Test: This test to assess hip flexor tightness - also known as the Thomas Test - is relatively straight forward. Find an elevated physical therapy table or simply any elevated padded surface - for example a bed. Now lie on your back with both legs dangling off at the gluteal fold (at the upper thigh). Pull one knee towards your chest while the other leg remains extended. Pull this leg to the chest until the lower back touches the testing table. Note whether the opposite leg rises off the table.
Movement: Lie down on an elevated surface with both legs extended off the surface at the gluteal fold. Your back, butt and the top portions of the thighs should rest on the surface, with the rest of your legs hanging off. Now take the knee of the leg to be stretched and pull it to the chest. Do not allow the opposite thigh to lift off the table - this will be a strong indicator of tightness in the hip flexors.
Repetitions: Hold for the full prescribed time on one side, then switch sides and repeat. This counts as one set.
Instructions and effects for the Lying Hip Flexor Stretch exercise 
Movement in Sagittal Plane