Heavy and weak legs. The root cause?

I am preparing for an SI physical test on 10 June. I am experiencing persistent heaviness and weakness in both legs. There is very little pain, but my legs feel fatigued, heavy, and do not generate enough power while running. My endurance and performance have declined significantly, and during long jump my legs get exhausted very quickly. I have consulted multiple doctors. X-rays of my legs and lower back were normal. I have been prescribed Vitamin D supplements, muscle relaxants, recovery supplements, protein, electrolytes, and adequate hydration. Despite this, the heaviness and weakness persist. I have also tried taking complete rest for 2 days as advised, but there was no improvement. In fact, after the rest, I was unable to complete even 1 round of running. The main problem is weakness and heaviness in both legs rather than pain. What could be the possible cause, and what tests would you recommend to identify the root cause? With my physical test on 10 June, what can I do in the