All great responses. While the definition of “core” is obviously somewhat ambiguous I continue to suggest that it refers primarily to the muscles that support and stabilize the pelvis and lower spine. But I agree with Karin in that strengthening the shoulder girdle musculature has a profound effect on the function and strength of lower back and abdominal muscles. The integration of movement and strength throughout the entire spinal region is clearly an essential consideration in any training regimen. Don’t forget the shoulder internal and external rotators!