If I Sit For 30 Min On The Computer And

if I sit for 30 min on the computer and want to get up, very diffiv=cult, legs feel like heavy, like numb, then after maybe 30 sec I can take steps. difficulty walking up and down steps, inhave no edema or sob, I do have arthritis in my back, and in both knees. what could I do to help myself, I ride my bike 15 to 20 min daily, do stretching every day, walking outside with my dog, go shopping,but feel better using carriage to hold on.