External intercostal muscle

The abdominal part of the left pectoralis major is the lower portion of the left chest's primary pushing muscle, attaching from the sternum down to the ribs near the abdomen. It drives shoulder adduction and flexion, crucial for powerful presses like bench variations and dips. Fitness enthusiasts target it for that defined lower chest line in bodybuilding.

The abdominal part of the right pectoralis major forms the lower section of the right chest's main power muscle, spanning from the sternum to the abdominal ribs. It excels in adduction and flexion for presses and flyes, key for balanced chest development. Gym-goers love it for carving out that teardrop lower chest aesthetic.

This cardiac muscle inside the right ventricle anchors mitral valve leaflets for proper heart pumping. Not skeletal, but endurance training optimizes heart efficiency. Matters for cardio performance indirectly.

Specialized head of left heart papillary muscle securing mitral valve for left ventricle ejection. Critical for oxygenated blood flow. Athletes benefit from cardiac hypertrophy here.

Innermost intercostals are deep rib muscles aiding forced expiration like coughing. Support breathing in high-intensity training.

Internal intercostals between ribs assist expiration and stabilize chest in lifts.

Papillary muscle in heart's left ventricle anchors mitral valve to prevent backflow. Endurance training strengthens indirectly.

The massive pectoralis major covers the chest, powering arm adduction, flexion, and rotation for bench presses and push-ups. It's the prime mover for upper body pushing strength in fitness. Building it creates that powerful chest aesthetic.