Recently I was working on my finger board downstairs in the house and discovered something interesting to me, if not very original. I can hang onto a single-pad edge with two hands and about 100 pounds added on without too much trouble. With no weight I can hold on with two hands for 45 seconds to a minute. So far so good.
Now hanging with one hand on the same edge, I can hang around 5 seconds with no weight or about 2 seconds with 5 pounds held in my free hand. Why this radical disparity? Good question and I am not sure I have heard or read any good explanation. I think it is partly biomechanical as the two arms, back and abdominal muscles working together help manage the load much better, probably through balance and opposition, than separated. But it seems to me that this finding also indicates that traditional modes of two-handed training require a huge overload for very little real payoff, especially since climbing requires that hands and arms operate independently of each other. So pull-ups, weighted or not, actually add very little to one's overall necessary one-hand strength.
So what are the implications overall? Well for me, isolating one hand for hanging on an edge certainly makes more sense than spending too much time with two-hand hangs. However even this has its limits as I will explain below, And these exercises are hard to do at all for many climbers as one-hand hangs have a very high initial strength threshold. One answer may be to use a climbing wall like a system board and work on holding one arm positions with bad feet. You can also find ways to lessen the load slightly by hanging and pulling with the free hand lower on a rope or sling. Uneven pull-ups on an edge seem like a very good idea as well.
Using a campus board can certainly make sense as well, especially as there is another factor to consider which I will call dynamic or active finger strength, in order to avoid the confusing term "contact strength." I would describe active finger strength as the strength needed to resist dynamic eccentric shockloading, the kind that is produced by grabbing a hold on the go at a distance, resisting a swing, as well as concentric loading as in pulling into the wall, resetting on the hold, and so on. This could be thought of also as finger power in that delivering force rapidly is of the essence. Typically finger strength in climbing is considered in isometric terms, i.e. a consistent load and position through the move. A simple observation of most single moves shows this is hardly the case. The initial load on the fingers is clearly much more severe compared to the later maintenance of the grip. Whatever can be done to limit the initial load (through technique, etc.) leaves more energy for successive moves. But this dynamic or active load continues throughout the use of one hold, since climbers are adding to the load by resetting their fingers, shifting their grip position, pulling their bodies in closer and of course pulling up to gain height. At every turn, body weight is shifted creating momentum and increasing force on the fingers. These increased forces rapidly induce fatigue which in turn increases force on the fingers through poor technique, clumsy grip, bad aim etc. resulting in failure on the move. The vicious cycle is experienced by all of us sooner or later.
To me this implies that relatively static strength tests like one or two-handed hangs are potentially misleading indicators of resistance against severe loads. They might provide a baseline but hardly indicate practical strength. The analogy might be that of "safe working load" with construction equipment, where the breaking strength of an item is reduced to 1/5 of the amount to indicate its "real" strength in use. So the ability to hang on with one hand for 5 or even 10 seconds is of dubious relevance compared to being able to very rapidly latch the same size hold 3 feet up and go another two feet from that with a minuscule foot. And two-handed hangs distort that strength measure even more since you almost never execute a move with two hands side by side.
The solution lies in several directions. The first is that static strength measures provide some quantifiable baseline with which to measure progress but little more and may be of surprisingly low relevance in actual practice. Second, in training for climbing you must find ways of safely shockloading fingers and arms in as sport-specific a way as you can. If possible each arm should be isolated to some extent when training finger strength. So, for example, double-dynos on a campus board are not typical in climbing and probably shouldn't be trained. In regular campusing, laddering where your arms are always going one or two (or more) rungs beyond each other should be standard. Third in actual climbing practice, finding ways to reduce the initial shockload of gripping the hold and any subsequent shifts of weight or position should be emphasized. Good foot placement is important here as with so much of climbing. Make sure that in every move, both hands are working together as much as they can to lessen the individual load, since (remember the initial finding above) two hands working together are many times stronger than one hand working alone. Obviously the more quickly moves are done, the less energy is expended hanging on, so using momentum to follow through can be very helpful.
Training arms and fingers in isolation is clearly more stressful and should be done primarily in the context of climbing, most of the time. Specific workouts that involve campusing, fingerboards, etc should be done when fresh and at least 3 to 4 days apart. Oppositional exercises and shoulder strengthening should be consistently done along with liberal stretching of forearms and even having bodywork done as needed. Most important of all is patience as the most useful and least injurious results come slowly and in small increments.