Three-dimensional matrices

Today I went to the trainer's because my quad still hurts. The trainer had me sit and push with my leg against his hand, do strides with my leg attached to a stretchy thing, and do lunges with my foot on a squishy thing. Then he asked me, "do you know what a three-dimensional matrix is?"

Needless to say, I was quite surprised at this question. "Yes," I said, unclear as to their relevance to the situation. "Are you sure? Three-dimensional matrix? You've done those before?" he asked, and I assured him that I knew all about three-dimensional matrices in math. "Oh! Math!"

It turns out that in the training room, a three-dimensional matrix is this thing where you do lunges except that you jump when you do them, and you do them in three directions (front and back, side to side, and 135° diagonally behind you). I did several sets of these.

Now I know four equivalent definitions for "compact," and two non-equivalent definitions for "three-dimensional matrix."