@JBS can you give more details on what kind of things on bars are hard on the back???
I'll just start off with this... everyone thinks it's only the extension. Can't you hurt your back by bending over and lifting something heavy? Well... there are movements like this on bars... like a Maloney throw.
At a high level... tons of stuff is now being talked about. In-bars stalders are a big one (read the comments on that thread)...
No one can really pinpoint it... is it the arch going in... does it have something to do with the position at the bottom of the circle... or is it the extension out?
We have had issues with kids when they start working Maloney's... and this is before they were catching and doing the huge reverse tap after the catch. Pain during the throw. This basically told us to stop and figure out what we needed to do. More core and posterior chain strength was what has really made the difference for us.
The reverse taps out of Maloney's and such. We have never had any issues with this one. The bigger issue seems to be when it is required for all gymnasts to learn whip tap front giants... definitely not the right technique for everyone.
Catching the low bar in an arch... like a pak. However... the same issue can happen with bails / overshoots or even straddle backs. It's not that hard to get in a weird position before the hand impact the low bar.
Also... the arch that can happen during the catch of a release skill... especially if they catch close.
Another huge one that we have seen is when coaches insist on the "narrow hand grip" on bars. This does not work for athletes with tight shoulders. When the hands are close... the shoulders do not open as much on a tight shouldered athlete and the forces migrate to the low back. There is no reason for narrow hands on bars... just slide them on top during the weightless time right before the pirouette.
Also... some athletes have more mobile hips in a straddle position. For this reason we teach all of our athletes to straddle tap at some point. They do not have to use it... but it may be beneficial for some.
Another one is mistakes on release skills where they for some reason do not let go. Like a Tkatchev that the athlete just jerks and kind of drifts over in a huge arch.
I would add some of the popular "timer" drills into that category above. Tkatchev timer to a big arch while hanging onto the bar... Jaeger into a hugely strange arch while hanging on the the bar... don't like either one of those drills.
At a low level... really not much... mainly just things done improperly. The movements at a low level usually aren't as aggressive. Bad casts come to mind... especially when athletes are pushed to do all straight body casting when they are not strong enough.
Not sure what I missed there or what I even typed... but there is some of it.