I've read 100 times here how you can't actually "tone" muscles by adding more reps. You can only add muscle mass and reduce body fat for your muscles to appear more defined.Lactic acid is a by-product of glycogen breakdown in the absence of sufficient oxygen. Ok, now let me explain what that actually means. During weight training where the reps are medium range (sets lasting somewhere between 30 and 90 seconds or so), the primary fuel is stored muscle glycogen, which is simply a long chain of carbohydrate. When glycogen is broken down, it produces something called pyruvate. Now, if there were sufficient oxygen available (i.e. you were doing aerobics), the pyruvate would be metabolized to produce more energy. Since there's not (weight training is an anaerobic activity, meaning there isn't sufficient oxygen available for the complete breakdown of pyruvate), the pyruvate is made into lactate, which then turns into lactic acid and a Hydrogen ion.
Lactic acid does not have any direct effect on growth of muscle tissue. However, it appears to have beneficial effects on connective tissue health and there is a correlation between increase in lactic acid and growth hormone (GH) levels. While GH doesn't really have any anabolic effects in adult human skeletal muscle, it can affect fat loss. The combination of glycogen depletion (from the high reps) and the increase in GH seems to help with fat loss.
But isn't this article suggesting that the added reps increase the very fat loss that leads to (the appearance of) muscular tone?
I might be mis-reading it though.