Elizabeth Jackson — with muscles flexed and mind focused — pinches a tiny handhold high on a wall at the Rocks and Ropes climbing gym. She loses her grip. And falls. But it's just a short plunge before she's caught safely on a rope anchored above. "Some of these routes are awesome!" Jackson raves when she is lowered to the floor moments later. "They're made like a real climbing route — like you'd find out on a rock." Designers of indoor climbing routes — known as route setters — fashion a sequence of hand- and footholds to create a vertical path that's challenging but doable. "A good route needs to be thought-provoking," says Jason Mullins, a veteran route setter and owner of Rocks and Ropes, 330 S. Toole Ave. "It requires the climber to unlock the correct sequence of moves." Mullins and fellow route setters Greg Rupp and Luke Bertelsen keep Rocks and Ropes fitted with 100 to 150 routes at any one time. They leave each one in place for eight to 10 weeks before designing a new version. The 35-foot-high routes — with color-coded strips of tape marking holds for each route — range from 5.4 to 5.13 in difficulty. Technical climbs are rated Class 5, and therefore the number 5 always precedes a decimal point — with the number after the decimal point noting the level of difficulty. The rating system differs from the traditional decimal system in that a rating of 5.12, for example, is higher than a rating of 5.6 or 5.8. "We have a room filled with thousands of holds — all different shapes and sizes — that we use to set the routes," says Bertelsen. He climbs on a ladder to install lower holds, dangles from a rope to fit the upper sections. "This climb calls for explosive moves. It's very well set — a good workout," says climber Roland Freeland after spidering up a 5.10 passage to the ceiling. Gigi Owen, sampling some routes with fellow climber Dorea Kleker, notes that climbing is a uniquely personal challenge. "Each route is different for each person because of size, strength and balance differences," Owen says. "That's what makes it so interesting."