With the Olympics taking place in Rio, we are hearing the words athlete, athletic(s) and to a lesser extent athleticism all around us. This group of words seems to be unusual among those containing the sequence /θl/ in that it is quite common to hear a schwa inserted between the two consonants. The Longman Pronunciation Dictionary includes this pronunciation, but marks it as stigmatized, while the Oxford Dictionary of Pronunciation and the 3rd edition of the OED online have this as an American pronunciation, without further comment. Strangely, the OED says that the spellings athelete, athaletic, atheletic, and athuletic all occur in regional US English, but gives no example sentences or other sources for this statement. Only the OED seems to have caught up with itself enough to include the variant for pentathlon and pentathlete, but not for heptathlon or heptathlete (decathlon has not been updated since 1933, and decathlete is not given at all – yet).
Other words containing the sequence, such as breathless, Athlone, Kathleen (i.e. whether the stress falls on the first syllable alone, on the second alone, or with secondary stress on the second syllable) do not seem to exhibit this possibility, so I wonder what makes athlete, etc. so special.
It can’t even be that this group of words is treated as if the -thl- was syllable initial, because this un-English initial sequence is often heard from monolingual English speakers when they attempt to pronounce the Welsh syllable llan- in such place names as Llangollen or Llandudno. I’m not aware of having heard a schwa between the two in these names.