Financial questions that need an answer

I has spine surgery a few months ago and have since stopped working while I stay home recovering…the surgery has not resolved my pain and I have gained weight since I am home everyday. I have applied for disability because of my severe back pain. I have multiple herniations in my cervical and lumbar spine, along with advanced degenerative disk disease that started when I was about 16 — I am now 31 and in severe pain everyday, to the point where I struggle to get out of bed most days. I would like to go to the gym to lose some weight by doing aerobic excercise. Will attending the gym have negative effects on my disability application? Many have told me it is not a good intent if a disability investigator were to see me working out at the gym — is this true? Is it illegal by law or by their standards to work out at the gym?