What Causes Temporary Weakness In Leg Muscles Upon Standing?

twice in the last couple of weeks, I've had difficulty standing after being seated fo a while. its as if my foot has fallen asleep, but there is no numbness or tingling. my foot/ ankle just doesnt work. The first time, I completely fell down, and today, I twisted my ankle, but was able to catch myself on the table with my hands. My muscles just stopped working. I was able to get up, and was fine after a few seconds.