Oh, no. . . I feel a long post coming on. . .
Julios, I have two replies for you:
1. A 15% increase in a muscle's girth can be harder to detect than you think.
2. Brian was talking about a 15% increase in a given fiber, not in total muscle girth.
First, for #1:
In an untrained subject, with little or no muscle development, and a fair amount of fatty tissues, the muscle is a relatively small portion of arm girth. You have the bone, veins and arteries, fatty tissue, skin (yes, it adds up when you wrap it all the way around; it's about 1/3 inch thick), nerve channels, lymph systems. . . Let's say the muscle is probably about 40% of the arm's volume, and let's say the arm's girth is 12 inches. Let's assume a round or cylindrical arm. The arm's height is 3.8 inches, the radius of the circle is 1.9. The cross-sectional area for that arm is pi*r^2, or 11.45 square inches. Since the muscle accounts for 40% of that volume, it accounts for 4.58 square inches, and the rest of the arm is 6.87 square inches.
A 15% increase in muscle volume increases that 4.58 to 5.27 inches; the total volume grows to 12.15 square inches (5.27 + 6.87; the rest of the arm does not grow.) At the new area of 12.15 square inches, what is the new arm size? The arm is now (2*sqrt(area/pi)), or 3.93 inches tall, an increase of .112 inches. Can you visually notice that? The circumference is now 12.35 inches, an increase of 2.9% over the original 12 inches. A 2.9% increase is not that big; if you take untrained people and put them through an exercise program that leads to a whopping 15% increase in muscle girth, they can easily lose enough fat to overwhelm that 1/4 inch girth gain. . . and they'll look like they gained no size at all. If they do all gain size, it's going to be less than 2.9% because of fat loss -- let's say it's a 1% increase. HARD to notice.
To tie this in to Aaron's comments, if I have a trial with six or ten individuals, and they get a average 1% increase, can I declare growth? heck, no. The trial is so small that I cannot declare growth with such a small gain in size. To confirm a 1% increase in size, I would have to prove this over a MUCH larger group of people. If they had grown an average of 15%, then I could easily claim success with just a few people. But for my 10-peron trial, what is my conclusion? No statistically significant growth.
(In a lean althelete with a significant build and a 15" arm, a 15% increase in muscle girth will be far more evident; for them, the muscle is alread a much larger percentage of of the arm's size, the fat will be less significant, and they won't lose much fat in the exercise.)
Now, on to point #2:
Anoop brings up the issue of neurological training. An untrained individual will have poor recruitment of muscle fibers when they begin training. Thus, few muscle fibers will be adequately trained to grow. But those few muscles fibers will actually grow quite well, and will do so immediately. Those muscle fibers can easily grow 15% in size, and that would be evident under a microscope -- but many of their muscle fibers would not grow that much, or not grow at all. The net muscle growth would then be much less than 15%, and as you know from point #1, 15% is already hard to spot in an untrained individual!
OK, enough of my rambling. I need to go dig some holes.