Knee + foot ascenders

Do any rock climbers use these? I use them at work to get up a tree without wasting arm strength and was considering bringing my ascenders with me climbing incase I want to do anything requiring rope climbing. Ive noticed everyone uses hand ascenders with loops for your feet. It's a great technique, but arborists tend to avoid it for the sake of maintaining upper body strength. Is there a safety concern in rock climbing when it comes to foot ascenders??

No, no safety concerns. I used to have a foot Gibbs ascender for caving systems. It's just easier to get the ascender on and off the rope if it is within reach.

No safety concern. I had a partner who used a system with one hand ascender, a micotrax on their chest, and a foot ascender. Too much going on for my taste but seemed like it saved him a lot of energy.

​Some people even prefer the frog way of doing things​​​

Ross Goldberg wrote: Some people even prefer the frog way of doing things

I definitely prefer the Frog system these days. The usual climber system is what used to be referred to as the Two Foot Texas system. It is of course not that efficient. The most efficient system would probably be the double bungie Rope Walker system, but it's not good at getting on and off the rope with ease and sucks on less than vertical terrain. 

The double bungee rope walker system has been replaced with an easy on and off knee ascender in the arborist world, check out the Mini Saka.

I also am an arborist, and have been a climbing guide for quite a few years. When I climb rocks I use my foot ascender on the right, a hand ascender with a foot loop on the left, and a micro trax. I connect to the hand ascender with a tether to facilitate switching devices for descent. It works well and all the tools have multiple uses, with the exception being the foot ascender. I’ve never noticed any safety concerns with it.

Our to go system is:
* CAMP Turbo Chest attached to the right of belay loop with twisted mallion and fixed with something like MTDE Garma;
* left jug with foot loop;
* right CAMP Turbo Foot.

Versatile setup. Keeping a Petzl Rollclip on the jug one instantly gets 2-to-1 hailing system. Just drop in a rope loop into the rollclip, unclip turbo foot, clip it to hailing rope end and kick the rope for a while. Click, click, click and in jugging mode again.