There are two different ways our muscles get the energy they need to perform during exercise: Aerobic and Anaerobic.
Our Aerobic (cardiovascular) system carries blood to our muscles with the nutrients the muscles need to create energy to climb or do any other activities. Once the Aerobic system becomes congested and can no longer supply what is needed to perform, the Anaerobic system takes over.
The Anaerobic system creates energy inside the cells of the muscle in a process called Anaerobic Metabolism. It takes hours to recover your Aerobic system once your arms get congested (a pumped or burning feeling). Your Anaerobic system can only create energy for a limited amount of time (30sec.-1 min.) after your Aerobic system quits.
It is important to train each system independently.
To increase your Aerobic endurance, you need to improve your body’s ability to bring fresh blood into a target muscle group (for rock climbers, that would be your forearms) and remove old/used blood. We can do this by increasing the blood pressure in the forearms for an extended period of time (15-20 minutes). This will put pressure on the veins, arteries, and capillaries, forcing them to expand and grow new branches. It’s kind of like upgrading the plumbing system and making it easier for the blood to flow in and out.
You need to climb, nearly constantly without stopping, for 15-20 minutes just below your “pumped” threshold (where there is no lactic acid burn). That is, climb 2-4 grades below your maximum ability making sure you feel fatigue when you’re done, but no burning.
If you start getting a burning or “pumped” feeling, stop and lower right away, then go to a different route, 1 grade easier, and continue for the rest of the allotted time. The burn means you are producing lactic acid, which means you have moved into Anaerobic Metabolism and are no longer getting the benefits you are looking for. Anaerobic Metabolism creates lactic acid but is also limited when lactic acid is present, which is why the duration of this energy source is so limited.
To increase your Anaerobic endurance, you need to increases your muscle tolerance to lactic acid. To do this you need to force your muscles to continue working with lactic acid present. Bouldering is a great way to train in this way -- short bursts of high intensity climbing with frequent but short rests.
The 4×4 is a great routine to train your Anaerobic endurance. Pick 4 bouldering problems, 1-3 grades below your max. Climb each one 4 times in a row with no rest before moving on to the next problem. Do not rest in the middle of doing your 4 repetitions of a problem; do not top out or rest, keep moving.
Between each problem, take 30 seconds of rest before moving on to the next set of 4. You will have done 16 problems in a short period of time. When you’re done your 4×4, your forearms should be burning. If not, choose harder problems next time. If you couldn’t finish, choose easier ones next time.
Make sure you have warmed up your body well before doing a 4×4.
Don’t do a 4×4 before a session of climbing. It’s not a warm up; it will ruin your session.
Do this at the end of a session or do a short session dedicated to just the 4×4.
Lactic acid is toxic to your muscles and long term exposure to lactic acid can be detrimental to muscle development. Don’t do high intensity training where you are exposing your muscles to lactic acid for long periods of time more than once a week; you don’t need it more than that.
When training for climbing, you need to make sure you are getting sufficient rest. Muscle and tendon strength is increased by overloading the muscles and tendons followed by periods of rest and recovery. Overloading without proper rest will cause degeneration and injuries.
Don’t climb every day; 3-4 days a week is more than enough to progress through the grades.