That really depends on what you mean by a "bad back". Do you have a slipped disk? Is your spine degenerating? Are your back muscles weak? Do you have kyphosis or scoliosis? The best exercises for losing weight are aerobic exercises. This use up the most calories and thus end up burning used fat. While running is one of the best aerobic exercises, its appropriateness for you would depend on its effect on your back. Swimming isn't as effective at burning fat as running, but it provides a good full body workout, is less of an impact on your joints, and is excellent at building your endurance. The water will also provide support for your back and other joints. Different exercises will strengthen different back muscles. For example, the reverse fly will work the upper back (rhomboids) and rear shoulder muscles, while the lat pull downs work the Latissimus Dorsi (upper back) muscles. Whatever you work into your back routine, don't forget to work supporting muscles (like the abdominals). The more muscles that are supporting your back, the less stress they have to bear and the easier it will be for them to grow. With any new exercise programme, you should consult a physician. You should also consult a fitness trainer who can personalize your workout for your back. [Added in response to bapuji] As I mentioned at the beginning of my answer, it depends on what the asker meant by "bad back". If the back has some inherent problems (like scoliosis, kyphosis, arthritis, etc), then rest will do nothing. My answer still is appropriate in answering the question how to lose weight and strengthen back muscles.

BKS Iyengar is this yoga guru from India who developed his own style (Iyengar Yoga) which is designed especially for infirm/aged people, polio victims, people with injuries etc. It uses all the healing, strengthening and flexibility enhancing properties of regular yoga but minimises risk of injury and overcomes challenges through the use of ropes, blocks, walls, bolsters etc and specially designed techniques developed to address these problems. If you find a good Iyengar Yoga practitioner in your area (ask for accreditation) and have a consultation so you can customise your treatment/practice sessions, I believe this low impact/high strength method utilising chiefly your own body weight and tackling the problem holistically will probably have better results than using free weights or gym equipment. Naturally, having your mind in the right 'place' helps as well, and yoga is great for this. I myself cured a very painful lower back problem by doing only a few private consultations with a yoga expert (Ryoho Yoga, not Iyengar, which is more physical and so suited me better) over a few months. He'd send me away with specific exercises for me to do for 5-10 minutes twice a day plus I attended 1 or 2 classes a week – by the end of it not only was my back better and my posture improved, I also had more energy, a clearer mind and found smoking cigarettes very unappealing, so much so that I had to quit! The link below is to the official Iyengar website. Ask around your neighbourhood though for any other types of yoga being practiced, and try to find that special person who's recommended by other people they've treated. If you click the sub-heading "Teachers" on the Iyengar site it will actually give you the contact details of accredited teachers in your state, and all over the world. http://www.bksiyengar.com/

Walking is a great exercise for losing weight and strengthening back muscles. Running can be good depending on the severity of the back problem. Ryoho Yoga is excellent expecially if the class is designed for either of these issues. Yoga exercises such as Boat, Bow and Bridge are great for losing weight however, it would be very wise to do these under the supervision of an experienced teacher especially with a bad back. Sit ups (always stretch through the heel and pull the toes back) are good for two reasons. 1. they help to strengthen the abdominal muscles, if the right sit ups are done then the psoas muscle which plays a major role in supporting the spine is strengthened. 2. sit ups contract the abdominal area. If one wants to lose weight there needs to be less food consumption. contracting in the stomach in this way will help to eliminate the need for excessive food. If you lie on your belly and lift the legs up off the ground making sure they are straight and changing the width of your legs you will definately help a lower back problem by strengthening the lower back muscles. If you lie on your belly and lift the torso with the arms either straight out or interlaced behind your head you will strengthen the upper back muscles. In the same position, hands behind the head and legs wide and swinging around to left and right you will strengthen the waist muscles in the back. If you give it a go you will feel exactly where these exercises effect. I also healed a very serious and debilitating back problem with Ryoho yoga, shiatsu and macrobiotic eating. Back problems can take some time to fix expecially if it is a chronic problem. you need to be disciplined and see the future as weight and pain free. Eliminating stimulants from the diet is also a good idea and will make the process of success much quicker.

Go swimming. You can work your entire body without too much strain while having fun in the process.

The books are starting to show a little age, but my wife and I really like "Callanetics" by Callan Pinckney. Her program is sort of a mix of yoga and pilates, for both flexibility, stregnth, and toning. The exercises are done without special equipment, and are VERY back sensitive, as she herself had some serious back problems. There are books for beginner to advanced, and other specific purposes, along with some videos.

I myself had a lower back fusion (S1,L5,L4 and L3). I just started to ride a bike and my back does not hurt at all when I ride. One thing that you can do is to strengthen your abdomen.

Get an exercise ball from Walmart or KMart. It provides an all over safe workout for the back. No danger of hurting immobilized joints from laying down doing ab crunches, sit-ups, .....

You best bet is swimming..if you can't swim then walking and yoga..infact you may want to combine the 3 for better results but take your time and seek advise from your doctor before commensing any exercise programme :D