Shoes that have good heel cushioning and are flexible.
1. Flexible vs Stiff - According to the American Academy of Podiatry, a running shoe should be flexible in the last third of the shoe (the toe end); there should be resistance in flexing the middle third. A shoe that is very stiff will alter running mechanics and could cause problems with push-off, leading to calf and Achilles tendon problems.-- Dr. Cathy Fieseler
Scores range from 1-100. 100 being the most (stiff, heavy, ...)
Stiffer shoes on the other hand are recommended for trail running.