Barbell Chest Press: begins with the full contraction of the pectoral muscles (to get the full contraction, the muscle must be stretched out as much as possible, so lowering the bar to the chest. Begin the contraction with the lateral [outermost on the left and right] part of the pecs, and move medially, towards your midline). This causes the arms (which make up the synergist muscles) to move inward (closer to your midline, adduction) and slightly away from you (in a slight arch). So the chest begins in a fully stretched position, and as it contracts, the force it produces moves the arms upward and towards each other, which in turn moves the bar. The force is created laterally, without the bar your arms would move towards each other and your hands would eventually touch. To lower the weight back down, you stretch the pecs along the same arch you raised the weight, until you can go no further (bar on chest).  There is no shoulder involvement in the force production of this movement. The only contracting muscles are the pectorals, although the triceps may help stabilize the weight.