Evie, I see your point.
Many fitness professionals include myself define the core as everything between the head arms and legs as you have stated and I include myself among the many. However, if we are going to define it according to the textbook, the shoulder girdle is not inclusive of it. This is where I stand corrected.
When I am training my clients and I observe that the core in the pure definition of core is weak, there is generally weakness and instability in the scapular. Hence, I like many other fitness professionals design programs to improve core stability and strength/endurance as well as the stability and strength of the shoulder.
Thank you so much for your thought-provoking question and best to you!
looking at all the answers that preceded my post, I personally include the training of the intrinsic stabilizers of the shoulder girdle in my definition of core training even though this is not according to textbook.
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!