If you've been anywhere near the climbing training conversation lately, you've probably heard a weird word floating around: CARCing.
It sounds made up. It kind of is. But the idea behind it is old, and the early results from people testing it are hard to ignore.
CARCing — sometimes called "cardio for your forearms" — is basically ARCing (aerobic respiration and capillarization) done away from the climbing wall. You squeeze something at low intensity for a long time. That's it. No climbing holds, no wall time, no trashed skin. Just low-level endurance work for your forearms while you do other stuff.
Why Would This Work If You're Not Actually Climbing?
This is the part that trips people up. If you're not on a wall, how can you be training for climbing?
The answer comes down to what limits your endurance as a climber. Research on climbing physiology points pretty consistently to peripheral adaptations as the bottleneck — the stuff happening inside the forearm muscles themselves, not your heart and lungs. If you're a reasonably healthy person with decent cardiovascular fitness, your ticker is almost never the reason you fall off a boulder problem on attempt three.
What matters is capillary density in the forearm, mitochondrial function, and how efficiently those muscles use oxygen. Your finger flexors don't care what they're gripping. They don't know if it's a 20mm edge, a foam ball, or a grip trainer. If blood is flowing through the muscle and the mitochondria are being stressed at the right intensity, the adaptations happen regardless of the tool.
There's an important distinction here. Isometric strength training is angle-specific — the position you train in matters. But endurance and energy system work? That's about what's happening metabolically inside the muscle. The mode of exercise is much less important for those peripheral gains.
One Person Actually Tested It (On One Hand)
The most interesting data point so far comes from a self-experiment where a climber trained only one arm with CARCing for four weeks, leaving the other as a control. Same bouldering sessions. Same hangboard work. The only variable was three hours per week of low-intensity grip squeezing on the left hand.
The protocol was simple: one hour per session, three days a week, aiming for zone 2 intensity (low enough to hold a conversation). No pump. Just heat, blood flow, and a dull ache after 20–30 minutes suggesting slow-twitch fiber fatigue. About 100 reps per minute on a grip trainer. By the end, that added up to roughly 102,500 reps and over 17 hours of total work on one hand.
Before and after, both arms were tested using a critical force protocol — one of the better ways we have to measure climbing-specific endurance.
The Results
Strength actually went up 3.4% in the trained arm. Nobody expected that. The control arm barely changed (up 0.7%). Whatever was happening, it wasn't costing strength.
Critical force — the main endurance metric and the whole point of the experiment — improved 7.1% in the trained arm. The control arm actually dropped 2.5%. So the untrained arm was losing endurance fitness during those four weeks while the trained arm was gaining it.
For context, climbers at the 9a level tend to sit around 45–50% critical force. The test subject started at 31%. Plenty of room to grow. But even accounting for beginner gains, a 7% jump in four weeks from zero additional wall time is noteworthy — especially for boulderers who typically skip endurance work because it cuts into project sessions.
Rate of force development did take a small hit. The trained arm was about 125 milliseconds slower to reach 80% of max strength. If you climb on small crimps where contact strength matters, that's worth paying attention to. For open-handed, slower climbing on something like Fontainebleau sandstone, it didn't seem to make a difference.
W' — the anaerobic work capacity above your critical force threshold — decreased in the trained arm. But this isn't a bad thing. It means the aerobic system picked up work that was previously being done anaerobically. Less lactate, faster recovery between attempts, more efficient energy production. The ceiling didn't drop. The floor just got raised.
How to Actually Do It
What to Squeeze
You need something you can grip at low intensity for extended periods. The original version used whatever was in the car — steering wheel, dashboard trim — but that's not exactly repeatable or ergonomic. A basic grip trainer works, though palm discomfort becomes a problem after about 30 minutes. Band-resisted finger curl trainers like the Dual Edge BXE are a better fit for longer sessions since you can scale resistance with different bands and train through a full range of motion without grinding into your palm.
The whole point of CARCing is that you can do it anywhere you've got a free hand. Walking the dog is a popular one. Watching TV in the evening. Sitting at your desk between emails. Waiting around at the crag between attempts. The bar for entry is low — you just need a tool and 20–60 minutes of time you were already spending on something else.
How Much, How Often
There's no established protocol yet. The four-week experiment used three one-hour sessions per week, which is probably the high end for most people. A more reasonable starting point is two to three sessions at 20–30 minutes each, building volume over time.
Keep intensity in the zone 2 range — low aerobic effort, not a pump session. If you feel pumped, you've gone too hard. You want warmth in the forearm, steady blood flow, and maybe a dull ache after 20 minutes. You should be able to hold a conversation (or, you know, actually drive a car) while doing it.
If you're worried about the interference effect — the risk that endurance work blunts your strength gains — keep CARCing sessions away from your hard climbing or hangboard days. Treat it like easy cardio for your forearms.
What We Still Don't Know
This is all early. One self-experiment and a handful of anecdotes is not a study.
The biggest open question is whether this works for someone who already has strong endurance. The test subject started with average critical force. Someone sitting at 45% might see diminishing returns. We also don't know the minimum effective dose — three hours a week produced results, but maybe one hour does too. Nobody's tested that yet. And most people doing this don't have a good way to measure forearm-specific intensity, so "warm, not pumped" is about the best guidance available.
There's also the rate of force development question. Does that 125ms hit get worse over longer training periods, or does it level off?
Who Should Try This
If you're a boulderer who avoids endurance work because it eats into your skin or your project sessions, CARCing looks like a real option. You get the peripheral adaptations without the wall time, the skin damage, or the mental drain of pottering around on easy routes for an hour.
If you're someone who wants to improve but only climbs a couple times a week and doesn't want to add more wall sessions, this fills a gap. You can do it during a dog walk.
If you're already doing structured ARCing on the wall and it's working, there's probably no reason to switch. But if you hate it — and a lot of people quietly hate it — this gives you a way to get similar benefits without the parts you dread.
The physiology supports it. The early data supports it. And enough strong climbers are quietly doing it that it's worth paying attention to. It's not proven, but it's promising, and the downside risk is low as long as you keep the intensity honest and build volume gradually.