What causes injury?

What causes overuse injury?

Injury is caused by stress. The main cause of overuse injuries is that our bodies cannot deal with stress when they are exposed to ‘too much, too soon, too hard’. Overuse injury is a complex phenomenon that is usually caused by multiple risk factors that cause excessive stress. On the submenu pages I look at some of these risk factors from a holistic perspective by including a range of risk factors that include anatomical to social risk factors. I discuss how underlying anatomical vulnerabilities interact with each other and psychological and social risk factors to lead to injury, and I also include spiritual dimensions.

On this page I discuss in a general sense how stress causes injury and then how risk factors can cause injury through excessive stress.


What is stress?

Since injury is caused by stress it is important to understand how stress works:

Wear and Repair

The human body is constantly exposed to physical stress, for example simply walking down the street causes your feet to absorb stress from contact with the pavement. (The same is true of the human mind, interacting with other people and dealing with the challenges of daily life will always cause psychological stress). This physical stress leads to ‘wear’ within the tissues. This is not a problem when the damage inflicted by wear is exceeded by the healing initiated by the body’s repair mechanisms. Stress causes injury when the process of wear exceeds that of repair.

Stress is not avoidable

Since even minor activities and movements cause stress, it is therefore impossible to completely avoid it. In fact, if you do try to avoid one stress you will often expose yourself to another stress which may or may not be worse than the original stress. For example if you were to try to avoid the stress that walking down the street inflicts on your body you may be causing extra stress by reducing the amount of exercise that you are undertaking. What you can do, however, is improve the way you handle stress and though sometimes it is possible to avoid a certain stressor I think that the main focus should be on how you improve the way that you handle stress.

The physics of stress

It is possible to use Newton’s laws of physics to explain how physical stress is generated.

1) an object in motion stays in motion, unless an external force stops it. So when you are running the external forces of things like the wind and friction exert an external force on your body which is the equivalent to a physical stress.

2) force equals mass times acceleration. When you are climbing stairs three times your body weight is passing through your joints and this force will increase the more weight you are carrying and the faster you are moving.

3) every force has an equal and opposing force. Going back to the running analogy, the more force that you hit the ground with, the more force that the ground hits your body back with.

How does stress cause injury?

Stressors (things that cause stress) vary according to their intensity and duration. A very high intensity and short duration stressor can cause an acute injury, a high intensity and long duration stressor can cause an overuse/misuse injury. However, in both cases the intensity of the stress can be varied by the way that the stress is handled. For example, a cyclist can fall off their bike in a crash but sustain minimal injuries if they roll when they hit the ground.  The way a stress is handled affects the intensity of the stress and therefore the severity of the injury.

Too much, too hard, too soon

Most overuse injuries are caused by suddenly increasing the stress on tissues by suddenly increasing the duration, intensity or the frequency of an activity see Poor technique.  This is because as explained above the process of repair cannot keep up with that of wear.

Stress can be silent

Unfortunately our bodies and minds don’t always tell us if we are under stress. Stress can be silent. For example, I can be reading text on a white page under a fluorescent light and feel fine, until the migraine strikes. This is because the visual stress caused by the rapidly flickering artificial light does not immediately cause much discomfort. I think that this applies to both mental and physical stress. I have noticed that when I spend time by myself I often do not feel stressed, but when I eventually return to interacting with people I can sense a feeling of built up isolation. Similarly with physical stress, if our bodies would tell us when we were putting them through too much stress it would be a lot easier to avoid overuse injuries. In the same way cartilage can be badly injured but you will not feel much pain because it does not have a nerve supply.

Stress likes to travel

Stress needs to be dissipated otherwise it will be transmitted to another area where it will cause damage.  Stress normally travels upwards or downwards or sideways i.e. it does not jump from one part of the body to another. For example, the stress from the feet can travel up to the knees and the stress from one knee can be transmitted to another knee. This is because the human body operates as a functional unit that is composed of sub-units. If one part of the unit is damaged it has knock-on effects to the other parts. Joints often work in pairs so if e.g. one knee is injured then the sub-unit of the knees has been injured and this puts more stress on the other knee and this stress is then passed upwards to other areas.

In society, stress may travel from one person to another – normally in a kind of downward motion from e.g. a person in authority to a subordinate.

Risk factors

In addition to doing things too hard, there are other things that affect the intensity of the stress, namely vulnerabilities aka risk factors. Everybody has underlying vulnerabilities that mean that their bodies or minds do not deal with stress, whether physical or mental, as well and this can make them more prone to certain overuse injuries in particular and possibly other kinds of injury too. These vulnerabilities are risk factors for injury. In the following pages in this section I look at various psychological, social and anatomical risk factors that all increase your vulnerability to stress.

Risk factors can be divided into intrinsic and extrinsic. Intrinsic risk factors are either anatomical, psychological/behavioural or connected to your technique. Extrinsic risk factors include the equipment you are using and social factors. Risk factors need to be identified and if possible addressed. In reality, the best ones to focus on are the psychological, technique and equipment risk factors because the anatomical and social are normally outside of your control, though you can develop strategies to handle them better. To understand how an injury works it is important to understand how different risk factors interact with each other. Relatively minor vulnerabilities can interact together with overuse to lead to an injury.

The model that I am using is widely used in sports medicine. I argue later in this website that this same model can be applied to mental health.

Further reading:

This is an excellent website about sports injury that also includes a page about overuse injuries.