An article in the Journal of Strength and Conditioning Research (Vol. 23, no. 2, 2009, pages 660-667) described the comparison of various hamstring stretching techniques done for 30 seconds, 5 days per week. There were 100 subjects ranging in age from 21 to 57. The most effective stretch over the 8 weeks for lengthening the hamstrings was a straight-leg passive stretch. For this stretch, a protruding corner of a wall, a tall piece of furniture or other object is needed. The subject lies on the floor with the crotch at the corner and, with both knees completely straight, keeps the inner surface of one leg on the floor next to the wall and raises the other leg as close to vertical as possible, resting the heel against the other wall forming the protruding corner. This position is held for 30 seconds. Then the subject repositions to stretch the other leg similarly. In each subsequent session, the subject tries to bring the stretched leg more vertical by getting closer to the wall. The passive stretch was more effective than an active stretch in which the subject's hip flexor muscles, rather than a wall, were used to pull the leg toward vertical.
The study must not have included extremely flexible people like dancers and gymnasts who, while keeping one leg flat on the ground, can easily raise the other straight leg beyond 90 degrees. However, such athletes can hold a passive straight-leg position beyond 90 degrees by either having a partner hold the leg or by using a strap to hold the leg in position themselves.