The deepest muscle on the left is the 
deep
bulbospiral muscle group which originates from the septal
portion of the mitral valve annulus. The deep bulbospiral muscle
layer divides the left ventricular wall, wraps around the aortic
outlet in a sphincter like form and re-attaches to the septal
portion of the mitral valve annulus.
The deep bulbospiral and sinospiral fascicles interdigitate
in the muscular IV septum. Upon ventricular contraction, the
orientation of the muscle bands causes shortening in the diameter
of the heart. The spiral muscles, upon contraction, also shorten
the heart from 
apex to base .