Injuries happen when your tissues (muscles, bone, tendons, and ligaments) can not withstand a specific load. This load can be sudden, or it can be gradual. Most people can think of a time they hurt themselves, picking up something too heavy and feeling a twinge in their back. The cause was the weight, and the effect was you hurt your back. Harder to understand injuries are when the cause builds up over time, cumulative loading. Both sudden and cumulative loading cause your tissues to go past their load-bearing threshold, and injuries occur.
Most running injuries fall into the second category of injury type, cumulative loading. Why? With every stride you take, you are placing 2.5-3x your body weight through your tissues. Think about it, if you weigh 150lbs, 2.5x your body weight is 375lbs. This 375lbs is also landing just on one foot, repeatedly, throughout your 5k run. Then do this day after day on tissues that are not used to that amount of load, hello injury.
Let's set the scene. You have been running consistently for two weeks without days off after a winter of inactivity. You are trying to run faster, increasing your pace every few days. You go out for another run and notice your knee is sore. You finish your run, hoping it will go away, but it is still painful even when walking. Nothing different happened during the run, and it was the same route you always do. So, why did you get injured? The answer is your tissues could not handle the amount of running you did over the last two weeks. You did not take the time to build up your body's capacity.
Loading your tissues is not a bad thing. Actually, without loading your tissues, exercise can not exist. Your tendon, muscles and bones love to be loaded, and it's how they get strong. Proper loading is key to injury prevention, with rest days sprinkled in so your tissue can make repairs and get stronger.
Several other factors play into your injury rate. If you are tired, stressed, over-training and eating poorly, these factors will lower your injury threshold. They reduce your injury threshold because they make it hard for your tissues to repair themselves. Taking 1-2 days off a week, improving your sleep habits, strength training and eating better are essential factors in remaining injury-free.
So next time you get an injury and are unsure why, ask yourself, "Have I been sleeping well?" "Have I been taking enough time off of my activity to allow my tissues to repair themselves?" "Am I stressed out" and "Have I eaten nutritious meals?" Then you might get an honest answer.