Injuries occur when demand exceeds capacity. It is not sexier than that. Sure I could elaborate and expand the definition using more academic words, but that wouldn't be practical. When the demand in the form of 'force'; force from an impact from the ground, another human or an object, is greater than the strength of the muscle, ligament, tendon or bone that experiences the force then an injury will occur.
Injuries are considered either acute or chronic. Acute injuries are considered sudden and often traumatic. A chronic injury develops more slowly and may last longer. Acute injuries may stop or dramatically limit physical activity. Whereas chronic injuries may be managed through physical activity. Regardless, both acute and chronic injuries happen because the demand or force experienced by the body is greater than what the body can handle. However, HOW an acute injury happens is different to HOW a chronic injury can happen. The difference is important to understand if you want to prevent either from occurring.
Here is a simple graphic illustration of the how acute injuries differ from chronic injuries.
On the vertical line (Y-Axis) is the load or force the body experiences from lifting weights, running, swimming, playing tennis, anything that provides physical stress to the bodies soft tissue. On the horizontal line (X-Axis) is time. The farther you move up or to the right from where the lines start (origin) the greater the load and the longer the time. The blue line represents the tolerance or strength of the soft tissue (muscle, ligament, bone, tendon). The green line represents the load applied by physical activity. In an acute injury, the applied load increases suddenly, and at the point, the load exceeds the strength of the tissue an acute injury occurs. For example, if you fall with great speed, or get tackled in an unexpected way you may experience a break of a bone or a rupture of a tendon or ligament as a result of the force being greater than the soft tissue could handle. That may seem fairly intuitive. However, chronic injuries are a bit trickier to understand.
A chronic injury is still the result of the load being too much for the soft tissue to handle, however, not always because the force is too much. The soft tissue can be weakened by poor mechanics and over time the strength of the tissue is decreased to the point where a force that seems minimal is enough to the cause an injury. For example, let's say you have been running your whole life or swimming three times a week for the past ten years. You have never had an injury as a result of the running or swimming you have done. Then one day 'ouch' that feels weird. Your knee starts to ache, or your shoulder gets a sharp pain during a swim set. You don't understand why all of a sudden what you have been doing so easily for so long is now painful. Essentially, the strength of soft tissue was weakened due to faulty movement mechanics or muscle imbalances. That is why proper technique is so critical and using your body the right way matters most.
The objective of any training program should be to increase the margin of safety between the capacity of the soft tissue and the load that tissue experiences in sport, work or recreation. Moving the right way and using your body the way it was designed to be used while engaging in a progressive training program that systematically loads the body to make it more physically capable is what you need for a long active life.
So the next time you experience an injury, whether it is acute or chronic remember the injury occurred because the demand was greater than your soft tissue could handle. You can't always prevent an acute injury, but the majority of chronic injuries can be prevented. It almost always is not what you are doing, rather HOW you are doing it and whether the intensity is appropriate based on your movement competency and capacity.