I am looking to practice a form of martial arts which will be good for reducing back pain abd work stress while improving general body fitness. Which would be best to start with?
Honestly, the best thing you can probably start with is not going to be a martial art at all. Instead, try the following:
One of the biggest culprits in back pain, knee pain, etc can actually be corrected by increasing the strength of the surrounding muscles. I had hip issues and years and years of martial arts did not fix my hip issues, 3 months of doing squats did. Not that a different martial art wouldn't have necessarily gotten me there, but squats were a much more direct route to fixing the underlying problem.
So start working out and engaging in strength training. This can be using weights (e.g., with a program such as Starting Strength or using a bodyweight system such as Convict Conditioning or Building the Gymnastic Body. There's also Fitness.SE, which is a great resource here.
Working out in general will be good for your stress levels. So any form of workout will help. Basically here you can go with any martial art, and which one will suit you best will depend more on your rapport with the individual teacher and how you personally like to de-stress than it will any specifics of the art itself.
I personally find that weapons work in particular is a great way of bleeding off excess stress, but you may not be the same and so it is worth playing around to see what works for you.
You may also find that you have good success with yoga, which frequently is more targeted in this regard than many of your martial arts will be, so it might be worth giving that a try, and it is something that is fairly easy to get started with. Or just go with what Tony Horton says and do your yoga!
You may also find that meditation helps here. Many martial arts have a meditation component if you want to integrate it with your martial arts practice, or you can pick it up separately. Talk to the instructors of different styles and see what they emphasize, or find a meditation/sitting practice and go from there.
Reducing stress, eliminating back pain, and improving fitness have little to do with martial arts styles. Instead, they have a lot to do with the physical culture that a given school adheres to.
In terms of strength, if a tai chi school has newcomers master one-leg squats, stone lifting, barbells, or gymnastics alongside forms practice, that could help. If they just do the form by waving their hands in the air, that will only get you stronger to the most minimal degree. The same goes for a judo, karate, or boxing program: if they encourage proper strength training, you'll get substantially stronger. If they just have you go through a few cursory bodyweight exercises as part of the class warm-up, your strength gains will be limited.
In terms of back pain, generally the problem is either medical in nature, or is caused by bad posture or weak back muscles. This means that it would be more productive to fix those issues in a strength program, with medical advice, or by addressing your postural issues.
In terms of stress reduction, any number of activities could help: cycling, yoga, judo, boxing, gardening, aikido...it really depends on what tickles your fancy. That's a very personal exploration.
Tai Chi. Any kind. Possibly even any teacher should do.
TaiChi is neat, because of the general focus on smoothness. The movements must be carefully orchestrated and done not only with the minimum effort possible, but also with efforts to relax.
The only way to relax fully, over time, is going to include fixing the posture, going as far down as the bones and tendons. The body ends up used to tightness and deformity over years of misuse, but the opening and closing and stretching and rotating required by the taichi movements help fix that over time.
The very simple (note: not easy) action of lowering oneself while keeping the back straight and relatively vertical does wonders for the muscles and mobility of the lower back.
Doing Luta Livre helped my back and posture problems a lot. A lot of different muscles are used in both static and explosive manner.
Luta Livre is -just like Brazilian Jiu Jitsu- a submission wrestling sport, although in Luta Livre no gi is worn. The athletes usually wear tight-fitting clothes like fight-shorts and rash-guards. Apart from that the two sports are very similar, even when the practitioners of any one might have you believe otherwise. It's probably a case of rivalry because of similarity. I've been told by both Luta Livre and BJJ fighters that the other art is all about using power and has less emphasis on technique.
LL is a pure grappling martial art - No punches or kicks are allowed and attacking orifices or single fingers is forbidden as well.
So when practiced respectfully it is quite possible to grapple without much holding back without hurting each other, so i guess it's a great stress relieve as well.
Luta Livre is a very safe sport if both opponents understand some basic rules. Namely that a dangerous lock or choke has to be applied with caution, that tapping out is not a shame and that the bout is over immediately after tap-out.
Relief from Work Stress?
Relief from Back pain? 1. Easy way - Go to a doctor, take medicines. 2. Better way - Do yoga. Sitting for half an hour a day with a straight ,stiff back. 3. Little tough but the best way- If you sleep on a soft cushion, get rid of it. Sleep on a hard surface. Do yoga. Do exercises that exercise your back bones.
Have fun :)
I highly recommend BJJ or Brazilian Jiu-jitsu. I sit on an office chair all day and use to have lower back pain. I even have an inverted table. I train BJJ and muay thai. But, BJJ movements feel similar to Yoga. There is a lot of stretching, and isometric contractions of the muscles. Plus being in the gaurd, and being in someones gaurd you are relieving back pressure. Similar to child's pose. I just know that I don't have any back pain anymore. Give it a try for 3 weeks.