People who do physical jobs can suffer from a host of injuries. One of the areas of the body that is likely to suffer from an injury is the knee. Knee injuries can be caused by an accident, such as a fall, or by the constant stress on the joint during the course of a normal work day.
If you work with electricity, you're bound to get shocked sooner or later. It's just the nature of the business. Maybe you are connecting two wires and you forgot to switch off the breaker, and you soon find out the error of your ways.