Injuries are caused when the loads are too great for the strength of the body. Unfortunately it is impossible to say what loads are excessive. This is due to individual differences in size, strength, geometry, age, and the individual response to training. For example, one athlete may be able to successfully train at 70 miles a week whereas another will have to limit mileage to 40 per week to avoid injury. Therefore, injury is caused from excessive loads for each individual.
Injuries can be acute or chronic.
Acute injuries such as a hamstring pull occur from one single big load. Chronic injuries occur from repetitive loads which in accumulation can become excessive over a whole training session or series of sessions. The major cause of distance running injuries are training errors. These account for about two-thirds of all running injuries. A training error is when you have attempted a training session that your body was unable to cope with.