@Dalaran1991
I have a trusty workout that I have done for years for my lower chest. I don't know how to describe it but I will do the best I can. It is a essentially a 'tension' type pull trough for your lower chest but to isolate you have to a do a odd position to get the max amount of tension on your lower chest.
People say "decline is best for lower chest" but the decline is dumb, and you will only push up serious weight unless you are in the static smith machine which is not superior to getting tension all over the lower part of the pec, go and try and do serious weight with the decline that with a free moving barbell and see how that works...
Evrey gym is different but most usually have the usual suspects for machines. There is likley a pulley apparatus and most gyms will have the twin towers with the two center pulleys:
and the exteriors/sides for your upper and lower back, triceps:
I prefer to use the exterior that you can use for triceps/biceps so it has to be adjustable to you can put it on the lowest height (if not use the interior, just know clowns will bug you so they can do some weak ass cable flys since your using one of the pullys). If your a tall mother fucker you can just do this on your feet. If your a short dude like me then you will need a boost.
This can be tricky as you have to get the right height to be able to get a good contraction as I will explain later. Those step benches where you can add on different levels will work. If your gym has step/squat boxes get a bunch of those. The first time you do this it will take a while to adjust the pully and the height on the boxes to achieve the right level of height.
The position:
*You want to have your back parallel to the pully cord, and be essentially facing away from the machine.
*You then want to square up your legs to have enough room to pull the pully rope you would use for triceps in between your legs. If this sounds awkward.. is because it is.
*Essentially if your height is right the pully cord will be grazing the bottom of your shorts, a half foot and some below your nuts. I don't want this shit grazing your sack but you want it below where it would be grazing your shorts which would be a few inches below your nuts if you wear them just above your knee.
Now you would grip the cord in the opposite fashion you would for a traditional tricep pull down. Meaning that on a tricep pulldown your outer (where your thumb connects) nuckles are crossing, so the opposite would be that inner knuckles (closest to your pinky) would be the opposite and would be in the crossing fashion, just like you would position them to work you biceps.
This is how you want to look. But don't be as far back as Lee in this photo. You want to have it in between the legs, but raised height allows you space to be able to focus and contract your pecs, which standing on your feet and so far away from the base of the machine would limit.
Any time I see this in a magazine the rare time they leave out the height factor. You only get so much tension from the base of your feet. The last picture the position is good but when he follows through, you want more spacing of your arms floating above your thighs, to get more of the chest into it.
The Movement:
Now position yourself, with light weight (start off with maybe 20lbs, very light) and try to squeeze and isolate your lower chest. Essentially try to use your lower chest to contract and move the weight. The contraction will only be for a short period, a few seconds but it will be 100% done by your lower chest and nothing else. Practice this until you can contract the lower chest in a clean movement. If your are successful with this the end movement will be a clean follow through with the cable thought your legs and your hands making following the formation of a "V". But the key is to not simply use your arms, biceps or any other muscles to move the weight, you have to focus the contraction on the lower pec.
You will know when you hit it, it will be strange the first time. Keep on doing this until you burn out your pecs. If it is to light, go a bit higher and do pyramid sets until your pec fuses up with blood flow.
It will burn like a bitch. Your chest will swell up and you will look like have a gorilla chest. Plus regardless of how over the top the movement looks it does the job. People always look at me funny when I do it, but then they see the pump after and stop staring. IF you like to look cool in the gym then don't do this as you won' look cool yanking rope under your nuts, but if you want that big lower chest this IMO is the best way, the best way, to isolate the lower section.
For the inner chest. It is impossible to isolate it. It just grows from stimulation of all the other parts of the pectoral sections. To get that 'line" its a matter of getting size and depth in your pec muscle fibers which have many layers and then cutting down. How your body type is also affects how pronounced it will look as well.