Muscles are classified as "fast-twitch" or "slow-twitch." Slow-twitch muscles are found in large or stabilizing muscle groups such as the abdomen or back. These muscle fibers can typically contract for long periods of time and are difficult to fatigue which makes them slow to respond in terms of definition.

Fatigue is important because it is a signal of overload that stimulates your muscles to adapt by getting stronger. When a muscle fatigues, the central nervous system initiates mechanisms to increase fiber recruitment within the muscle and eventually cause the muscle to hypertrophy (that is, get bigger) to allow more force production. This neuromuscular adaptation takes place very rapidly and is why the first few weeks of strength training see large increases in strength without any noticeable change in size.

Your stomach muscles, for instance, are slow-twitch and basically contract all day long because they stabilize the body. Essentially, they have been working out for 16-plus hours a day since you were able to sit up by yourself and are therefore in good shape. The downside is that they are difficult to overload during training; therefore hypertrophy (the visible gain of muscle definition) comes slowly. Typically, it takes two to three months of regular sit-ups to see any visible change in stomach muscles definition.

The back, on the other hand, has a greater variation in fiber type. The upper back and shoulders respond faster to regular weight training than the stabilizing muscles of the lower back. After four to six weeks of targeted exercises like military presses, side dumbbell raises, seated rows and even lat pull downs, you will start to see muscle tone in the shoulder area. As for the gluteus maximus, which is roughly even in both slow- and fast-twitch fibers, results come quickly because you can use larger weights to overload these muscles faster.

Fast-twitch muscles, found in the arms and legs, contract quickly and usually respond better to strength training than slow-twitch fiber muscles because they're easier to overload and fatigue. They also experience greater increases in size.

Of course, just one session in the weight room won't do the trick. If you've seen bodybuilders doing biceps curls before a competition, they're not trying to get a quick fix of muscle definition. This exercise increases blood flow to the muscle which will definitely cause it look bigger but expect substantive results to take at least a few weeks.

It is this difference between slow-twitch and fast-twitch muscles that explains why it is so difficult to get perfectly cut abs and relatively easy to develop bulging biceps and calves.