Warming up is the crucial first step. For a climber, the most important muscles to warm up are those in the forearms that control the all-important finger strength. Jumping right onto a difficult climb with cold fingers and arms generally results in a “flash pump.” This is the phenomenon of an almost instantaneous swell and cramp that turns the forearms into useless chunks of pumped-out muscle.
This can happen in a ridiculously short period of time and can leave a climber unable to even tie shoelaces-for hours. Warm up your arms by gradually massaging your forearms and flexing your wrists and fingers. Then start with very easy climbing.
Gradually load your arms by climbing first on lower-angled terrain, then progressing to steeper and steeper walls. Also, start with large handholds and slowly move to smaller crimpers as you warm up. At the first hint of a pump, step off the wall and rest.
A good warm-up for most experienced climbers is simply to climb a couple of routes that are easy and straightforward. Repeating climbs that you have done several times before also helps because you know what to expect and will not be gripped by the fear of the unknown.
The other sure way to get a flash pump is to start the gym session on a climb that is scary because that leads to gripping way too tightly and climbing slowly. The warm-up period should last 10 to 15 minutes, or until you are feeling warm and loose, and all, jitters have been worked out.
Traversing: One of the very best ways to warm up in a gym is by traversing. Traversing is climbing sideways across the wall. Most gyms allow climbers to traverse below a line that is 8 feet or so above the ground. That way the climber is never high off the deck, maybe a foot or so at the most, so ropes are not required. Traversing is a great activity for climbing in the gym by yourself.
A good traversing technique has the climber moving with the hips parallel to the wall, shuffling hands, and feet in one direction or the other. Try to stay on the inside edges of your feet. Try not to cross your hands or feet. Imagine Spiderman moving sideways across the wall, hips in close to the wall, and moving slowly and smoothly.