They have a malfunction somewhere in the chain of chemical reactions by which glycogen should be broken down into pyruvate. This does not happen, and glycogen is therefore stored in excess quantities in muscle and other tissues. These 'unusable stores' may cause problems themselves, but in addition there is an acute lack of pyruvate when it is needed to provide energy. Strenuous exercise, eg sprinting or running for a bus, tends to bring on symptoms. Swimming, for instance, may be painless if you are doing it gently, but painful if you are exerting yourself.
Symptoms are mainly muscle pain and stiffness.
They come on during exercise and stop you from continuing. The pain occurs in the working muscles, and therefore is often felt in the legs. Stop exercising when you feel much pain otherwise you will be damaging your muscles. Each patient is likely to have a type and variety of exercise that they can perform safely. It happens because you started off using carbohydrates for energy and then switched to fat.