A quick warning. This is a long post and parts of it will be geekie!! But I hope it will give you an insight as to how injuries occur and what actually happening to the injured tissue.
OK here goes….
There are 2 types of injury that the body can sustain:
- Acute injuries, which occur after single traumatic event and usually include sprains, fractures and dislocations.
- Chronic injuries that occur over a period of time, usually as result of repeated micro-traumas to tissues. Such injuries include stress fractures and tendonitis.
From a biomechanical perspective it is the chronic injuries we are most concerned about, although movements compensating for painful acute injuries can lead to chronic injuries e.g. if a person has a painful foot they will alter their gait to minimise weight bearing on the painful foot which can lead to increased stress on other tissues.
The Mechanics Bit
There are internal and external factors which can increase tissue stress.
Internal factors are specific to the person and include alignment of bones, muscle imbalance and weakness, reduced flexibility and instability of joints. Often it is not possible to change these factors, or if any change can be made it can take considerable time.
External factors are more controllable and include; poor technique, improper equipment and training errors such as sudden changes in duration or frequency of activity and type of activity.
For all elastic materials, which include most of the tissues within the human body, the 4 following factors apply. (Don’t worry about the equations, they’re just for geeks).
- Stress is a measure of how hard the molecules with in material are being pulled apart and can be written as: Stress = load/area.
- Strain is a measure of how far the molecules are being pulled apart and has the equation: Strain = increase in length/original length.
- Young’s Modulus is a measure of a materials stiffness or elasticity. It can be worked out with the following equation: Stress/Strain.
- Hooke’s Law states the extension or compression of a spring is in direct proportion with the load added to it.
F = –k X
X is the displacement of the end of the spring from its equilibrium (start) position.
F is the restoring force exerted by the material.
K is the spring stiffness.
In basic terms Hooke’s law means:
- When a stress (force) is applied to a material such as the achillies tendon, it will stretch.
- The amount of stretch, the difference between the original length and the stretched length, is known as strain.
- The amount of strain is proportional to the applied stress.
- When the force is removed the achillies tendon will return to its original state (see fig. 1).
Hooke’s law holds true until the material e.g. achillies tendon is stretched beyond its elastic limit. At this point the material will either:
- When the stress is removed the material will not revert back to its original position.
Fig.1 Typical Stress/Strain Curve
The Biology Bit – What actually happens in the damaged tissue.
Cells in healthy living tissue are constantly subjected to various external mechanical forces. These forces greatly influence various cell functions such as protein secretion and reproduction rate.
Different tissues are subjected to different stresses e.g. tendon cells are usually subjected to tensile forces (stretching) and bone cells are subjected to compression.
Mechanical loads from physical activity are extremely important and beneficial to bone and muscle as they cause stem cells to proliferate faster, which builds mass and strength, reducing the risk of injury and conditions such as osteoporosis.
Laboratory tests have shown if mechanical loads are too great, stem cells reproduce at an increased rate, however some stem cells mature into other types of cell.
e.g. tendon stem cells under low mechanical stress will increase the rate of cell division and the new cells will mature into tendon cells. Where as tendon stem cells under high mechanical stress will increase rate of division but some will mature into non-tendon cells such as fat cells and bone cells. This results in fat accumulation, mucoid formation and tissue calcification which are typical features of tendonopathy.
If high mechanical loading continues, the number of non-tendon cells will continue to increase. This leads to a decrease in tendon strength, resulting in a greater chance of more severe and persistent injury. At this present moment it is unknown how much degeneration has to take place before pain is felt and how much force has to be applied to tendon stem cells to cause them to mature into non-tendon cells.