Re: Can you use Knee Pads in Aikido?
Very bulky pads can cause more strain on the top of the ankle as your knees are elevated when you are in seiza, or the bulk can lead to strain on the side of the knee with side falls if it prevents your leg from landing flat. Most pads aren't that bulky.
Something that goes completely around the knee tightly can bunch up under the knee or interrupt circulation to the lower leg. Stretching and range of motion without the pads on later can help restore this.
I had a lot of suwari waza coming up through the ranks, and pads saved my knees from bleeding too much but I never found a perfect pad. Aside from these two issues above, pads are not harmful IMO. Samurai armour - look up some of the designs of the suneate, they definitely did not have a problem with knee pads.
Some schools only practice pins from seiza or shikko dachi. Some, and it seems the older schools moreso, do standing pins or pins with one knee up. Don't bludgeon your knees through an injury. Modify as necessary.
Don't drop to the asphalt on your knees in a real situation, and be mindful that you will fight as you train.
In getting ready for grading, I found that to be ready for suwari waza and hanmi handachi I trained five to ten minutes regularly, then stopped for the day. I stopped with any sharp pain too. Not training these movements for months, then doing a marathon session to get up to speed is the most painful and damaging approach and ultimately counter productive in my experience.
Much further afield than your question, but hopefully that gives you what you need.