Left lumbar back problem lasting 10 months: what's the diagnosis (then what's the therapy)

[neurospast]

1st thing I would think of would be the Latissimus Dorsi which origins at the top of pelvis and inserts onto the Humerus. It is responsible for internal rotation of the shoulder and if shortened you cannot abduct nor flex your arm fully above horizontal. It would also cause a spasm on the exercise you have been doing. Another option would be the Quadratus Lumborum which neither would like your move. Both muscles tend to shorten if other muscles tend not to do the job they are made for and as a result other muscles feel the need to take over (eg Lattisimus and Q-Lumborum). Having said this; core control which is essential for pilates has to be taught carefully before you really start with pilates otherwise every move you make can cause movements of joints which go beyond their range of movement resulting in symptoms as pain. The latter is just a suggestion. By the way if you have irritated a muscle well enough it will cause you problems again and again (just the same when you irritate a stressed person; get out of his/her way!)