Why is it, when you run a long distance your legs feel heavy? Is this due to blood being pumped into your legs more or – Secondary 11

Question: Why is it, when you run a long distance your legs feel heavy? Is this due to blood being pumped into your legs more or is it because they ache? Lactic acid?
- Keywords:
  blood,
  exercise,
  lactic acid,
  pain,
  running
Asked by jiggles to Amylou, Maria, Mark L, Mark U, Faye, Martin, Mus, Pete on 21 Jun 2012. This question was also asked by ells8.
- Mark Uphill answered on 21 Jun 2012:
  
  Hi jiggles, it’s been a long time since I’ve run a long way…. I think there are probably a number of reasons why your legs could feel heavy, but if this is unusual or painful it might be worth checking it out…

Question: Why is it, when you run a long distance your legs feel heavy? Is this due to blood being pumped into your legs more or is it because they ache? Lactic acid?