Muscle imbalances and repetitive strain

I'd like to ask if anybody else has suffered from either repetitive strain, upper/ lower back strains or pains, or muscle strength imbalances that they think might be attributable to playing bass.

I have pain in right trapezius and muscle weakness in right arm which I think might be caused by pinched muscles in right arm, due to holding shoulder up and forward whilst playing (I haven't found an alternative to this position yet).

Does anybody else know which muscles are susceptible to imbalance through overuse playing bass, and good ways to correct the postural imbalances? Or a resource for this?