Jim, I’m going to take a WAG about it. Remember, I’m just guessing…here goes.
A relatively mildly acidic cellular state actually favors oxygen transport from the blood to the interstitial spaces and across the cell membrane, and therefore oxygen is more available to the mitochondria to “do their thing” called the Kreb’s Cycle. The Kreb’s Cycle is a very aerodynamic bicycle made up of Kallikrien, Ribonucleic acid, Eosinophils, and Bat spit…OK, I just threw that in there to make sure you know I’m just making up this answer, I haven’t fully researched it, but I wouldn’t be surprised if my answer is actually correct.
When you first begin exercising, you do indeed increase the temperature in the muscles…thus the term Warm-up. Increased temperatures shift the oxyhemoglobin curve to the right, meaning, oxygen is “unloaded” to the tissues more easily. Increasing acidity also results in the same rightward shift characteristics in the oxyhemoglobin curve. Guess what? So does an increase in CO2 (which shouldn’t be surprising, because CO2 can be considered to be an acid, as CO2 forms carbonic acid using carbonic anhydrase as the enzyme to speed this reaction along.)
Anyway, I’m going to guess that some of the other exercise-induced changes that begin to occur to assist one in continuing to exercise (such as gluconeogenesis, increased cardiac output, lubrication of the muscle sheaths, dilation of the arterioles/capillary beds, etc.) start to all cuminate at a point of near-maximum efficiency…this is a point where you actually are starting to get some increasing acid buildup at the cellular level. Probably mostly lactic acid. As long as it is a mildly acidic state, it is NOT bad for your muscle function, and it’s very good for oxygen transport.
So, you have all these systems peaking at about the same time, and majically, you experience the “second wind”. It’s interesting that this second wind really exists, but, what is more interesting to me is that the second wind is also a signal that things are going to start coming apart soon…it’s a sign that you’ve reached maximal efficiency, and unless you do something to preserve this effort level, you can step over the cliff and get into decreased performance levels pretty quickly…or, you can ease up just a hair, and the second wind hangs around a LOT longer. A LOT longer.
Or, by “second wind”, are you referring to something associated with eating legumes, cabbage, and chili?