result. Conversely, athletes in repetitive or lower load sports (such as swimming or cycling) may not have been exposed to these protective loads.
Low bone density places an athlete at risk of developing a bone stress injury, given the reduced capacity of their bone to resist loads and recover from accumulative damage associated with training.
Nutrition (calcium & vitamin D)
Calcium provides strength to developing and remodeling bone, whilst vitamin D promotes the digestive absorption of calcium. These nutrients are considered highly important in young developing athletes, in addition to those completing high training loads and therefore having higher bone remodeling activity in recovery. Vitamin D levels have been inversely related to time lost due to bone stress injury in NCAA collegiate runners.
Bone stress injuries can be highly challenging for any athlete, resulting in a long recovery process and time away from the sport they love. It is therefore important to consider a range of factors and have a high index of suspicion for bone stress when working with endurance athletes.
If you have any questions or would like any guidance about preventing or managing an injury, please contact us.