Are you constantly stretching your hamstrings but still feel that they are tight? It might be counter-intuitive, but there's a good chance that they are just tired and NOT tight.

Normal range of of hip flexion is 70-90 degrees with a straight leg. Most of the yogis and patients that complain to me about "tight" hamstrings are within this range. If their hamstrings were actually tight or short, they probably would not be able to reach this range of hip flexion. So... what's the deal then? Why do they feel so tight?!
That tightness that is felt may actually be the hamstrings saying, "Stop working me so hard!" Many people can have inhibited glutes and core muscles because we spend so much time sitting. The hamstrings act as synergists to the glutes that help posteriorly rotate the pelvis, extend the hips, and keep the low back in neutral. If the core and glutes are slacking off, the hamstrings have to pull extra load. Sitting can also irritate the hamstrings by putting pressure on the sit bones (ischial tuberosities) right where the hamstrings attach to the pelvis. All of this means that the hamstrings are actually overworked, sore and possibly strained rather than "tight."
That tightness that is felt may actually be the hamstrings saying, "Stop working me so hard!" Many people can have inhibited glutes and core muscles because we spend so much time sitting. The hamstrings act as synergists to the glutes that help posteriorly rotate the pelvis, extend the hips, and keep the low back in neutral. If the core and glutes are slacking off, the hamstrings have to pull extra load. Sitting can also irritate the hamstrings by putting pressure on the sit bones (ischial tuberosities) right where the hamstrings attach to the pelvis. All of this means that the hamstrings are actually overworked, sore and possibly strained rather than "tight."
If this is the case, you're better off activating the glutes, core, and hamstrings to "wake them up" and return them to health. Then that tight feeling will disappear and you'll be able to stretch and increase hamstring flexibility. Bottom line... if you have been constantly stretching your hamstrings, and they still feel tight, try this: sit less, extend (the hips) more, and STRENGTHEN your hamstrings!
Namasté
Namasté
Leg stretch picture from http://cdn-media-1.lifehack.org/wp-content/files/2014/07/Leg-Stretch1.jpg
Pelvic tilt graphic from Kinesiology of the Musculoskeletal System by Neumann
Hip flexion graphic from http://www.mhhe.com/socscience/hhp/fahey7e/labs/imgs/Lab5_1_Ave_Hip_Flexion_Straight_Leg.gif
Pelvic tilt graphic from Kinesiology of the Musculoskeletal System by Neumann
Hip flexion graphic from http://www.mhhe.com/socscience/hhp/fahey7e/labs/imgs/Lab5_1_Ave_Hip_Flexion_Straight_Leg.gif