Yogi Beware: Make Your Practice Safe
Once you're in Padmasana, see if there's any disparity between the heights of your knees. One knee is usually a bit higher—typically, the one you fold into the pose last. This usually isn't a problem unless the difference is large, in which case you're probably creating strain in the knees and would be better off sticking with preparatory work for the time being.
If either of your ankles is "sickling" (the joint curves and the foot rolls over its outer edge the way it would if you sprained your ankle), you're increasing mobility in the outer ankle ligaments, which is where you want stability. You're also increasing the risk of spraining your ankles. Instead of curving, the outer ankles and heels should be directly in line with the outer shins.
TO AVOID INJURY ON SEATED FORWARD BENDS, including Paschimottanasana, move into them by tilting your pelvis, not your spine, forward. Your pelvis should rotate easily toward your thighs, the back of your pelvis should slant toward the floor, and you should feel the stretch in the meat of the hamstring muscles at the middle of the back of your thighs (not at the back of your knees), at your sitting bones, or in your lower back.
If you've been diagnosed with disk disease or if you have pain radiating through your buttock(s) and/or down your leg(s), avoid seated forward bends until you consult a health professional and an experienced yoga teacher about whether these poses can be healthy for you now. If they give you the go-ahead, follow their personalized practice guidelines very carefully.
Also avoid seated forward bends if your lower back rounds backward when you bend forward; this means you're creating the forward bend from your spine, rather than from your pelvis. If your pelvis and sacrum slant forward in Paschimottanasana, you can probably proceed safely with seated forward bends. But if your pelvis and sacrum slant back when you try to bend forward (or if your chest collapses, your shoulders hunch, and your upper back rounds significantly), you should do more preparatory work. All this rounding is a strong sign that your spine is moving but your pelvis isn't.
TO PRACTICE MARICHYASANA III SAFELY, make sure your pelvis rotates in the same direction as your spine during the twist. To do that, sit in Dandasana (Staff Pose), with your spine long and your legs straight in front of you. Place your weight toward the front edge of your sitting bones so your pelvis and lower spine don't slump backward. (You'll try to maintain this alignment throughout Marichyasana III.)
Next, bend your right knee toward your chest, placing the sole of your foot on the floor near your right sitting bone. On an exhalation, hug the right leg with your left arm and slide your left leg and the left side of your pelvis several inches forward. Do not hold the pelvis stationary as you turn the spine. Doing so separates the sacrum from the ilium; the sacrum is pulled with the rest of the spine into the twist, while the pelvis remains behind, creating an overly loose joint and pain associated with sacroiliac dysfunction.