Stress fractures are amongst the most frequently diagnosed injuries in athletes. Some reports suggest that the rate of tibial stress fracture could be as high as 33 – 55% of all diagnosed stress fractures.

Running stresses are applied to the bone in two different ways;

1) External ground reaction forces (GRF) which result from the impact of the foot with the ground and;

2) Internal muscle reaction forces which are applied to the bone in response to the impact loading.

Vertical ground reaction force has two peaks when running with significant heel strike. The first peak is a rapidly applied transient shock created as the foot collides with the ground, while the more slowly applied second peak is the maximum absorption vertical force (see the graph below).

It has been suggested in the past that stress fractures occur as a result of high magnitudes of impact loading on bone, during physical activity. However it is now thought that it is not the magnitude of the loading on the bone but the rate at which the force is applied. This theory would suggest that it is the first rapidly applied transient impact load that is implicated in bony injuries in the athletic population.

So how can we reduce the amount of loading of ground reaction forces?

It seems that excessive over striding will increase the impact loading on the foot and the tibia which will increase bending stress on the tibia. By reducing the landing distance from the centre of mass and the angle of the tibia, the first GRF peak and loading rate is greatly reduced. This will reduce the external stress on the bone and also reduce the internal reaction forces of the muscles in the lower limb.

As part of a post stress fracture rehabilitation program it is important to look at key factors that could indicate the presence of high rates of impact loading while engaged in physical activity. It is important to formulate a plan that can help to reduce the shock applied to the tibia.

Case History

A young athlete was offered an opportunity to compete in the New York Marathon but only had 9 weeks to prepare for the event. At the start of his training program he was poorly conditioned but had just brought a new pair of running shoes and thus felt quite prepared for the training ahead.

In earnest training started but as the running distances rapidly progressed to 15km per session over only two weeks, pain started in the medial left shin. This pain was subsequently diagnosed as a tibial stress fracture.

This runner committed two very significant training errors. The impact loading was increased too rapidly and starting a training program with poor muscle conditioning was also a problem.

Running training resumed after 8 weeks of rest at which time a running assessment showed that our runner had a very excessive over striding style. We would suspect that this over striding along with the rapid increase of training and poor muscle conditioning significantly contributed to bony injury. After completing a programme designed to retrain the brain and body, running restarted with a reduced stride length, increased muscle conditioning, and a safe training programme. I hope he is able to compete in the next New York Marathon.

Running technique, injuries and running corrective exercises will be discussed at the Running Technique and Injuries Workshop in June/July 2011.