What Causes Stress Fractures?

Believe it or not, stress fractures are actually not that unlike most other common running injuries. While bone is of course fundamentally different in structure to muscle and tendon, it is just like all tissues in as much as that our bones too experience a great deal of stress and strain as we run. Appreciating what causes stress fractures is the beginning of being able to recover quickly and return to running.

Bone can break-down and fracture under repetitive stress, in much the same way as muscles strain and tear when overloaded. Metatarsal stress fractures, tibial stress fractures and stress fractures of the hip are not at all uncommon in runners.

Running is of course a high impact activity. However, the human body is well suited to absorb the demands of training and racing, while everything is in balance. When there is a flaw in the system, or a mistakes are made in training, the injuries such as stress fractures, sometimes also known as hairline fractures, can unfortunately occur.

Stress Reactions vs Stress Fractures

In recent years, stress fracture diagnoses have expanded to include the sub-category of ‘stress reactions‘.

Think of stress fractures and stress reactions as a bony equivalent to the scale of severity used to describe muscular strains.

On one end we have a simple strain and on the other, we have a complete muscle tear or rupture. You can think of a stress reaction is essentially a ‘bone strain’. There is a clear breakdown in the bone structure that can be seen by bone scan, but it may not show up on a traditional X-ray. Left untreated, this reaction can progress to a clear stress fracture.

By comparison, true stress fractures or hairline fractures will show up on X-ray and bone scan alike because it is more severe.

The symptoms for both are similar: Pain during your activity that is gone at rest, swelling and pain that is mostly spot specific, but might radiate.

Understanding What Causes Stress Fractures

Let’s take a closer look at stress fractures in an effort to better understand what causes this frustrating running injury. When we understand why stress fractures occur, we can better avoid them.

External Contributing Factors

Of course, it’s very quick to blame the ‘wrong shoes’ or biomechanical issues. We have to also remember that sudden changes in training are probably the most common factor in runners sustaining stress fractures. Tibial stress fractures as well as stress fractures at the foot or hip are frustratingly common. When we dig into the history of a given case, a change in training load is often present.

Changes such as those in training intensity, volume, terrain, footwear, diet or even sleep can all be important factors in what causes stress fractures.

For the sake of this discussion, let’s assume that there have been no big changes in your training. Your running gear and diet hasn’t changed. Also that you have no family history of osteoporosis or bone density issues.

What does that leave us in terms of potential causes?

Decreased Shock Absorption

While the minimal-or-maximal running shoe debate rages on, the reality is that our muscles remain our primary shock absorbers. If you run with an efficient running stride, your muscles can dissipate much of the impact with each stride. This particularly occurs during the early part of stance phase in running gait.

That said, even before your foot hits the pavement, muscles are working to decelerate the swing forward of your leg. This happens in preparation for contact with the ground. When your foot strikes the ground, other muscles go to work to absorb the impact.

At the ankle, the muscles along the front of our shin contract to keep our ankle from collapsing. The quads contract to keep our knee from buckling under our weight just like the glutes do at the hip.

While you’re able to maintain an efficient running technique and you don’t overstride, the impact forces these major muscle groups have to work against are manageable. However, if form is compromised (usually with fatigue) the impact forces tend to increase with each stride. At this point, with poor running form, more impact is transmitted through our bones.

If you let your form slip, shock will be less efficiently absorbed and the chance of something breaking-down will increase. As far as bony injuries are concerned, stress fractures are often the result.

Mobility Restrictions

In order to walk normally, you need to be able to extend your hip. You also need to be able to fully straighten your knee and move forward over your ankle and foot.

If you do not have the motion to meet these requirements, you’ll probably be compensating as you walk. The resulting compensation may reduce your body’s ability to absorb the shock of impact.

This is equally true when it comes to running where those mobility requirements are even greater to allow for adequate shock absorption and stability. For a sport like running, cumulative fatigue and insufficient recovery can exacerbate this problem.

Muscular Imbalances

Like mobility, there are minimum strength requirements we must meet to be able to walk and run efficiently. Not only do we need the strength and stabiliy to repeatedly hop from one leg to the other (which is basically what running boils-down to), we also need to propel ourselves forward.

To make it more complex, we need to do that on varying terrain, at varying speeds, for hours on end. No matter how you look at it, that’s a lot of work. Add in cumulative fatigue and insufficient recovery and it’s easy for imbalances to start building. This can create compensatory movement patterns, sometimes resulting in adverse loads on areas of bone which aren’t able to cope with the stress.

Insufficient Recovery

As endurance athletes, our muscles are constantly in a cycle of breaking down and rebuilding to adapt to our training and racing demands. Scary as it sounds, this is a good thing, and the reality of how training adaptation works.

Just like our muscles and bones, the recovery process itself can be overworked as we ramp-up our training and race prep.

Eventually, this can result in the recover process stalling-out completely, and over training creeping in. Let’s also not forget, most of us aren’t full-time athletes. Not only do we need to recover from our sport, but we also need to recover from the stresses of our day-to-day life.

Improper recovery will lead to a compounding of any one of the issues discussed above. Developing an effective recovery strategy will help you keep running successfully for years to come!

Other Bone Health Factors Not to Be Overlooked

As mentioned briefly above, a history of osteoporosis in your family, dietary factors and the female athlete triad all have to be taken into account when understanding what causes stress fractures.

Now check out the second part in this series… Are You at Risk of Stress Fractures? [Video].

Last updated on March 2nd, 2021.