word order - Can the move of frequency adverb from middle to the start mean something?

Most of the frequency adverbs can live at the start, in the middle and at the ending of the sentence.

In Does adverb placement affect meaning?, in the answer https://english.stackexchange.com/a/83402/16498 is analyzed the difference between the end placement and others. But what is the difference between the start and middle placement?

Does these placements make some difference for the meaning:

Sometimes I fly.
I sometimes fly.

Do those sentences mean the same or not? If not, is there any difference in feeling, mood or something else? Is there some context in which they have different use?