Bone Stress Injuries (BSI) in Runners
Bone stress injuries (BSIs) are among the most complex and commonly misunderstood injuries in endurance sports. Despite their prevalence—comprising over 10% of all sports-related injuries and up to 30% of running-related injuries—many runners attempt to train through them, often delaying proper diagnosis and increasing recovery time. This article will help you understand the key aspects of BSIs, including risk factors, symptoms, diagnostic challenges, and recovery timelines, so you can take proactive steps to protect your bone health.
What is a Bone Stress Injury?
BSIs occur when a bone is unable to withstand repetitive loading, leading to a breakdown in its structure. These injuries exist on a continuum:
Stress reaction – Early-stage inflammation and microdamage in the bone.
Stress fracture – A progression from stress reaction, where a visible fracture line develops.
Unlike typical fractures caused by a single traumatic event, BSIs result from cumulative stress exceeding the bone’s ability to repair and adapt.
Why Do Bone Stress Injuries Occur?
Bone is a dynamic tissue that constantly remodels in response to mechanical stress. This process involves:
Bone resorption (breakdown of old bone tissue)
Bone formation (rebuilding and strengthening)
For bones to remain strong and adapt to increasing training loads, the following conditions must be met:
1. Adequate Energy Availability
Low energy availability (LEA) occurs when athletes fail to consume enough calories to support both training and essential bodily functions. This condition is central to Relative Energy Deficiency in Sport (RED-S) and can lead to:
Decreased bone mineral density (BMD)
Increased bone resorption
Impaired healing and higher BSI risk
2. Hormonal Health
Hormones play a vital role in bone metabolism. Disruptions due to LEA can result in:
Low estrogen/testosterone – Impairs bone formation and strength.
Amenorrhea in females – Irregular or absent menstrual cycles are linked to decreased BMD and increased BSI risk.
Hypogonadism in males – A condition where testosterone levels drop, affecting bone health.
3. Vitamin D and Calcium Levels
Vitamin D is essential for calcium absorption and bone remodelling.
Calcium is a critical component of bone structure. Deficiency increases the risk of BSIs.
4. Training Load Progression
A sudden increase in mileage or intensity without allowing time for adaptation can overload bones, leading to injury. Gradual progression is key to preventing BSIs. It's crucial to consider total bone load, not just individual activities. Even if running volume increases gradually (e.g., 3x per week), additional rugby play can cumulatively overload the bone, leading to excessive stress despite each activity seeming sensible on its own.
5. Sleep and its effect on Bone recovery
Sleep is a critical regulator of bone remodelling and recovery following mechanical loading, directly influencing the balance between osteoblastic bone formation and osteoclastic resorption. During slow-wave sleep, the body secretes pulsatile growth hormone (GH), which stimulates insulin-like growth factor-1 (IGF-1), a key mediator of osteoblast proliferation and differentiation. Osteoblasts are responsible for synthesizing osteoid, the organic matrix of bone, which later mineralizes to enhance skeletal integrity. Disrupted or insufficient sleep leads to dysregulation of GH and IGF-1, reducing osteoblastic activity and impairing bone formation. Additionally, sleep deprivation increases circulating cortisol and parathyroid hormone (PTH), both of which promote osteoclastic activity and accelerate bone resorption. Chronic sleep restriction has been linked to lower bone mineral density (BMD) and decreased expression of genes essential for bone matrix production, such as RUNX2 and COL1A1. These disruptions contribute to an imbalance in bone turnover, weakening skeletal structures and increasing susceptibility to bone stress injuries (BSI). Given that BSI occurs due to an inability of bone to sufficiently adapt to repetitive mechanical loading, optimizing sleep duration and quality is essential for maintaining bone strength, supporting osteogenesis, and reducing injury risk in athletes and individuals with high training demands.
High-Risk Sites (More prone to complications and delayed healing)
High-risk stress fracture sites, such as the femoral neck, anterior tibia, navicular, and proximal fifth metatarsal, carry a significant risk of complications due to their limited vascular supply and high mechanical loading. According to research in the British Journal of Sports Medicine (BJSM), these fractures are prone to delayed healing, non-union, and progression to complete fracture if not appropriately managed. In particular, femoral neck stress fractures can lead to displacement and avascular necrosis, while navicular fractures often present with subtle symptoms but can result in chronic pain and functional impairment if missed. Given these risks, early diagnosis using MRI and strict offloading protocols are essential to prevent long-term complications and ensure successful recovery.
Femoral neck
Anterior tibia
Navicular
Fifth metatarsal
Patella
Medial malleolus
Low-Risk Sites (Typically heal reliably with proper management)
Tibial shaft
Fibula
Calcaneus
Metatarsals (excluding fifth metatarsal)
Sacrum
Recognizing the location of pain and its progression is essential for early detection and intervention.
How Are BSIs Diagnosed?
Common Symptoms:
Localized pain – Pinpoint tenderness over a bone.
Pain at rest or at night – A hallmark of more severe BSIs.
Pain that worsens during a run – Unlike tendon injuries, which often improve as a run progresses, BSI pain typically worsens over time.
Swelling or palpable callus formation – Indicates the body’s attempt to heal.
Imaging for Diagnosis:
X-rays – Often appear normal in early-stage BSIs.
MRI – The gold standard for detecting stress reactions and fractures, classified using the Fredericson grading system:
Grade 1: Periosteal edema (early-stage stress reaction)
Grade 2: Bone marrow edema visible only on T2-weighted MRI
Grade 3: Bone marrow edema on both T1 and T2-weighted MRI
Grade 4: Cortical fracture line, indicating a full stress fracture
Diagnosing bone stress injuries (BSI) is often more complex than many assume, and these injuries are frequently missed, increasing the risk of complications. As mentioned earlier, pain is not always present at rest, and its severity often does not correlate with the extent of the injury. In our clinic alone, we have recently identified 12 BSI cases, three of which were partial or complete fractures—remarkably, one of these fractures presented with the least reported pain or symptoms. This highlights the need for a thorough clinical assessment rather than relying solely on pain levels. The decision to refer for MRI should be based on both the patient’s reported symptoms and their individual risk factors for BSI. For example, a patient with mild but persistent stiffness around the anterior hip and thigh, combined with a gradual increase in running volume, may not raise immediate concern. However, if they have a history of nutrient deficiencies, previous stress fractures, menopause, or other risk factors, they should be considered at higher risk and investigated accordingly to prevent progression to a more severe injury.
Recovery Timelines and Management
The severity of a BSI determines recovery time:
Grade 1: ~11 weeks
Grade 2: ~13.5 weeks
Grade 3: ~18.8 weeks
Grade 4: ~31.7 weeks
Recovery times for bone stress injuries (BSI) vary widely depending on individual factors such as injury severity, anatomical location, and personal risk factors. High-risk sites with poor vascular supply, like the navicular or femoral neck, typically require longer healing times and more cautious management. Additionally, biological factors such as age, sex, hormonal health (e.g., menopause, low energy availability), and nutritional status (e.g., vitamin D and calcium levels) significantly impact bone remodeling and recovery. Training history and overall load management also play a role—athletes with a history of previous stress fractures or those returning too quickly may experience prolonged recovery. Given these variables, rehabilitation should be individualized, with return-to-sport decisions guided by symptoms, imaging, and progressive loading protocols.
Key Principles of BSI Recovery
Unloading the Bone – Avoid weight-bearing activities until pain-free.
Addressing Underlying Factors – Energy availability, hormonal health, and training errors.
Gradual Return to Running – A structured reloading program under professional guidance.
Strength and Conditioning – Progressive loading to improve bone resilience.
Conclusion
BSIs are serious but preventable injuries. Recognizing the signs early, addressing risk factors, and following a structured recovery plan can significantly improve outcomes. If you suspect a BSI, seeking a professional assessment is crucial to ensure a safe and timely return to running.
