Lase oma sisemine sportlane välja. - Release your inner athlete.
oma – reflexive poss. adj. (refers back to preceding pronun).
cf. Õpetaja ütles õpilasele, et ta loeks oma raamatut. 'The teacher told the student to read his own [the student's] book.'
Õpetaja ütles õpilasele, et ta loeks tema raamatut. 'The teacher told the student to read his [the teacher's] book.'
Ma loen oma raamatut! - I am reading (some of) my book. Sa loed oma raamatut! - You are reading (some of) your book.
sisemine – adj. inner (ending mine should not be confused with suffix -mine, the latter signifying 'act of doing', as in luge+mine 'reading' (from lugema 'read'), söö+mine 'eating' (sööma 'eat'), etc. You know -mine attaches to the stem of the ma-infinitive and not the da-infinitive, because it's söömine and not süümine* (da-infinitive is süüa)). The breakdown of sisemine is actually sisemi+ne, -ne being an adjectival ending, with sg. genitive -se and sg. partitive -st.
sportlane – n. sportsperson. It can stand for both male 'sportsman' and female 'sportswoman', though if one wishes to specify 'sportswoman' one can use the female form of the suffix -lane (sg. gen. -lase, sg. part. -last), which is -lanna (sg. gen. -lanna, sg. part. -lannat). However, it must be said that this is a rare usage, especially in the singular. A female athlete would call herself sportlane, just like in English where more and more actresses refer to themselves as 'actor'.
Where I have most heard the use of the -lanna suffix is in the word sõbranna 'female friend', what straight women in English call a 'girlfriend'. It can also be used with nationalities: cf. eestlane, eestlased 'Estonian person, Estonians'; eestlanna, eestlannad 'Estonian woman, Estonian women'. The stand alone noun maa ('land') used in compound nouns and the suffix -lane are very productive. Ex. Iiri+maa, iir+lane 'Ireland, Irish person', Vene+maa, vene+lane 'Russia, Russian person', etc. (Be careful of the dropped final vowel in the first part of iirlane - not iirilane*. Cf. Soome, soomlane 'Finland, Finn' and Eesti, eestlane 'Estonian, Estonian person').
välja – adv. 'out' as in Lind lendas aknast välja. 'The bird flew out through (lit. 'from') the window' and väljapääs 'exit' (pääs 'escape' < pääsemine/pääsema 'escape n./vb.' ; 'way, passage'. Cf. sisepääs 'entrance' as in sisepääs kaubamajja 'entrance to the department store'. The double j here is not a mistake, majja is the sg. illative case form of maja 'house, building' – the illative is often expressed by a lengthening of a vowel or consonant of the word, and may not be indicated in the orthography. For example kool 'school' (nom. long o), kooli 'of the school' (gen., long o), but kooli 'to the school' (ill., overlong o in speech, but not in orthography)).
Not to be confused with the orthographically identical, but phonetically different välja, the genitive form of the noun väli 'field, empty space, expanse'. The latter has a long l, which you will notice is not indicated by the orthography. Estonian has a few nice, little, juicy gems like this. For example, take the two orthographically identical words palk 'wage' and palk 'log'. The latter has a palatalised l. Their genitive and partitive forms are also different: wage: palk, palga, palka ; log: palk, palgi, palki. And so you know, the palatalised l continues also through the case forms of log. There is also the partitive case form of väli, which is also välja, but with a short l.
Handy reminder of the above: välja (short l) = 1) adverb meaning 'out' ; 2) partitive case form of väli 'field, empty space, expanse'. välja (long l) = genitive case form of väli.