While it is true that these Krav Maga classes are designed for any "Joe Schmoe" to do, like another poster said, I would seriously consider not signing up for it with your condition. It is not for everybody.
You don't become flexible with the stretches and warmups that you do before training at KM, you need to have some degree of flexibilty beforehand. With an arthritic condition in your back, I can think of few things worse for you than KM.
They will require you to sign up and agree to pay for six months. While you can get out of the contract by demonstrating a medical condition with a doctor's note, I would consider something less stressful than Krav. Just my 2 cents, but I would not like to see someone getting hurt or wasting their money.
Go to Columbia and talk to Jeff Mount, lay it all out for him and he will give you a straight up yes or no if it something he and KravMD thinks you can do without causing more issues.
The one thing I always liked about the Columbia guys is that they would work with you to make sure you, as an individual, got the most out of the class and that the techniques worked for you.
Of course my wife and I went during the day (morning) when we had 8 - 12 people in each class instead of the 20+ they have at the evening classes.
Talk to Jeff, he won't BS you.