Adho Mukha Svanasana - Downward Facing Dog
The way I often do Down Dog in Yoga is already modified. I tend to be rounded in my low back (lumbar spine) due to short hamstrings and posteriorly tilted pelvis. If I try to straighten my knees and bring my heels down to the ground, I will be rounded in the low back.
In Down Dog, I look for neutral spine and not rounded, therefore I need to bend my knees and lift my heels up. When I teach this Asana, the biggest benefit of Down Dog is not to stretch hamstrings, but to create traction in the spine.
Our spine is not one long bone, but a lot of vertebrae - smaller bones stacking on each other with jelly discs between them to absorb shock as we move. In every single activity during the day that we do, we tend to be in an upright position and the spine is affected by the gravity of the Earth. The gravity of the Earth compresses the vertebrae down into each other. There's no daily activity that can reverse this process, and even in Yoga, even if you do inversions, your spine is still compressed. But Decompression is something Down Dog can do: with the spine hanging kind of upside-down, hanging from the pelvis and all the vertebrae are free to fall down toward the floor. This reverses the gravity impact.
With that being my main goal in practicing Down Dog, I strengthen my legs so that my hips can ¨hang¨ from them, letting loose of the spine in a neutral position for the spinal traction. I'm interested in making the spine neutral, which is very difficult, and then hamstring stretch comes secondary. It's actually very hard to keep neutral spine in Down Dog. To stretch the hamstrings, there are plenty of other Yoga poses that can do a better job, more simple and effective. But Down Dog is the only Yoga pose that can reverse the gravity impact on the spine.
Sometimes we need to bend the knees and lift the heels very much to keep neutral spine in Down Dog. You can see my post about Ardha Uttanasana, variation with the wall, to see the relevance.
I don't use props for Down Dog in my Yoga class because there are so many Down Dogs that it would take so much time and effort putting the props in and out throughout the whole class. Moreover, the best modification is to bend knees, lift heels. But if you want to try it, put blocks underneath your heels and press the heels down towards the blocks, making your legs stronger and therefore pushing the hips up higher and more backward.
¨But then when will I ever get my legs straight in Down Dog?!?!¨ Be patient, in Yoga Asana practice, we do a lot of Yoga poses and among which, a lot of hamstrings stretching poses such as: Pyramid Pose, Triangle Pose, Uttanasana, Paschimottanasana, Hand to Big Toe, etc. After a period of practicing Yoga Asanas, your hamstrings will gain flexibility and lengthen - it's a result of your persistent practice, and then, you can straighten your legs in Down Dog!
Bhujangasana - Cobra Pose
Urdhva Mukha Svanasana - Upward Facing Dog
Personally I prefer Cobra Pose than Upward Facing Dog, and I rarely teach Up Dog in Yoga classes for beginners (most of my Yoga classes). I think Cobra pose is a very effective backbend Yoga pose. Up Dog is more complex for adding the arm strength and shoulder strength into the backbend. Especially in a Vinyasa Flow, moving the shoulders through Chaturanga, Up Dog, Down Dog can be way too much for Yoga beginners.
A common modification for Cobra Pose is the height of the pose. Above is high Cobra.
High Cobra, low Cobra or medium Cobra or whatever, you can modify your Cobra pose in different ways so that every time you do Cobra you are bending your spine in a new way and therefore gaining flexibility throughout your spine. That's better than doing 100 Cobra poses that hit into one bendy spot on your spine. For Yoga beginners who haven't figured out their range of movements, you can start with a low Cobra Pose. However, that doesn't mean a High Cobra is more advance than a Low Cobra. Many healthy bodies can only backbend to a limited extend.
Wrists problem, or weak wrists and shoulders in general, which shows in shrugged shoulders when you do Cobra Pose. Then you can do Sphinx Pose instead, with your forearms on the ground. Forearms are stronger than hands, so now you are more stable. Notice that even in Sphinx Pose, we are still opening the chest forwards as we press the forearms and hands down to the ground to find more space in the chest and extension in the upper spine.
One modification is to hold the pose with your fingertips. This pose is also called Seal Pose. This pose gets rid of the force on your wrists, and make your back and shoulders work harder to hold the pose. When you straighten your arms in this fingertips variation, the backbend becomes deeper and you stretch more in your front body.
To many Yoga beginners, pressing the hip bones down to the hard ground is painful. We can fold a blanket and support underneath the pelvis. Probably a bolster under the rib cage to lift the pose up, bend yourself a bit more and still being soft and safe, no suffering. The arms are working less, and you can even lift your hands from the ground to only use your back strength for the backbend.
Upward Facing Dog is very strong, you need your whole body strength here to lift up. However, not everyone has this much strength, and in many cases, even when they can lift their body up, they can't control the engagement of the legs and belly, which make it too much bend in the low back, or shoulders lifting up to the ears.
When I teach Upward Facing Dog, I would put my toes down to the ground (the first image). Then, the leg muscles are stronger to hold the pose better. Moreover, I can move directly to Plank and Down Dog without relocating my feet.
Or we can come back to the soft bolster under the hips to help to lift us up into Up Dog. Instead of sinking down to the bolster, try lifting yourself away from it (even though it is still supporting you) to create more strength in legs and hips.