I'm a bodybuilder and I'm not sure if under muscle placement is best with lifting heavy weights. Advice? (photo)

Thanks for your question. Under the muscle (dual plane) (especially at 350cc) will give you a more natural look but you may notice animation when you lift weights and occasionally, some patients may notice a small reduction in pec muscle strength.
Are you having a breast lift as well. From your pics, it may be something to consider?

I have a couple of triple crown bodybuilders with under the muscle with no issues for competition. After I measure and discuss goals with my patients, we then look at pictures of women (my patients) with similar measurements with different size and style breast implants. They also get to see what they would look like when the breasts have fully dropped and fluffed and in clothes. You get to see actual results and the surgeon's experience and not a virtual idea of what can be achieved. Women tell me that this process is very helpful in determining what size and style breast implant is right for them. You should also be aware that results of surgery can change in the future with significant weight change and/or stretching of skin from the milk of pregnancy. Dr. Ted Eisenberg, Board Certified Plastic Surgeon and Author. RealSelf Distinguished Hall of Fame. Philadelphia, Pa., USA.

Since you do have some tissue, going over the muscle will work fairly well for you. You still may have somewhat of an edge showing through in the upper part.