The first example I see often is usually along the lines of "Go get 'em!".
In personal training muscles are often spoken of in a shortened manner, for example triceps are called tri's. Would this be valid use of an apostrophe?
Regarding the triceps: It seems to be perfectly acceptable to use an apostrophe to denote plural in such cases. Personally, I would also argue it often helps eliminate confusion.
Consider:
bi: a bisexual person
plural: bis or bi's
(AHD)
Or:
a: the first letter of the English alphabet
plural: a's or as
(Webster's Unabridged)
In go get 'em the 'em part is a contraction, so it uses an apostrophe to indicate omission. Contraction is defined by Webster's Unabridged like so:
a shortening of a word, syllable, or word group by omission of one or more sounds or letters or by the reduction of two or more vowels or syllables to one — used especially of shortening in the interior of a word (as e'er for ever) and of shortening of enclitics (as 'll for will in they'll) and proclitics (as 't for it in 't is).
I think the apostrophe is usually used when a word is shortened by cutting out the front.
"Go get 'em."
"I been working out my 'ceps a lot."
"I gotta let my 'rents know I'm coming home late."
When a word is shortened by cutting off the end, it is not usually accompanied by an apostrophe.
"Grab your sneaks before we go."
"What are the stats on that player?"
"My abs are still sore from the core workout yesterday."
tris
would be confusing. However, if it's in written form why wouldn't you spelltriceps
out fully?