My legs give way

This started nearly a year ago, no pattern, occurs randomly but quite scary when it does. It has only happended when I get out of bed, I sit up on the edge of the bed first, go to stand up and my legs completely give way. They're not asleep, heavy, crampped or tingly before I stand up but when I do they completely go from under me- not like when you 'go over' on your ankle etc, they just completely collapse like a rag doll. Last episode was this weekend but starting to worry now as when I go down theres no warning and it really hurts, as I hit the floor like a dead weight! Also worried about it happening when I'm out as seems to be occurring more frequently (although to date only ever when get out of bed)
Pretty healthy apart from this. 28 years old.
Any ideas?

[Nassim Abi Chahine, MD]

A blood supply problem could affect the nerve, th motor neuron and subsequently the muscle. A brain angiogram could be a good exam to rule out any vascular problem causing local hypopefusion and subsequent motor deficit on ortostatic hypotension.