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 the 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).