noma

Thank you for your contributions. I understand that I'm probably splitting hairs here and pure isolation is impossible and several muscle groups are involved. What I realy mean by that is that the "bulk" of work is done by a certain muscle group. In the above example of Pec.Major: -the flexion involves (not only but) primarily P.Major and Front Deltoids, with Deltoids carrying the "bulk" of the load; -the adduction engages (not only but) primarily again P.Major and Lats, with, approximately equal sharing of the load between the two; -the medial rotation with the humerus being parallel to the body engages primarily P.Major and Subscapularis, with the "bulk" of the work done by P.Major (e.g. if one bends the arm at the elbow joint to the side at a 90 degree angle and overcoming resistance turns it inward, the P.Major would carry most of the load). Would you agree with that?

My understanding is that medial rotation of the upper arm , specifically, when the humerus is parallel to the body, is the action that best isolates* the pectoralis major muscles (as opposed to flexion or adduction). Could someone please confirm or refute this. Thank you. *By 'isolation' I mean that the movement engages the smallest number of other muscle groups.