Most people get tired, aching legs at some time in their lives - usually from standing or sitting for long periods.
Those most likely to suffer are people who stand a lot without exercising their calf muscles, for example shop assistants, hairdressers and nurses. Even long-distance travel can cause discomfort and potentially more serious problems such as deep vein thrombosis.
When you walk or run, the calf muscles in the legs help to pump the blood in your legs back to the heart
However, during long periods of inactivity, blood can pool in the veins of the leg causing a feeling of heaviness and sometimes swollen ankles. This in turn can lead to aching and tiredness.
If these symptoms are allowed to persist over a period of time, it is quite likely that varicose veins will develop.
hope that helps,