Kurmasana (Turtle Pose) is one of the more memorable floor postures of the Ashtanga Primary series with it's distinctive shape and sneaky strength. Kurmasana tones the abs, spinal extensors, and quadriceps as it stretches the hamstrings and opens the hips in preparation for Supta Kurmasana, one of the prerequisite postures for moving on to the Intermediate series.
To enter Kurmasana from Adho Muka Svanasana (Downward Facing Dog), step or jump the legs around the upper arms and lower to the floor. Take a moment to work the knees even further up the triceps, then walk the hands back at an angle equal to that of the legs. Straighten both the arms and the legs as you take the chest and chin to the floor. If the chest comes down without any discomfort felt in the back, flex the feet and press the legs straight until the heels lift from the floor.
Lifting the heels requires a surprising amount of strength from the quadriceps and hip flexors. This flexion may put stress on the low back if there is not sufficient abdominal strength to maintain extension of the spine. Be sure to lead with the heart and turn the gaze forward to encourage this extension. Actively press the palms into the floor, engaging both the triceps and the biceps to prevent hyperextension of the elbows. If pressure is felt at the joint even with the arms engaged, bring the hands forward to the level of the shoulders and slide the thighs a little further up the arms. Those with any history of hamstring injuries might consider pointing rather than flexing the feet here to avoid pulling too hard on those tight hammies.