I can’t believe I saw your post! Omg! I can’t provide any answers, except say that I had almost identical symptoms years ago when in my 40’s. I don’t think anyone understood just how stiff, sore and weak my leg muscles were. It was torture. I was very worried. I had all the tests you describe and they weren’t helpful except to exclude certain things like ALS, MS, etc. I saw Neurologist, orthopedic, primary, endocrinologist (have Type 1 diabetes), and rheumatologist who would have diagnosed me with polymyalgia rheumatica , except, she said I was too young, according to her. That theory has now been challenged from what I read. So, I never got an answer or diagnosis. I had other issues too, like frozen shoulders, muscle knots….anyway, they all resolved over time, even the sore, stiff, weak legs. Eventually, I was back to working out. Before it happened, I ran 3 miles a day, plus biking and strength training.

I hope you find answers. Be your own best advocate. Keep trying to get help. I gave up too early. Oh, also, ask for B12 test. That’s bit me from behind and I’m not sure how long I’ve had it. I’m suggesting to everyone, since I had no warning.

Best of luck with everything. I look forward to seeing what you discover. Take care!