Psoriasis
Psoriasis is a skin condition that may affect about 10% of people with ankylosing spondylitis. It is an immune system disease that causes raised, scaly patches on the skin. Most often, these patches are itchy and cause redness and inflammation of the surrounding area.
If you are noticing any such change in your skin that is also associated with back pain or muscle strains, you should consult your physician or skin care specialist for a proper evaluation and diagnosis.