Attested mainly in telic sense, reflecting an ancient root aorist in the past tense and thematic root "imperfective" in present. Parallel to Proto-Slavic*tęgati, Proto-Slavic*tęgnǫti which convey atelic nuances.
Černyx, P. Ja. (1999), “тя́га”, in Istoriko-etimologičeskij slovarʹ russkovo jazyka [Historical-Etymological Dictionary of the Russian Language] (in Russian), volume 2, 3rd reprint edition, Moscow: Russkij jazyk, page 278
Vasmer, Max (1964–1973), “тя́га”, in Etimologičeskij slovarʹ russkovo jazyka [Etymological Dictionary of the Russian Language] (in Russian), translated from German and supplemented by Oleg Trubačóv, Moscow: Progress