Hummmm I used to think a muscle cant be isolated in differnet ways, but then it wouldnt explain this.
While sitting/standing up right and if you place your left finger on the bottom of your right pec and push your right arm outwards in front of you (like a no weight seated bench press), you will feel the lower part of your pec slightly contract but if you then extend your arm downwards (move hand diagonly downwards while seated or standing), you will feel a much fuller contraction in your lower pec.
Try to simulate a standing/seated bench pressing movement with your elbow/arm plane of motion.
Also if you do a diagnal extension upward you will not feel hardly any contaction in your lower pec.
While sitting/standing up right and if you place your left finger on the bottom of your right pec and push your right arm outwards in front of you (like a no weight seated bench press), you will feel the lower part of your pec slightly contract but if you then extend your arm downwards (move hand diagonly downwards while seated or standing), you will feel a much fuller contraction in your lower pec.
Try to simulate a standing/seated bench pressing movement with your elbow/arm plane of motion.
Also if you do a diagnal extension upward you will not feel hardly any contaction in your lower pec.