Deep thoughts? Dancing to the songs in their heads? The onset of Tourette’s? Restless legs syndrome?
Sheer boredom?
To be honest I have no idea why I do it. I’d gladly stop if I could but it’s a compulsion.
Because it’s comfy, I suppose. Also it really helped when I took up drumming and discovered that for twenty-six years I’d conditioned myself to have a really speedy kick drum foot, so perhaps I was unconsciously training in speed metal :smack:
It’s not natural for humans to sit still for long periods of time. It’s a modern convention to just sit on our asses all day. And when doing so we naturally get restless sometimes. That’s my theory.
Restlessness. It helps to stem off the desire to run away screaming from a situation where you’re forced to stay put for long periods of time.
Being restless, as others have said. I never start shaking my leg at the beginning of a class, but when there are ten minutes left and the teacher keeps going on and on and I just want him to finish so I can leave…that’s when it starts.
They’re being forced to sit when their full of energy that needs to be used.
For me, it’s just pent-up energy, I think. Not that I’m so much restless as I just never stop moving.
I’m the most appalling leg-jiggler. I tend to blame my low blood pressure - I figure my body’s just trying any way to keep the stuff circulating.
I don’t know if this is related, but I also can’t bear to stand still for long periods of time. If there’s a long wait at the train station (ie more than two minutes) I’ll be the one pacing up and down the platform from end to end - I can do that for hours without a problem but ten minutes of standing still in one spot and I feel like my heart’s thumping out of my chest.
I’m jiggling as I type this, of course…
Like other forms of fidgeting, I think it helps focus my mind - seriously. And unlike fiddling with something at hand, it leaves my hands free to type.
There are a surprising number of hits on the subject! I googled “why jiggle leg when sitting” and one of the first hits (from the Google summary) suggested it looked like Parkinson’s!! (more info on the page said that nope, it was probably unconnected)
I suspect it’s all a form of self-stimulating behavior - careful how you google that, some hits are for >>that<< kind, though most are for the neurological sort as seen in autism. One in particular was fascinating in that it discussed stims that had become acceptable - like playing with hair or with a piece of jewelry.
Of interest -I find that when I’m jiggling a leg, or doing something else that takes just a little of my attention (e.g. playing minesweeper or Bejeweled on my PDA), I do a MUCH better job of listening to a speaker. Strange. It’s almost like I need something focused - but not TOO much so - for my brain to keep it from wandering everywhere and totally ignoring the speaker.
I have no idea. I don’t do it as much as I did when I was younger, but it’s a habit – I’m not aware I’m doing it until someone points it out to me.
For me - it’s because of my Restless Leg Syndrome. Yes - If I didn’t have it, I’d make fun of it too. And to be honest, I do make fun of it pretty regularly since it sounds like I’m part of an SNL skit. Basically - it’s like the feeling you get when you sit in a tight airplane seat for 6+ hours without moving - a compulsion to move and try to release the tension. Except that when it’s bad - it happens all the time. Before I knew what I had - I would bounce my legs a lot in an effort to release the tension. Now I know it doesn’t help me any and actually seems to make it worse. Luckily - I’m able to control it through regular exercise, so as long as I’m running 4-5 times a week, it doesn’t bother me. Otherwise . . . . . .
Because the meeting…shake shake shake…just will not…shake shake shake…end! Oh god, more slides? I’m done reading the bullet points move on already…shake shake shake…
I try not to do it when other people are around. But when I’m alone in my office- hey, it’s exercise of a sort that I don’t find too difficult or boring (like I do most types of exercise- at least I can look at interesting stuff online while I do this).
Because I’m one those impatient, insufferable, fidgety people who gets annoyed when people walk too slowly down the grocery store aisles, and wants to cause bodily harm to people who use the elevator to move up one floor.
It keeps the circulation going in the legs to prevent deep vein thrombosis.
I don’t do it nearly as much as I used to, but the funny thing is that I can only do the sewing-machine thing with my right leg. Not only can I not really do it with the left, the left is unsatisfying.
As a teacher I can honestly say we want the class to end just as much as you do, but I’m usually standing/walking around so I don’t need to shake my leg or anything
I’m also one of those drummers that everyone hates. Sitting at a desk with nothing to do I tend to drum out little beats I have going in my head. That pisses people off 
I do it all the time, and always have. My husband says I even do it in my sleep.
I assume it’s because of the ADHD, but it could be I’m just fidgety. I also pace rather than stand still and fiddle with stuff endlessly. Like Zsofia, my right leg is the dominant fidgeter.
Interesting fact: Leg shakers and other fidgeters perform better on standardized tests. But don’t ask for a cite – I remember this being reported in a credible news source years ago, back when I fidgeted more.