It might be nervousness, but I bounce my leg up and down a lot of the time with out noticing. Just fidgeting. But if you notice a guy doing this only when he's sitting near you, I'd say he was probably feeling nervous - which is a good thing, because that probably means he likes you.
It's a form of fidgetting, I think.
actually, scientifically the answer is that we are nervous, and its a way for us to settle down, oddly enough, ud think shaking would be worse
but take it for what it is
if he's nervous, he likes you
If he was just sitting next to you while studying don't assume that he was doing it because he was nervous. Chances are he wasn't, he was simply doing it out of habit. I do it when in my room studying by myself. It's something to do, I can't just sit there and concentrate unless I'm distracting myself (if that makes sense). In fact I'm doing it right now. You may be hot, but I'm not entirely sure your that hot :P
Leg shaking, especially as it relates to hormonally charged males, is a warning sign of imminent gastric profusion. I recommend that upon noting this sign, you clear the area immediately! Or, simply light a match.
IF you mean bouncing their leg up, and down they're probably nervous. And some people just don't notice that they're doing it.
It's just a habit. On occasion, it means the guy has to pee, is nervous, or is stressed.
i sort of bounce my leg up and down but I don't even realise I'm doing it
some of my mates said they do it on purpose for something to do with the friction =/ seems a bit weird to me =L