To achieve this stretch, start out on your hands and knees. Slowly widen your knees out as far as they can go, and make sure to keep your lower legs in line with your knees. Your lower legs and feet should be parallel with one another. Next, ease your upper body forward on your forearms and reach forward until you feel the stretch. Hold this pose for about 10 seconds.
Keep it a one joint stretch. Many people want to jump right to performing a hip flexor stretch while flexing the knee. This incorporates the rectus and the psoas, but I find far too many people can not appropriately perform this stretch. They will compensate, usually by stretching their anterior capsule too much or hyperextending their lumbar spine.
A sedentary lifestyle can lead to having weak and tight hip flexors as they are always in the shortened position. Tight hip flexors can lead to a limited range of motion, poor posture, lower back and hip pain, and even injuries. These muscles need to get a workout when you are standing and doing movements such as raising your leg to climb stairs, run, or ride a bicycle.