Another reason that flexibility is so important in cycling is your lower back. When your hamstrings are/get really tight, it will cause lower back pain which will affect your power that you can produce and obviously lessen your comfort level. Additionally hamstring tightness can also translate into knee pain. All your muscles are connected and are designed to work together, when one set of muscles becomes tighter than others, especially ones that work in a contraction/elongation relationship (hamstrings and quads), your other supporting muscles/ligaments/tendons will negatively be affected.