4x4s: training power endurance

The 4x4 is one of the most common structured workouts climbers use to build power endurance — the ability to keep pulling hard moves after your muscles are already fatigued, rather than pure one-move strength or all-day stamina.

What a 4x4 is

Pick four boulder problems around or just below your flash grade — steep, powerful, without good rest positions. Climb all four back to back with little or no rest between them (that's one "4"). Rest around four minutes, then repeat the full circuit three more times (the other "4"). Sixteen problems total, done in four fatigued bursts.

Why it works

Sustained near-maximal climbing burns through short-term energy stores fast and produces lactate faster than your body can clear it, which is what makes forearms and shoulders "pump out." Repeating that stress with only partial recovery trains the same adaptations sport scientists target in anaerobic interval work: your muscles get better at buffering and clearing lactate, and at keeping recruiting fibers even as the burn sets in. The result isn't more raw power — it's holding onto more of your power for longer once you're already tired, which is exactly the wall you hit deep into a boulder problem or a redpoint crux sequence.

4x4 vs. ARC training

They train different systems, and most training plans use both rather than picking one:

Climbers who train this way

4x4s show up across the sport, from gym rats to World Cup competitors — usually 1–2 sessions a week alongside strength and technique work, not as the whole plan.

• Adam Ondra runs a "4x4x4": four sets of four problems with minimal rest inside a set, only about a minute between sets, sometimes in a weight vest — a much harder variant built on top of decades of base fitness, not a beginner starting point.
• Competition boulderers commonly use power-endurance circuits in the same family, slotted into a periodized training block after a base/power phase.

The core protocol scales down fine for club and gym climbers — the elite variants (weight vests, near-zero rest) are what to grow into, not start with.

Running 4x4s with Boulder Blitz

Boulder Blitz's three preset modes aren't arbitrary speeds — they're built around this exact progression, and each one sets both your climb time and your rest time together (mode speed × climbs per round). Set Rapid to four climbs per round and you get four minutes of climbing paired with four minutes of rest: the classic 4x4, at its original proportions. That's the right place to start — it teaches you the pattern before you try to push it.

Once four-minute rest windows start to feel generous, move to Blitz. It halves both sides of the clock — two minutes climbing, two minutes resting, over the same four fatigued bursts — which is a noticeably tighter version of the same workout for climbers who've built some resilience to the format.

Bullet compresses things further still, down to 15 seconds a climb, and at that point it stops being a pure power-endurance drill: there's no time left to work a sequence, so it rewards climbers training decisiveness and speed of movement as much as pump tolerance. Whichever mode you pick, Boulder Blitz keeps the clock and the round count so you can stay on the wall instead of watching a phone.

Grouped block patterns (four fatigued climb/rest rounds, then a longer reset before the next block) are the kind of custom interval pattern Training Mode is meant to cover — see the voting board on the Home page if that's a mode you'd want prioritized.