Let’s talk about Muscular Endurance Training (ME Training) as it apples to Rock Climbing, and all the same could be applied for ice climbing as well:
The basic concept of Muscular Endurance is to do the sport specific work in ‘more difficult conditions’ (to quote the god-father of ME training, Verkoshensky’s, definition). You simply overload the working muscles beyond what you’d ask them to do in the ‘race’ or ‘event’. I first witnessed ME training in Rock Climbing way back when…(early 90’s, I’m not sure exactly) watching (slack-jawed) Scott Franklin doing laps on Churning in the Wake, a 5.13a wearing a weight vest. Soon after that Scott Franklin became the first American to establish a 5.14a.
ME Training works, but it has to come on top of solid fitness and health. I’ve heard Scott J tell a story about a very strong British climber that used to do laps on the traverses around boulder wearing a diver’s weight belt though most climbers struggled to finish the same traverses unencumbered. I’ve seen a lot of other good rock climbers (5.13+ climbers) doing this at Smith Rock over the last two decades.
Climbers have to be careful that they do this at an easy-enough grade relative to their maximum grade to make sure they don’t hurt their fingers. They also need to be smart about applying this type of training at the right time in the training, it has to come on top of a regular diet of high-volumes of climbing
I myself have done this workout on my Treadwall, using finger friendly holds. We’ve also had a lot of people doing this type of climbing with a little weight in the backpack (it doesn’t take that much weight!) I know that Josh Wharton who also does this type of ME training on his Treadwall.
A bit off topic. Scott Johnston reminded me of this: Eric Horst, author of Training for climbing, uses extra weight for his ‘max strength’ type training with a systems wall. He uses a max strength protocol (few reps, lots of rest) but keep in mind that this is very different than a ME workout. An ME workout would need to be have ‘on times’ close to the length of the event, so climbing 30 meters, then resting a shorter period, then climbing 30 meters again would be a sample ME protocol.