Navasana helps us strengthen our core muscles, which hold the pelvis in neutral. There are several modifications for this pose. Choose the position that allows your pelvis to remain in neutral (not rounding in the lower back) and the chest open. With practice you will be able to proceed through all of the variations of this pose.
Sit with the knees bent and the soles of the feet on the floor.
Bring your hands behind the thighs and lift through the chest. Feel for a neutral position in the pelvis. The sitting bones will be grounding into the mat.
Activate the core to hold the pelvis in neutral and straighten the arms. This is the first variation of Navasana and will serve to safely strengthen the core muscles while keeping the lower back from rounding.
If you are feeling strong and stable, release the hands from behind the thighs, reaching the fingertips forwards with the palms facing in. If the lower back rounds, return to the previous variation.
Next, we can come onto the tips of the toes with both feet, lifting one foot from the floor, keeping the knee bent. You can lift the foot until the shin is parallel with the floor, as long as the lower back does not round. Take several ujjayi breaths here and then switch feet.
For an additional challenge, you can bring the arms behind you with the fingertips on the mat. Try your best to limit the amount of weight you transfer into the arms. This is an intermediary step that will offer additional strength and support for the next few variation. After lifting one leg at a time from the floor with success you may want to try lifting both legs. With the arms behind you for gently support, lift both feet from the mat, keeping the knees bent.
At this stage, draw the legs in towards each other to keep them active. Be sure to also keep the feet engaged by activating standing feet.
When this feels stable, the next variation is to bring the arms forward to shoulder height, reaching the fingertips towards the toes. Keep the chest open and the lower spine strong and straight.
The full pose simply straightens the legs. Remember to keep the legs active by drawing them towards each other with standing feet. If the lower back rounds or the hip flexors complain, return to a previous variation.