I work out. I try to prep in late winter (I've recently started my annual workout) to tone up for our busy riding season. I notice that my legs will shake if I have to put out a lot of effort.
It has to do with the ability of the muscles to sustain a contraction. In better shape = longer contractibility. Not so hot for strength and endurance? Muscles go into anerobic contraction and that gives you the twitching and often soreness.
If you're not in pretty good condition, as you put out effort not all the muscles that are needed (within a group for a motion) will contract effectively. Even slight imbalances will be noted as twitching/shaking.
Kind of hard to explain briefly, for me. I'm better at drawing pictures!