Simply put, this is the athlete. This shape is similar to the trapezoid shape that the “average” build may have, but with more extreme proportions. Moderate to heavy muscle definition means that your shoulders are broad; with a narrow, defined waist, and muscular arms and legs.
While slimmer guys may find that they can throw on a medium shirt right off the rack and walk out, you need some extra assistance when it comes to shirting. Considering your chest is broad, the conventional wisdom is that you should wear a v-neck. In theory, this creates a natural visual line that narrows your chest and draws the eye to your trim waistline. While that's all well and good, V-necks are admittedly pretty corny when not worn under something else, so instead opt for crewneck shirts that utilize graphics, or horizontal stripes. Horizontal stripes (like Breton Stripes) broaden the figure. While this is ill advised if you're packing extra weight around your middle, the fact that your stomach is lean means that you can aesthetically add to this part of your figure without looking wide.
Always look for shirting that can accommodate your brawnier upper body, chest, and arms, while going for a tailored waist. If you're looking for a department store trick, try going for a shirt that says “slim fit,” just buy one size larger than you normally would.
When it comes to bottoms, look for “slim” items, but not skinny. While your time on the treadmill may leave you able to fit into something skinny, you're pants have a high chance of looking like running leggings—or worse, like you skip “leg day.” This doesn't mean you should only wear straight leg trousers, just be mindful of when too skinny is, well, too skinny.
For suiting, opt for double breasted suits, but be cautious. Double breasted jackets make the body look bulky in the upper body, but when sculpted, can support the added visual weight of the shoulders and chest, while emphasizing a narrow waist. Think of a dark colored, unstructured suit. You want to be aesthetically supported, but not disproportionate.