Heavy Legs. My performance safely?

I am preparing for an SI physical test on 10 June. I am experiencing persistent heaviness and weakness in both legs. There is 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. The main problem is that my legs feel weak and do not respond properly during physical activity, even though pain is minimal. What could be the possible cause, and what tests would you recommend to identify the root cause? Also, with my physical test on 10 June, what can I do in the remaining days to improve my performance safely?