I'm curious what the exact cause is of dramatic heat generation that comes along with muscle activity. Can anybody explain this in understandable language?
First, it might be helpful to talk briefly about what heat is. It is a form of energy - molecular energy. Basically, heat is the amount of jiggliness of the molecules of a substance or object. When we look at a hot object, it doesn't apear to be moving any more than a cold object, but on a scale much smaller than we can see, it's molecules are moving faster than in a colder object. Read this for more information if you want.
There are at least two things going on in muscle contraction (as well as in other metabolic processes) which produce heat. The first is the chemical reaction that powers muscle contraction. You probably don't want to get into great detail, but basically there is a molecule which your cells use to store power, which we abbreviate as ATP. When our muscles use that power, an exothermic chemical reaction occurs that "burns" ATP, breaking it into two pieces (ADP and phosphate). That reaction releases energy. Some of that energy is used in the actual movement of the muscle. But some of it just "jiggles" the nearby molecules. Again that jiggling is the very definition of heat.
The second source of heat is probably much smaller: friction. In short, just about any time anything moves, nearby molecules get jiggled some more. Imagine trying to walk through a room packed full of harps. Everywhere you move, some of those strings get plucked and vibrate. It's a little bit like that. When your muscle fibers tighten and flex, they rub past each other and jiggle each others atoms. All of that jiggliness is heat, and the jiggling continues until it can transfer to some other medium, as when, sweat evaporates from your skin.
This is oversimplified, but hopefully clear.
When we eat food, metabolic processes break down larger, multi-carbon molecules (like glucose) into smaller molecules. This process is called catabolism, and it releases energy. Here's a simple example of a catabolic reaction:
AB -> A + B + energy
For more on this principle, look at the section on Le Châtelier's Principle on this page from Washington University.
Our body can use the energy released to drive more chemical reactions. Specifically, our cells make a molecule called ATP, which is used all over the cell when energy is needed. However, our cells can't convert 100% of the energy from catabolic processes into chemical energy, some of it is lost as heat (or entropy). When we use ATP, a high energy phosphate group is hydrolyzed (chemically split) from the ATP molecule (ASPU, look at the arrow on the right for energy release):
This releases energy, some of which (during exercise) is used to make muscles contract, but much of it is lost as heat. We heat up when we exercise because the muscles are being used more than at rest, which means more ATP is being used to contract the muscles, which means more heat is being released by the ATP hydrolysis reaction.