You asked if I would elaborate on the notion that lactic acid training ruins economy.
Economy, as Jack Daniels, expert, will tell you is the measure of oxygen cost for running at submaximal paces. Oxygen cost or economy is the result of skill, fiber makeup (types), conditioning, illness or health, drugs in the blood stream, hormones, mental relaxation or lack of relaxation, and by-products of metabolism such as lactic acid.
Lactic acid training, as mentioned earlier, does cause muscle fiber and blood chemistry to change from equilibrium in proportion to quantity in the body per unit of time. So, if you are jogging along and not producing much lactic acid above resting then your equilibrium is not severely effected, but if you run a fast 400m then your body is wading in lactic acid and your equilibrium is messed up. Quickly your body will try to restore equilibrium by oxidizing lactic acid. The problem with lactic acid training is that it tends to put a person over the edge, so to speak, causing severe acidosis for extended periods of time. As soon as lactic acid rises, economy does too (worsens). Your body simply wants to get rid of the acidosis and speeds up respriatory processes. After running for quite a while in an acidic state, your muscle fibers start to break up their aerobic enzymes and the inner membranes of the mitochondria (aerobic powerhouse of the cell). Neuman and Gohlitz wrote that acidosis in muscle cells (above 7 mmols...or about 8km race pace for the average Joe or Jane runner) reduces mitochondrial oxidation potential by 3-7% immediately. Result? More oxygen is required to run along than before. More oxygen needed (eroded economy) results in loss of performance capacity. Indeed, it is wise to not do lactic acid training in the days just preceding a distance race because economy will not be optimal. Additonally, lactate threshold erodes from too much acidosis. If you goof up your lactate threshold, you goof up economy and you can kiss your goal of peak performance good bye.