Before I got into climbing, I used to lift weights, and I've carried over some concepts from that experience into my climbing training. I'll attempt to summarize some of that here.
Lets start by defining some basic terms relevant to climbing:
power The maximum force you can exert (how quickly can you do work).
strength The ability to exert force.
endurance The ability to continue to exert force.
power endurance The ability to continue to exert maximum force.
isometric strength The ability to exert force while not contracting or lengthening a muscle.
contact strength The ability to begin exerting isometric force quickly.
These are basically the things we as climbers want from our muscles.
The force/duration spectrum
Exertion exists on a spectrum. With the highest levels for force generated, it's not possible to continue generating that force for long, so the duration of exercises at this level of force is limited. As we decrease the level of force generated, we can generate that force for longer periods of time. As a very loose rule of thumb, we can guess where we are on the force/duration spectrum by how many reps we're doing:
- 1-2 reps: power.
- 3-5 reps: strength.
- 6-8 reps: hypertrophy.
- 9-20 reps: endurance.
20 reps: cardio.
However, the number of reps doesn't tell the whole story. There are some exercises which are by nature ballistic: in these exercises, the force is primarily generated for a brief period during the lift, and while there might be some continued force after that, the weight is carried on primarily by momentum. We can sometimes contrast these lifts to similar lifts that are slow and controlled throughout the movement:
- Deadlift or kettlebell swing: ballistic, good morning: slow and controlled.
- Pendlay row: ballistic, forward row (bar not touching ground): slow and controlled.
- Jerk: ballistic, overhead press: slow and controlled.
- Clean: ballistic, curl: slow and controlled.
- Box jump: ballistic, squat: slow and controlled.
In the case of the box jump, we literally can't exert force after the initial exertion because we're no longer in contact with the ground. In the other ballistic motions, it's common to simply drop the weight after completing the motion, because attempting to control the weight on its way down would practically guarantee injury. This is worth reiterating: while people might complain about the noise or sound the lunk alarm at certain gyms, with many power lifts, dropping the weight is the correct form. It's possible, of course, to deadlift and set it down in a slow and controlled manner, and this is a valid exercise, but critically, it's a strength exercise, not a power exercise. A slow, controlled motion can build strength, but it simply cannot build the ability to exert that strength all at once. It also doesn't build the coordination necessary to contract large muscle groups in sequence quickly. There's some translation between strength and power, but maximal power can only be reached by training power directly.
Power exercises are rarely worth doing in sets of more than 5. Coordination breaks down and you're risking injury. If you can do more reps of a power exercise, it's probably better to do more weight instead.
The next thing that jumps out about this spectrum is hypertrophy. What's that? Hypertrophy is muscle volume. This is something body builders care about, but for climbing, it's actually a downside; muscle size is loosely correlated with some of the properties we want, but maximal muscles size focuses in on the outlier, a muscle which is disproportionately large in relation to the power, strength, and endurance it can give us. This size comes with a big downside: weight. As such, you'd think climbers wouldn't ever want to work in the 6-8 rep range, but it turns out that range can be useful for working power endurance. More on that later.
Cardio isn't directly climbing related, but it's not a bad thing. First of all, it's healthy. And while it may not directly contribute to climbing, it can help avoid fatigue: good cardio means that you may arrive at your climb less tired from the approach hike. As long as you're not burning nutrients necessary for climbing on your cardio, cardio is mostly a good thing.
All this is to come back to the elephant in the room: strength. Even without training power or endurance, strength translates well into power and endurance.
There's some effects to power training which are not achieved by strength training, such as building different energy systems and fast twitch muscles, but often power is a matter of coordinating the strength that is already there so that muscles fire in sequence, rather than building some new property of the muscles. Coordination can be trained more quickly than muscle adaptation; strong muscles can be made powerful easily.
Likewise, strength translates well to endurance. A study I saw (TODO: find that study again) said something like that if you are operating at 20% of your max effort, you don't fatigue significantly over repetitions; if you're 5 times as strong as you need to be, you don't need endurance. And while it's often unrealistic to be 5 times as strong as you need to be to perform a climbing move, there's some evidence (TODO: look that up too, I think it's from Eric Hörst) which shows that endurance can be trained quickly: while strength for a climb might take many training cycles over years, it's possible for someone with the required strength to train endurance to near-maximal potential with a two week training period before their intended performance period.
All this gives us a focus: while we'll spend some time training power and endurance, our primary focus is strength: as we near a performance goal, we can test and fill in gaps in our power and endurance as needed if we have the requisite strength.