Lie on back and lift knee up. Grasp behind thigh near knee with both hands. Pull knee close to chest.
Extend knee while maintaining knee close to chest. Hold stretch. Repeat with opposite leg.
Far leg can be bent at knee and hip particularly if hip flexors are tight and greater spinal flexion will not irritate existing lower back injury. Alternative method (pictured low right) involves pulling leg toward chest with knee nearly straight. Also see Lying Hamstring Stretch with towel.