Abdominal part of left pectoralis major

Pars abdominalis musculi pectoralis majoris sinistri · Břišní část levého velkého prsního svalu

Muscles

Looks like

The lowest, smallest slip of the chest muscle, starting from the sheath of the front abdominal muscle below the ribs.

Adds to pulling the raised arm down and across the body, as in climbing or swimming strokes.

Powered by the pectoral nerves. Strain here is rare; sudden pain with a pop and bruising in the armpit suggests a pectoral tear.