Here is Brian M traversing his work-in-progress bouldering cave. There are no visual tricks or slight of camera here. He is clinging to the ceiling! In the background, there are vertical wall sections slightly obscured by his body. Towards his feet (right half of photo) there is a wall with an 18-degree back-angle.
His goal is to put up a few more horizontal "cave" sheets up, and basically traverse the ceiling while inverted...using the holds, I-beam, pullup bar, or whatever else he can grab. As you can see, he's grabbing what only allows for a three-finger hold.
In the background, you can see his hanging Rock Rings.
The bouldering cave is most taxing on grip. However, during the course of even a simple route, the entire body becomes engaged at one time or another. Climbing is a highly specialized skill. It's a blend of technique and fitness. More and more functional gyms are gravitating towards climbing for their cross-training or skill development.