▁thor ac ic ▁n erves ▁ ▁Vari ation ▁The ▁s ternal is ▁mus cle ▁may ▁be ▁a ▁variant ▁form ▁of ▁the ▁p ector alis ▁major ▁or ▁the ▁rect us ▁ab domin is . ▁Some ▁fib ers ▁are ▁occasionally ▁connected ▁with ▁the ▁cost ox i ph oid ▁lig aments , ▁and ▁the ▁side ▁of ▁the ▁x i ph oid ▁process . ▁ ▁Function ▁The ▁rect us ▁ab domin is ▁is ▁an ▁important ▁post ural ▁mus cle . ▁It ▁is ▁responsible ▁for ▁flex ing ▁the ▁lum bar ▁sp ine , ▁as ▁when ▁doing ▁a ▁so - called ▁" cr unch " ▁sit ▁up . ▁The ▁rib ▁c age ▁is ▁brought ▁up ▁to ▁where ▁the ▁pel vis ▁is ▁when ▁the ▁pel vis ▁is ▁fixed , ▁or ▁the ▁pel vis ▁can ▁be ▁brought ▁towards ▁the ▁rib ▁c age ▁( post er ior ▁pel vic ▁t ilt ) ▁when ▁the ▁rib ▁c age ▁is ▁fixed , ▁such ▁as ▁in ▁a ▁leg - hip ▁raise . ▁The ▁two ▁can ▁also ▁be ▁brought ▁together ▁simultaneously ▁when ▁neither ▁is ▁fixed ▁in ▁space . ▁ ▁The ▁rect us ▁ab domin is ▁ass ists ▁with ▁breath ing ▁and ▁plays ▁an ▁important ▁role ▁in ▁resp iration ▁when ▁force fully ▁ex hal ing , ▁as ▁seen ▁after ▁exercise ▁as ▁well ▁as ▁in ▁conditions ▁where ▁ex hal ation ▁is ▁difficult ▁such ▁as ▁em phys ema . ▁It ▁also ▁helps ▁in ▁keeping ▁the ▁internal ▁org ans ▁int act ▁and ▁in ▁creating ▁int ra - ab dom inal ▁pressure , ▁such ▁as ▁when ▁exer cis ing ▁or ▁lif