What Causes Weakness In Legs While On Prednisone?

I was diagnosed with severe PMA and was put on prednisone 5m. one year ago. I still have some problems especially weakness in my legs. I was told to gradually wean myself off the prednisone and am down to 1/4 of a pill. I have been going to water aerobics for an hour every morning. Other than that I am very healthy. My age is 83. Can I ever expect it get over this PMA. Thank you . YYYY@YYYY