Heavy legs can have various causes, ranging in severity from 'serious' to 'critical'. Finding the true cause means ruling out or confirming each possibility – in other words, diagnosis.
Diagnosis is usually a complex process due to the sheer number of possible causes and related symptoms. In order to diagnose heavy legs, we could:
Do your legs ever feel 'heavy', making it difficult to stand up, climb stairs, walk or stand?
Possible responses:→ Don't know
→ Sometimes slightly
→ Often slightly / sometimes severely
→ Often severely
Tired and heavy legs may indicate that the return blood flow from the legs to the heart is impaired. Gravity and impaired veins combined with possible heart problems result in oxygen-depletion in the legs, producing that heavy, tired feeling.