Hitting Hard on the Heels May Hit Hard on the Bones

Written by John Sloane

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.

Stress fractures occur when sub maximum strain is applied to the bone at a frequency that normal repair processes of osteoclasts and osteoblasts can’t handle.

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.

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