Your legs could be feeling tight for a number of reasons. Your muscles could be seizing up as you run due to dehydration and a lack of magnesium. You could also be experiencing u003ca href=u0022https://pubmed.ncbi.nlm.nih.gov/12617692/u0022 target=u0022_blanku0022 rel=u0022noreferrer noopeneru0022u003eDOMSu003c/au003e after a previous workout that you haven’t fully recovered from.