[FONT="]He was excited; his shoulders and waist moved before he seemed to want them to, but his head remained aloof and socket-eyed. It was a repulsive insect movement...
vs.
[/FONT][FONT="]Excited, his shoulders and waist moved before he seemed to want them to, but his head remained aloof and socket-eyed. It was a repulsive insect movement...
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1. The first sentence doesn't really work, as it messes up the structure.
You have: [He was excited]-->[[Clause 1] - but -> [Clause 2]], when the excitement is really only part of clause 1, and the incongruity between body and head is what looks eerie.
2. There are two ways to read the "excited" in the second sentence:
2. a) (unlikely) People make the connection between "him" and "excited". In that case, it depends on the reader if they're okay with it or not. The more "theory-polluted" the reader, the more likely they'll mind. Thus, you'll get a disproportionate number of "no"s on a writing site. It can be an acquired aversion (though, it can also be a natural aversion).
2. b) People read "excited" as referring to the shoulders. In that case, we have figure of speech: a metonymy. It's clear that
he is excited, but it's the shoulders that express it. Since you have contrast with the head, that actually works pretty well for me. Shoulder = excited; head =/= excited. Watcher = grossed out.
3. Something
is off in the sentence. "seemed to want them to" - here you're explicitly referring to what a watcher infers about the motivation, and that interferes with the image. So, rather than getting rid of the "Excited", I'd get rid of the "before he seemed to want them to" line, focussing on the image. Both, "excited" and "before he seemed to want them to" pretty much refer to the same state, but only the former is - via metonymy - tied into the image. The latter is also more wordy. So my suggestion:
[FONT="]Excited, his shoulders and waist moved [forward; or any other phrase that indicates a direction; if he's standing still, how about "quivered" or some such word?], but his head remained aloof and socket-eyed. It was a repulsive insect movement...
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