For any gymnast worth his weight in spandex onesies, the front lever is as basic as a layback. For climbers, on the other hand, the front lever is a royally difficult pose that very few of us could do off the couch. Still, it is one of the most attainable gymnastic positions for mortals and slouches alike, and practicing/training to do one will yield the core strength of an iron rod. Wunderbar!
The front lever is when you hold onto rings or a hangboard with straight arms and make your body rigid and perfectly horizontal. The main requirement for a front lever is total body tension, and attaining this insane core fitness will help us climb and rest better on steep routes by making it easier to direct body weight through the feet. Core strength also helps us check our bodies when we swing out from the wall. If you've ever seen Chris Sharma's feet cut, his body swings out but only to plumb vertical, as if he hits an imaginary wall behind him. Core strength is what keeps him from peeling off and falling.
All climbers could benefit from a stronger core. You can get there by training to do a front lever. The best apparatus to practice on is a set of gymnastic rings. If you don't have those, you can use a hangboard or a pull-up bar at a gym.
The front lever is difficult because you are leveraging the majority of your body weight around the fulcrum of your shoulders. Moving the weight closer to your shoulders (i.e., bringing your legs up to your chest) shortens the length of the lever (your body) and hence reduces the force (strength) needed to maintain the position. This concept gives us a basic progression of poses that will help us work up to the full front lever.
Try to hold each of the positions described below for 10 to 15 seconds before moving to the next one. Do three to five sets four times per week. Always strive for perfect form before moving on.
Both legs to chest: This is the first position to master. Grab the rings/hangboard and bring your knees up to your chest. Try to flatten your spine. Bring your shoulders down away from your head. Pull hard with your shoulder blades, trying to squeeze them together. Keep your spine straight by looking up into the air as opposed to down at your toes.
One leg out: Starting with both legs at your chest, try to straighten one leg slowly. One leg remains tucked to your chest while the other is pointed out. Tuck your coccyx down by straightening your spine and squeezing with the core muscles of the abdomen. Do one set with the left leg for 10 to 15 seconds, and another set with the right leg for the same period. Hold the position, and maintain the form: one leg straight, back flat, face pointed up.
Both legs out: Once you can hold a front lever with either leg out, work on slowly bringing the next leg out. The trick is to pull your shoulders away from your chest: Flex your shoulder blades and squeeze them together. Maintain a flat spine. Don't forget to breathe. A tense core should not interfere with the diaphragm.