Upward facing dog usually follows low push-up in the sun salutation sequences. It is a backbend and chest opener which invigorates and energizes the core of the body, encourages extension through the neck and chest, while warming the arms and grounding the hands.
Now, if your lower back feels ok, let's try the full expression of upward facing dog.
Test the flexibility in your lower back by trying cobra pose first, a variation of upward facing dog. Lay down flat on your mat on your belly. Relax for a moment and tune into your breathing. Notice the rise and fall of the inhales and exhales in the belly.
Bring the palms to the floor just under the shoulders. Press into the hands and lift the upper chest off the ground to any height that feels comfortable. Use the lower back strength to lift, rather than lifting by pressing into the hands. Let the legs be soft and relaxed on the floor. Come down and repeat, as you gradually lift higher, putting more weight into the hands.
Bring the palms to the floor on either side of the belly button. Spread the fingers wide and place equal weight onto the entire surface of the palms and fingers. As you inhale, press the weight of the upper body into the hands and lift straight upward as far as feels comfortable. Puff the chest through the arms, widening the collar bones. Make sure the palms are lined up just under the shoulders. Adjust the alignment of your shoulders by shuffling the tops of the feet either back or forward. Press the shoulder blades flat onto the back, relaxing the tops of the shoulders away from the ears. Lift the crown of the head up toward the ceiling and breathe deeply for 3-4 breaths. As you feel comfortable here, you may lift the pubic bone and tops of thighs off the floor by engaging through the legs.
Upward facing dog is a back bend and heart opener. These kind of poses tend to increase energy when you're feeling sluggish. Opening the chest in this vulnerable position promotes openness to new ideas and tolerance of others. Think of it as the opposite of arms crossed over the chest, a position we sometimes take when scared, unsure, or dissatisfied with a situation. The depth of the breath will enhance these effects, so be sure to take deep breaths once your in the posture. Rotating the shoulders back and down away from the ears frees the neck to release tension and irritability. This pose creates mobility in the lower back and stretches the abdominal muscles, which strengthens awareness of the core of the body. Awareness of the core body can impact the depth of breath, release anxiety, and promote emotional strength and resilience when practiced regularly.