01/14/2014 at 05:00 PM EST
We all know about the bicep curls behind her famous Michelle Obama Arms
Now, as the First Lady
turns 50 this week, throw in a downward dog for good measure.
In her only interview on the occasion of her milestone birthday Jan. 17, Mrs. Obama tells PEOPLE she's tweaked her workout routine
with an eye toward her 70s and 80s.
"I'm seeing myself shift from weight-bearing stuff – even though that continues to be important – and the heavy cardio and running, to things like yoga that will keep me flexible," says Mrs. Obama, who will be feted Saturday at a dance party hosted by President Obama at the White House.
With her retooled workout, the First Lady says she focused on balance and flexibility "so that I'm not falling and breaking a hip one day."
For more of our interview with the First Lady – including her thoughts on Botox, menopause and becoming a grandma – pick up the latest issue of PEOPLE, on newsstands Friday