Hi S.C., thanks for responding
I know that the standard thinking in the fitness community is that barbells are better than machines, but I lift alone in a nice home gym I constructed in a spare room in my house, so for safety reasons as well as space considerations, I don't do any big barbell movements. Instead, I have machines, as well as a dip station and a dumbell rack. Furthermore, I was a serious lifter in my teens and late 20's, so I've already experienced my genetic potential as far as muscle building is concerned, and at this point in my life, I don't think it would make a big difference on wheather I used free weights or machines.
I do know that free weights help with the stabalizing muscles better than an isolating body part machine.
I asked my question from a physilogial point of view regarding the relationship between the shoulders and lower back in lifting. Like I said, the weight that I was using didn"t feel that heavy to me. I do buy your theory that my core might be weak. I do the exercise seated.