Re: training with an injury?!
A lot depends on the particular details of the back injury. Lower back injuries can be due to muscle pulls, vertebral body damage or disease (degenerative/arthritic vs compression fractures vs spondylolisthesis or canal stenosis), disc herniation or torn disc annulus fibrosus, SI joint inflammation, scoliosis, or even referred pain from other problems which may be unrecognized, such as hip joint disease, sciatic nerve entrapment at the piriformis triangle, and even kidney disease, among others. The specific problem needs to be accurately identified and a solid treatment plan formulated by someone who understands spine conditions as well as your training demands and interest in continuing to work out. I think that Sports Medicine trained docs/health care providers are your best bet in this regard. I would be extremely wary about attempting any specific therapy or further training based on the advice of someone who lacks fundamental knowledge of spine conditions and also hasn't evaluated you with at least a physical examination and possibly even imaging studies (X-ray, MRI, etc. if warranted). Get evaluated by a professional, and if things aren't improving, seek additional opinion from the most qualified individuals that you can find. Oh, and learn soft ukemi. Take care of your back now, so that you can stay active and do aikido, or whatever else you want,into your 70's, 80's, or longer. Good luck. John