"We can tell what issue most people are going to have when they walk in the door," says Allen Sparks, an instructor and five-year veteran at iFly San Diego, an indoor skydiving facility.

We're sitting outside of the glass wind tunnel, waiting for my group's turn to fly, and I ask Sparks what he might expect—hypothetically—from a dude in his late 20s who considers himself athletic.

"The ideal place for your hands is above your head," he says, positioning his arms in a triangle, like he's going through a body scanner at the airport, "and guys with a lot of shoulder muscles tend to tense up and hold their hands at their forehead." Curious whether my shoulders qualify as muscular enough, I glance down.

"And is there a certain background that transfers well to skydiving?" I ask. I dove (into a pool) in college and am hoping that will give me an advantage.

"Pilots and scuba divers," he says without pausing.

I scrunch my brow.

"Pilots understand the mechanics of it," he explains, "and scuba divers know how to relax." He points to a guy in the tunnel who has his hands out, as if to do a pushup. "You can't muscle your way through this. You have to let the wind do the work."

A few minutes later, I'm called to fly with a family of five. The tunnel operator will adjust the wind speeds based on each flyer's mass, starting with the youngest, who can't be older than five years old. iFly prides itself on accessibility—they accommodate people in wheelchairs, almost any age ("three to 103," say Allen), and anyone up to 300lbs—but some body types are better predisposed for flying. Too much bulk, whether it's fat or muscle, makes it harder to develop the proprioceptive body awareness required to maneuver in the air.

All dressed in baggy jumpsuits and wearing helmets, we march in and sit down on a bench outside the tunnel. My instructor, Nick Riedel, motions for the kid to approach the door. Standing inside, on the mesh wiring above the wind-generating fans, Nick puts his hand on his hip. When the kid is halfway into the tunnel, he floats upward, and Nick slides him through the triangular space between his arm and body. The kid smiles and wiggles while his family takes pictures.

You can't muscle your way through this. You have to let the wind do the work.

Next, Nick points to me, and I walk up to the door. His hand isn't on his hip, so I'm not sure how he'll spot me, but as soon as I enter the tunnel I'm airborne, and he's holding my suit from the side. As quickly as possible, I try to hit the position we'd practiced beforehand: arms above my head, my chest back and relaxed, and my legs in a V-shape and slightly bent. Five seconds in, when I finally notice Nick again, he's pointing to my arms. With a slight grin, I extend them fully, past my forehead.

I expected to feel as if I were free-falling, but as I become more or less stable, the sensation is more like weightlessness, a slight floating and falling without the stomach-turning adrenaline I'd get back in my collegiate diving days. On my second and third flights, Nick motions for me to drop one shoulder, which sends me spinning until I bump into the wall.

After he pulls me back into the middle of the tube, he motions to drop the other shoulder to spin the other way. Instead, I twist my hips, bend at the waist, and instantly drop toward the mesh wire. Flying? Not so much.

After my last flight, Allen shares his advice: "Almost everybody has a strong side," he says (for most people, it's the left), "and it's almost impossible for them to spin the other way at first."

That left me with a few questions. Namely: How in the hell is a guy supposed to get better at indoor skydiving—especially if you can't just "muscle your way" through it? He gave me five key training tips—concrete things I could focus on at the gym to improve my performance in a sport without barbells, pullup bars, or even really gravity.

Give it a read here: Five Fitness Training Tips of Competitive Skydivers.