I
You can run a hundred meters in 13 seconds. Really ?
In this first paragraph, "really" is categorized as a content disjunct (one of the four categories of adverbials in A Comprehensive Grammar of the English Language: adjuncts, disjuncts, subjuncts, conjuncts.). Content disjuncts are also called "attitudinal disjunct".
Here is what CoGEL has to say about this adverbial.
(CoGEL § 8.130 Note) [c] Some content disjuncts can be responses to questions or can be used as a comment on a previous utterance: usually accompanied by yes or no:
A: They have returned to San Francisco.
B: Very wisely. ['They were very wise to do so']
A: They won't be coming back.
B: Unfortunately, no. ['It's unfortunate that they won't]
A few style disjuncts (honestly, literally, seriously, truly, truthfully) are used as verbless questions:
A: I'm going to resign.
B: Seriously? ['Were you speaking seriously when you said that?]
The content disjunct really is commonly used in this way:
A : I'm going to resign.
B: Really? ['Is that so?']
A particular intonation contour is not obligatory, but certain adverbials have a characteristic intonation when used as responses; for example, following 'I hear you may soon be emigrating', somewe might reply with one of the following […]: CÊRtainly, QUÌTE (BrE), Yes, inDÈED, AbsoLÙTEly, […]
The role played by disjuncts, that is, as adverbials, differs widely from that of adjuncts, which are to be rekoned with on an equal footing with other clause constituents (S, V, O, C); instead, disjuncts apply to the sentence as a whole. This content disjunct concerns the degree of truth (incertitude).
Disjuncts are often found at the initial position in a sentence, although they can appear in any position, which is in keeping with a full enunciation of the sentence as below.
- Really, you can run a hundred meters in 13 seconds?
- You can run a hundred meters in 13 seconds, really?
II
You can run a hundred meters in 8.6 seconds. Really.
(CoGEL § 18.55) Apart from the emphasis given by information focusing, the language provides means of giving a unit purely emotive emphasis. We have noted, in […] a number of features of this type. They include exclamations […], the persuasive do in imperatives […],
interjections […], expletives and intensifiers […], including the general clause emphasizers such as actually, really, and indeed. A thorough study of emotive expressions would involve examining figures of speech such as simile, hyperbole and irony [which is not the concern of grammar].
In this case, "really" appears to be again a content disjunct, but used ironically.
figure of speech, irony
use of word in a way that conveys a meaning opposite to its usual meaning.
- You can run a hundred meters in 8.6 seconds, really. ("You can run a hundred meters in 8.6 seconds, of which I am (quite/absolutely/…) certain.", whereas it should be "That you can run a hundred meters in 8.6 seconds is doubtlessly untrue.", or something similar.)
The punctuation is more likely a comma, as full stops indicate the end of a complete thought, and as the enunciation is not complete without "really".