There are certain muscles that respond faster to weight training than others -- ones that seemingly shape up after one round on the Nautilus circuit. If you want quick, visible proof of your workout, know which body parts to focus on first.

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.
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Muscle fatigue signifies overload, which is necessary for muscle growth. 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 explains why the first few weeks of strength training result in large increases in strength without any noticeable change in size.

Your slow-twitch stomach muscles, for instance, contract all day long as they stabilize the body. So you could say that you work them out for 16-plus hours a day, and they are therefore in great 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 muscle 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 pulldowns, you will start to see muscle tone in the shoulder area. As for the gluteus maximus, which has about equal amounts of slow- and fast-twitch fibers, results come quickly because you can use larger weights to overload these muscles faster.
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Fast-twitch muscles, found in the arms and legs, contract quickly and usually respond better to strength training because they're easier to overload and fatigue. They also tend to develop in size at a much faster rate.

Of course, just one session in the weight room won't do the trick. If you've seen bodybuilders doing bicep 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 more substantial results 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.