Bone Stress Injury (BSI): What Active People Should Know and How to Prevent It

Exercise is the foundation of health, but if you are not careful it can also harm your body. This article looks at one of the bone injuries athletes may face — bone stress injury (BSI) — and how to prevent and treat it.

The basics of bone stress injury (BSI)

A BSI is an overuse injury caused by repetitive weight-bearing activity (such as running, jogging, or marching) and insufficient recovery time. It means the bone cannot tolerate the repeated load, producing localized bone pain and tenderness. BSIs commonly appear in people who love running, track and field, endurance training, and military training, as well as in gymnasts and dancers, and they can also occur in healthy people who are starting new training or training too intensely.

Risk factors for recurrent BSI or prolonged time loss include relative energy deficiency in sport (RED-S), disordered eating, metabolic bone disease, and overtraining syndrome. A BSI may also serve as a warning sign in high-risk individuals, indicating that the affected athlete is at risk of low bone mineral density.

In a normal, healthy bone, exercise or prolonged loading stimulates bone cells to regenerate: the body selectively breaks down parts of the bone and then, over 1 to 2 weeks, forms new bone tissue. The new bone structure becomes stronger and denser and adapts better to new activity or loading. However, if we continue high-load or high-impact activity during this regeneration period without giving the bone enough recovery time, the overloaded bone accumulates microfractures and eventually develops into a bone stress injury.

Bone stress injuries vary in severity. The earliest findings are periosteal edema and bone marrow edema. In more severe cases, a distinct fracture line appears. Fractures account for more than 10% of all exercise-related injuries, and in running this may be as high as 30%.

Bone stress injuries and stress fractures most often occur in the lower limb, with a recurrence rate as high as 22%. This type of injury can be divided into two main categories, low-risk and high-risk, which can be classified by anatomical site or by imaging.

BSIs often occur during periods of high training demand and can arise in elite athletes during adolescence and adulthood. Adolescence is a critical period for bone health: by age 18, about 90% of bone mineral density (BMD) has been accumulated, and peak bone mass (PBM) is reached at around the early thirties. BSI is associated with these two developmental milestones — the adolescent growth spurt (peak height velocity) and the development of peak bone mass (PBM). A bone stress injury may be the initial presentation of an underlying multisystem condition and may also be part of relative energy deficiency in sport.

Low-risk BSIs are susceptible to compressive stress and can usually be treated with activity modification. High-risk BSIs, on the other hand, are susceptible to tensile forces and occur in areas with poorer blood supply. They are prone to delayed or poor healing and are more likely to progress to a complete fracture at a later stage.

Risk factors for bone stress injury

These include low energy availability, such as menstrual irregularity, low body mass index, evidence of metabolic disturbance, and a history of high-risk bone stress injury.

Exercise-related risk factors include non-weight-bearing sports, overtraining syndrome, insufficient recovery, sports with weight requirements, and medication use (corticosteroids equivalent to prednisolone over 2.5 mg/day for 3 months).

Dietary and lifestyle factors include smoking status and excessive alcohol intake.

Diagnosing a bone stress injury

The diagnosis of a bone stress injury is usually made through a combination of physical examination, imaging, and bone scanning. Common symptoms include localized bone pain and discomfort that may worsen with running or walking, and may be accompanied by swelling and a feeling of instability.

The gold-standard method for diagnosing bone stress injuries (BSIs) is magnetic resonance imaging (MRI). Compared with conventional radiography (X-ray), MRI is the most sensitive and specific imaging method. Even when clinical signs and symptoms of a bone stress injury are present, X-ray images may look normal in the first few months. The injury usually has to progress to a fatigue fracture before subtle changes are seen on X-ray.

By imaging early with MRI, a bone stress injury can be detected so that appropriate measures can be taken to avoid subsequent fracture and delay. A stress reaction shows as swelling (bone edema) on MRI. More recently, diagnostic ultrasound has been used increasingly, and it can help with point-of-care detection and subsequent follow-up.

For athletes diagnosed with a bone stress injury, the clinician must measure bone mineral density and assess endocrine function. According to the recommendations of the American College of Sports Medicine (ACSM), a Z-score below -1 in female athletes in weight-bearing sports is considered low bone mineral density. For male athletes, no standard has been established, but the International Society for Clinical Densitometry (ISCD) regards a Z-score below -2 in both sexes as low bone mineral density, which can be used as a reference.

A recent BJSM-related study proposed a risk-stratified assessment for BSI in adolescents. This pathway includes:

1. Initial medical assessment (stage 1): performed by a sports medicine physician to confirm high-risk factors (dietary, medical, fracture history, and developmental).

2. If necessary, consider laboratory blood tests +/- bone mineral density assessment (DXA scan) (stage 2).

3. Contact a local pediatric rheumatology or specialist medical unit (stage 3).

Criteria for referring a patient from stage 2 to stage 3, that is, referral to a specialist center:

• A history of one or more high-risk (femoral neck, sacrum, pelvis) BSIs.

• A history of two or more low-risk BSIs (all other BSI sites) in the past two years, or absence from training of six months or more due to a BSI in the past two years.

• A lowest bone mineral density Z-score of -2 or lower.

• A family history of osteogenesis imperfecta.

• Menarche after age 15 or persistent secondary amenorrhea.

• Confirmation on the relative energy deficiency in sport (RED-S) clinical assessment tool (CAT2).

• Confirmed celiac disease.

• Low energy availability (below 30 kcal/kg/FFM).

• Other comorbidities or significant oral glucocorticoid use.

Treating a bone stress injury

Treatment involves understanding and correcting the factors that caused the injury, using a two-stage approach that includes activity modification and a gradual return to sport. The recovery time for low-risk bone stress injuries can vary depending on whether it is a simple stress reaction or a stress fracture that has already progressed, but the average healing time is 6 to 8 weeks.

Stage 1: Pain control

First, control the pain using analgesics and physical therapy. Because non-steroidal anti-inflammatory drugs may have a negative effect on bone healing, they should not be used. Weight-bearing walking within a tolerable range is allowed for daily activities, but in the early management of a low-risk bone stress injury the specific sport needs to be temporarily stopped and activity modified so the tissue can heal and pathology is limited.

Low-intensity pulsed ultrasound (below 0.1 W/cm2) is better supported by research, as there is some basis for its beneficial effect on complete fractures. However, the benefit of low-intensity pulsed ultrasound for acute BSI is still unclear. Extracorporeal shockwave therapy and electromagnetic therapy are also considered as ways to promote BSI repair, though current research is mainly limited to refractory BSI.

Stage 2: Physical conditioning

Cardiovascular endurance training: maintaining fitness during recovery from a bone stress injury is important, because endurance athletes show a decline in cardiopulmonary function within 2 weeks of stopping training. During recovery there are many ways to maintain cardiopulmonary function, such as swimming, cycling, deep-water running, and anti-gravity treadmill training.

Dietary adjustment to improve bone health: ensure adequate calcium and vitamin D intake. All athletes with a bone stress injury should be assessed to ensure adequate calcium and vitamin D intake. Based on age and sex, the target values published by the National Academy of Medicine (US) are as follows:

• For ages 9 to 70, a daily intake of 600 international units of vitamin D is recommended.
• For those over 71, a daily intake of 800 international units of vitamin D is recommended.
• For ages 9 to 18, a daily intake of 1300 milligrams of calcium is recommended.

Stage 3: Axial loading exercise

When an athlete has been pain-free in daily activities for five consecutive days, a specific progressive axial loading program can begin, with the aim of introducing controlled, progressive load at the injured site. Several different progressive loading programs can be individually tailored; these are usually 30-minute sessions spaced by rest days. These types of exercise are essential for optimizing bone loading so that it can tolerate the repeated, maximal loads of high-impact activities (such as running). Here are some examples of the various exercises that can be included in a cross-training program:

• Marching
• Single-leg squats or skater squats
• Weighted carries using a hex bar
• Cycling
• Treadmill walking
• High-intensity resistance training (HIRT)
• Gait retraining

Stage 4: Return to sport

The first phase is 30 minutes of walking, gradually increasing jogging time (jogging at 50% of normal speed) and reducing walking time. The second phase gradually increases jogging speed until jogging at normal speed for 30 minutes. The third phase is running two days in a row, then resting for one day. The final phase includes individualized jogging until a full return to the required running activity.

Progression through each phase depends on the level of pain at the bone stress injury site. If the athlete can complete a training session without pain and without underlying symptoms (that is, no pain that day, that evening, or the next morning), they can safely move on to the next phase of the program, knowing the injury can tolerate the load. If pain is felt at the bone stress injury site, the athlete needs to stop the training session immediately and, at the next session, return to the previous phase they were able to complete successfully.

The ideal training program progression is usually linear; however, in practice things may fluctuate back and forth. In the early stages of the rehabilitation program the pathology may be more troublesome, so it is important to pay attention to pain feedback.

Managing high-risk bone stress injuries

Most bone stress injuries (BSIs) are low-risk, because they heal readily without complications or the need for interventional treatment. However, BSIs at certain sites can be challenging to diagnose and treat, leading to delays and making them prone to poor healing or progression to a complete fracture. Recommendations for these high-risk sites depend on factors including the location of the BSI, the presence or absence of a cortical defect on imaging, the duration of symptoms and/or pathology, and the level of running (elite versus novice).

Prognosis and prevention of bone stress injury

There is not enough research validating classification by anatomical site. A 2023 BJSM meta-analysis found that the navicular bone of the foot (127 days; 95% CI 102 to 151 days) and the femoral neck (107 days; 95% CI 79 to 135 days) took the longest to recover, while the posteromedial tibia (44 days; 95% CI 27 to 61 days) and the fibula (56 days; 95% CI 13 to 100 days) recovered the fastest.

Overall, more than 90% of athletes were able to return successfully to their sport. Complication rates were highest at the femoral neck, tarsal navicular, anterior tibia, and fifth metatarsal, and lowest at the fibula, pubis, and posteromedial tibia.

Some past studies have examined the link between running biomechanics and tibial stress fractures. Although some biomechanical variables reached conclusions in individual studies, after meta-analysis there is currently no statistically significant difference in the relevant biomechanical variables among runners.

Final reminders for preventing bone stress injury

• Maintain good posture: poor posture leads to uneven loading of the body and puts stress on specific areas. Keeping good posture helps reduce the load on the body and lowers the risk of bone stress injury.

• Balanced exercise: don’t focus on only one type of activity; aim for balanced exercise that includes endurance training, strength training, and flexibility training, which helps different parts of the body develop evenly and reduces injuries from overuse.

• Adequate rest: give the body enough rest so the bone has enough time to regenerate, avoiding injuries from overuse.

• Balanced nutrition: make sure your diet includes enough calcium and vitamin D to help keep bones strong. You can also include foods containing nutrients such as potassium, magnesium, and phosphorus to help maintain the body’s balance.

• Appropriate training: a suitable training program helps the body gradually adapt to load and reduces the risk of injury. The training program should be individually tailored to your situation, because a program that fits you protects the body better.

• Watch for abnormal symptoms: if you notice abnormal symptoms in a part of the body, such as pain or swelling, rest promptly and have a professional check it to avoid the injury worsening.

• Wear appropriate shoes: choosing sports shoes that fit you can reduce the load on the body and lower the risk of exercise-related injury.

References

• Return to sport following low-risk and high-risk bone stress injuries: a systematic review and meta-analysis. Br J Sports Med. 2023 Apr;57(7):427-432.
• Biomechanics associated with tibial stress fracture in runners: A systematic review and meta-analysis. J Sport Health Sci. 2022 Dec 5;S2095-2546(22)00116-8.
• Management and prevention of bone stress injuries in long-distance runners. J Orthop Sports Phys Ther. 2014 Oct;44(10):749-65. doi: 10.2519/jospt.2014.5334.
• The MSK Playbook: Bone stress injuries

Further reading

• Rehabilitation exercise
• Sports injury overview
• Runner topics
• PRP prolotherapy
• Rehabilitation exercise prescription
• Pain management