Hi , thank you for your question! I hope other members will be able to chime in with their experiences.
Have you already spoken with your doctor about this? We're a non-clinical team, unable to give any medical advice. I searched on our site to see if anyone has mentioned muscle wasting, but wasn't able to find anything in particular.
Looking more into muscle wasting, just generally speaking, most places online seem to cite lack of physical activity as a reason for decreasing muscle mass. As I'm sure you're very aware, physical exercise can be difficult to do for many people in pain with PsA. This article briefly mentions this under the heading, "Progression and the later stages": https://psoriatic-arthritis.com/clinical/stages/. But again, it would really be best to speak with your doctors because there are other conditions that could potentially be the cause of muscle weakening, a couple of such can be found here: https://psoriatic-arthritis.com/clinical/thyroid-disorders/ & https://psoriatic-arthritis.com/clinical/autoimmune-myositis/.