erinja:"siaj gluteoj skuiĝas" doesn't work here. "si" and forms of "si" (like "siaj") can never be a subject.

It's referring to the pronoun which wasn't inserted at the start, as it would be in a normal sentence, i.e. because it started with the infinitive. I have seen this form used elsewhere.

Oniaj?

Sorry, I'm not seeing it. Which pronoun are you referring to?

The problem is that you have more than one verb, so it would have to be a pronoun going with "skuigxas". If you have seen this form elsewhere, I suspect it was wrong, or else it might be that the situation is not quite as similar as you imagine. Or else I am missing something massive here but I just don't see how 'si' can work in this context, in a grammatically correct way.

Yes erinja is correct. In the phrase "tiel forte ke siaj gluteoj skuiĝas", "siaj gluteoj" functions as subject, but 'siaj' cannot be used as part of the subject so the phrase is simply wrong. It makes no difference whether the referent is explicit or implied.

I would just say "Ruliĝi sur la planko ridante (tiel forte ke la gluteoj skuiĝas.)" No need for a possessive pronoun.

erinja:"siaj gluteoj skuiĝas" doesn't work here. "si" and forms of "si" (like "siaj") can never be a subject.

Mi pensas ke, li volis diri "sxiaj" ne "siaj"<br />
<br />
[en] I think that he tried to say "her", not "one's"

I didn't understand the conversation of Bruso and J_Mark; searching Google, I found internet slangs:
LOL: Laughing Out Loud (or Lots of laugter ?)
ROTFLMAO: Rolling On The Floor Laughing My Ass Off.

Anyway, "ha ha ha" is onomatopeia, LOL is abbreviation, and there is another type: mimesis, which imitates the situation by voice. For example Japanese has the following:
Kera kera warau; gera gera warau (warau=laugh) : similar to ROTFLMAO
others:
niko niko (laughing with a happy face)
kusu kusu (whispering)
hera hera (foolishly)
and so on. I believe some other languages also have such expressions.

erinja:Sorry, I'm not seeing it. Which pronoun are you referring to?

When one teaches English, one often teaches an infinitive form and the student then changes it up so that it will be used with the right pronoun. That's very common, I do it all the time when I teach English. Dictionaries do it too. So you can't see the pronoun because it's not there.

So I could teach 'to pick someone up from the airport.' This phrase itself is never used on its own, except in this situation. The student or whoever has to add the pronoun, the 'someone' and change other bits. (eg. He picked his friend from the airport, I will pick you up from the airport, etc.) I suppose the 'la' is okay, but in real life if one were to convert this sentence as I've described and replace the 'si' with the right adjectival pronoun, it won't do to use 'la', from this didactic point of view. The 'siaj' is subbing for, to be precise, miaj/viaj/liaj/ŝiaj/ĝiaj/iliaj, so regardless about what your grammar books say, I think it works fine in this situation; which to reiterate is not something that people would actually say or use in a real situation; they would substitute the pronouns and change the tense of the verb etc. in the way that they want. Oniaj is okay, if you don't like this unorthodox usage. Ties might be a happy medium, along the lines of 'la' as described by Tomm.

Please PM me if you disagree; if you think I'm making a bad error or making a confusing expression and are trying to correct it so that it is more correct or clearer, it ain't working doing it this way.

J_Marc:The 'siaj' is subbing for, to be precise, miaj/viaj/liaj/ŝiaj/ĝiaj/iliaj, so regardless about what your grammar books say, I think it works fine in this situation

That it is "subbing" makes no difference. The simple fact is "si" and "sia(j)" can never be subject. They are the subject in your phrase, hence the phrase is wrong. Case closed as far as I'm concerned.

Bear in mind it's not only grammatically wrong but virtually unseen usage, except among newbs who are making a basic error. So I'm not sure in what sense you can say that it "works fine", beyond the fact that some errors can be understood perfectly well.

To pick someone up from the airport is a grammatically correct fragment. It could be considered an element in a valid sentence, for instance, "I am departing now to pick someone up from the airport"

These model sentences may be fragments but they are not grammatically incorrect; though they are incomplete, they contain no elements that are incorrect. But "si" wouldn't be used as a subject in a model sentence in Esperanto, since that would give the false impression that it could in any circumstance be correct to use a form of "si" as the subject (which, obviously, it isn't correct - ever).

If you really wanted to use a 'neutral' subject you'd have to say "oniaj gluteoj" or "ies gluteoj".

If you see someone using 'si' or 'siaj' as a subject in teaching, as a stand-in for a variety of pronouns, I suggest that you suggest to that person that they stop. It gives a misleading impression that it could ever be correct in Esperanto to use 'si' in this manner as a 'neutral' subject. You don't want students to get used to hearing that, because it's never correct to use 'si' in that way, and you don't want their sense of the language to start developing that this usage is somehow correct, the way that "to pick someone up from the airport" is correct.