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."
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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