Athletes typically push themselves beyond barriers and past personal limits. And for competitive climbers, the force they defy is Earth's gravity itself.

Climbing is an evolutionary holdover. As our ancestors learned, if the animal trying to kill you is unable to climb a tree or rock, that's where you're safe. That survival skill is rarely called upon in modern life, but it remains in our DNA, often surfacing as a passion among kids and speaking to the innate joy of going up!

Competitive climbers never lose that love, even as they struggle to perch on a postage-stamp-size piece of real estate, leap from one egg-size outcrop to another, or scale inch by inch up a sheer wall, then an inverted wall, then a nearly upside-down wall.

These spider-men and -women flex taut muscles in their backs, shoulders, arms and legs -- muscles that are soft and undefined on non-supers like the rest of us.

And more indoor climbing walls are opening to fulfill that love, with international competitions following.

Compared with natural cliffs and boulders, indoor walls are a design science. Climbing holds are strategically placed by a "route setter" who plans for the climb to get progressively harder and weed out all but the strongest competitors.

Routes are also designed to be "visually spectacular" for audiences, as setter and professional indoor and outdoor climber Robbie Phillips put it, to "make climbers do interesting moves" such as long reaches, dramatic jumps and odd contortions.

Professional climbing walls create an even playing field for competitions, even if that field is vertical. And in September, many of the best climbers descended (or ascended) upon Scotland's Edinburgh International Climbing Arena, one the largest indoor climbing centers in the world and host to one the sport's half-dozen annual climbing World Cups.

Wall climbing ... and running

As competition, indoor climbing has three main disciplines: speed climbing, bouldering and lead climbing. Bouldering, climbing a designed boulder-like structure (as opposed to a wall), is a burst of power over six or fewer moves.

Lead climbing up a wall is a sport of endurance and technique in which climbers are tethered by a rope in case of falls. Speed racing is a head-to-head match going straight up at a dizzying pace.

Speed still matters in lead climbing because the climb must be done in under 6 minutes. As difficult as it is to get to the top, time pressure doesn't help -- not that you can take too long anyway, because a climber's strength will quickly run out from just hanging.

For lead climbing, audience appeal lies in the acrobatics designed by the route setter, but also in the tension of climbers trying to make it to the top of a 50-foot wall. You see the struggle, the missteps and the harrowing transitions.

Most competitors slip (at the Edinburgh World Cup, around 70% failed to reach the top in the qualifying round, and closer to 95% fell in the more difficult route of semifinals), so when they do reach the top after a very difficult final stretch, the crowd erupts.

In the final round of this World Cup, not a single competitor made it to the top, though several came very close. The winner is the one who makes it farthest.

Speed climbing may be the most fun to watch as the athletes frog-hop and monkey-climb a slightly inverted wall with seemingly no effort. It's an all-body power surge of less than 10 seconds up a wall of 15 meters (nearly 50 feet), pitting country against country. The fastest at the Edinburgh World Cup scaled it in 5.89 seconds, just shy of the world record, a feat that made the audience of about 700 gasp and cheer.

Speed winners are determined by several rounds of single elimination. A false start is an automatic loss, with no do-over. For some of the men at Edinburgh, a false start was the difference between a gold and silver medal in the dramatic last round.

Climber strategy (called "route reading") happens just before the competition begins. They come out of isolation to see the wall's structure for the first time and have an "observation." Standing on the ground and looking up, their arms reach into the air, plotting the movements in their imagination. "Left foot there. Reach to that one next. Cross over there," they seem to be thinking.

There are various holds: crimps (a small edge), pinches, pockets, slopers (the big rounded shapes) and large structures called volumes that change the angle and dimension of the wall itself. Each is challenging in its own way. The slightest misstep or miscalculation, and the climb is done.

As interesting as it is to crane your neck and watch the best climbers scale geometric obstacle courses, even more awe-inducing are the paraclimbers doing the same with prosthetics, missing a limb or two, blind or having survived a stroke or other mobility issue.

For those without a limb, there are fewer positions available, and weight is divided among available hands and feet. "Missing a limb is missing an opportunity for weight shift and purchase points," explained Phillips, who tested routes at the Edinburgh World Cup competition. "Some are able to use the stump of a leg or arm, but that's not the same purchase power of fingers or toes."

Visually impaired climbers were led to the wall by their guides. The rock music blaring in the arena was shut off, and the announcer asked the crowd for silence so that the climber and their guide could hear each other by headset during the competition. The guide described in detail what the climber couldn't see, like where their feet and hands were in relation to a given hold. They quickly worked out a strategy at each move.

For out-of-reach holds, climbers with sight must carefully connect hand and eye at their destination in space. But a blind climber must make small leaps of faith. Once they shift their bodies, they hope to quickly find a finger or toe hold before they fall.

A mecca of indoor climbing

Although the equipment and terminology is basically the same, indoor and outdoor climbing are different sports. Indoor climbing is generally a lot more physical and has evolved beyond just training for outdoor climbing into a separate discipline for those who compete or use it as exercise.

Outdoor climbing is a lot more varied, given different rock types, and includes additional skills such as roped sports climbing and Alpinism (bagging high mountain peaks).

The Edinburgh International Climbing Arena appeals to both types. What's odd, or perhaps inspired, is that its climbing structures are bolted onto actual dolerite rock faces that make up two sides of the enclosed structure. Inside, the cliff is wet in places, and fungus and shoots of wild grass grow out of the rock.

It's basically an adult playground. An obstacle course of logs and rope netting dangles from the roof. A third-floor café has a wall of observation windows. And some of the climbing walls are whimsical; one colorful tower is reminiscent of a helix-shaped Jenga game.

The World Cup is a festive and well-organized affair, carrying the drama, thrills and heartbreak that often come from watching any athletes performing at the highest levels.

Contestants were mainly from the United States and Europe, but in general, the best tend to be the Japanese, Austrians and Slovenians. Some have Olympic dreams, and others are enticed by prize money, up to 3,620 euros (about $4,200) at the World Cup. But all come for a deep love of the sport.

Men outnumbered women in Edinburgh, but not by much, and the age range roughly spanned teens through 30s.

An announcer pumped up the cheering crowd. When the power went out for hours on the first day of competition, the play-by-play commentary was improvised on a bullhorn. That, combined with a brogue, made it entertainingly incomprehensible.

Popularity is going up, and up

Indoor climbing is on the ascent, no matter how you define in. Participation is growing as more and more climbing gyms open in cities around the world; Edinburgh alone has five.

Corporate sponsorship has turned the sport pro; one industry publication called it "the sport that will dethrone CrossFit." And speed climbing achieved the sporting world's highest honor: acceptance into the Olympics. It will make its debut at Tokyo in 2020.

It's also one heck of a workout. Climbers are built like gymnasts: strong but agile, and lightweight because the handicap of added weight and the physics of gravity require it. The lighter you are, the easier it is to hold positions and make transitional leaps. And because most of the effort is in the upper body, arms are the one area of larger muscle growth.

Mentally, climbing requires intense focus. The sport itself is a lesson in mindfulness practice. But off the wall, it's a very social sport, in which climbing clubs often become supportive second families. And competitive climbers travel frequently, a bonus for most.

The best training is probably the most fun: hours on the climbing wall. And because it's such a new sport, the level of techniques is still largely undeveloped. There are a couple of tools that have been around since the 1980s, such as the finger board and campus rungs that replicate some holds. But any variety of weight-bearing exercises (hiking, stair-climbing, running, dancing) will promote agile muscle growth.

There is inherent risk in climbing, but that's where proper training is key. Climbers learn how to use safety ropes and harnesses and avoid injury from falling. It's possible to sprain an ankle or wrist on descent, but that's rare. A 2001 study found that the most common injury in indoor climbing is overuse, such as a pulled tendon, and a 2013 study declared indoor climbing a low risk for acute injuries.

Less serious but more common is pain from the extended use of key muscles. "That tight, weak, swollen feeling in the forearms while climbing," explained Phillips, "that comes from the accumulation of lactic acid combined with restricted blood flow." Competitors are often seen getting arm massages before or after climbs.

Besides general fitness, the main requisites for starting the sport are cost and access to a rock face or climbing gym, which often has single-entry or monthly membership fees. There is needed equipment, too: climbing shoes, chalk bag and chalk, close-fitting athletic clothes, a harness, various ropes and carabiners.

But the child-like appeal is clear. And competitively, in what other sport, besides maybe stair climbing, do you eventually come out on top?