Start kneeling on your mat with knees hip-width apart and hips directly over knees. Press your shins and the tops of your feet into the mat. Bring your hands to your low back, fingers pointing down, and rest palms above glutes. Inhale and lift your chest, and then slowly start to lean your torso back. From here, bring your right hand to rest on your right heel and then your left hand to your left heel. (If you can't reach your heels, turn your toes under; it will be easier to reach your heels in this modification.) Press your thighs forward so they are perpendicular to the floor. Keep your head in a relatively neutral position or, if it doesn't strain your neck, drop it back. Hold for 30 seconds. To come out of the pose, bring your hands to your hips and slowly, leading with your chest, lift your torso as you press the thighs down toward the floor.
Apply the above concept to your hips. When you sit, your hips are in a "flexed" position. Therefore, the muscles that flex your hips are in a shortened state. You probably spend at least a third of your day sitting down. Think about how much time those hip flexor muscles stay shortened. A lot. Over time, they become tighter and tighter until you look like the old man in the picture. So unless you want to look like that, perform the stretches shown below.
Grade II (moderate): A larger tear in your muscle that makes it difficult to move and causes a moderate amount of pain, especially when you move the affected muscle, swelling, and tenderness. You may have 5 percent to 50 percent loss of function and you may be limping. You can't go back to sporting activities until the tear is completely healed. These injuries can take anywhere from a couple weeks to a few months to heal, depending on how bad they are.