Perhaps the answer to this is different depending on what you're doing so I thought I'd post it in the general section.
If I stand for a long time or hold a posture for a long time I generally feel fine but eventually dwindle away. At that point if it's been a while since moving I lock up any muscles I've been using in a painful cramp, even if while standing they generally feel quite relaxed. For example I just did San Ti for about 20 mins on one leg and I can hardly move my left arm as its not used to anything like that. It was absolutely fine until the moment I stopped.
I note that when Bruce trained with Wang Shu Jin he fell many times and had to be picked up so I'm thinking it's not all bad.
A must read for those who are new to qigong, those who are already practitioners, and anyone interested in complementary medicine or self-help.