system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why do some people shake their legs when sitting down? What are the benefits?
gpt
People's muscles 'twitch' constantly to keep toned. Lack of twitching leads to loss of tone over the entire body. Some people's 'tone twitching' is at a lower frequency but higher strength than others. So they are shaking their legs to maintain toned muscles - they just shake them more but less frequently than most people do. 

In fact most people 'twitch' in such a fashion that it's undetectable and they don't really know they are doing it.

Same reason behind fidigting.

The only benefit of being a 'slow twitcher' or fidigter is burning slightly more calories a day. So they tend to be thinner.