▁and ▁stabil ize ▁the ▁shoulder ▁joint . ▁ ▁Str ucture ▁It ▁attach es ▁med ial ly ▁to ▁the ▁in fr asp in ous ▁f ossa ▁of ▁the ▁sc ap ula ▁and ▁later ally ▁to ▁the ▁middle ▁fac et ▁of ▁the ▁greater ▁t uber cle ▁of ▁the ▁hum er us . ▁ ▁The ▁mus cle ▁ar ises ▁by ▁f les hy ▁fib ers ▁from ▁the ▁med ial ▁two - third s ▁of ▁the ▁in fr asp in at ous ▁f ossa , ▁and ▁by ▁tend in ous ▁fib ers ▁from ▁the ▁rid ges ▁on ▁its ▁surface ; ▁it ▁also ▁ar ises ▁from ▁the ▁in fr asp in at ous ▁fas cia ▁which ▁covers ▁it , ▁and ▁separ ates ▁it ▁from ▁the ▁ter es ▁major ▁and ▁ter es ▁minor . ▁ ▁The ▁fib ers ▁conver ge ▁to ▁a ▁tend on , ▁which ▁gl ides ▁over ▁the ▁later al ▁border ▁of ▁the ▁sp ine ▁of ▁the ▁sc ap ula ▁and ▁passing ▁across ▁the ▁posterior ▁part ▁of ▁the ▁caps ule ▁of ▁the ▁shoulder - joint , ▁is ▁inserted ▁into ▁the ▁middle ▁impression ▁on ▁the ▁greater ▁t uber cle ▁of ▁the ▁hum er us . ▁The ▁tra pe zo idal ▁insert ion ▁of ▁the ▁in fr asp in atus ▁onto ▁the ▁hum er us ▁is ▁much ▁larger ▁than ▁the ▁equivalent ▁insert ion ▁of ▁the ▁su pr asp in atus , ▁the ▁reason ▁why ▁the ▁in fr asp in atus ▁is ▁involved ▁in ▁rot ator ▁cu ff ▁tears ▁about ▁as ▁frequently ▁as ▁the ▁su pr asp in atus .