What Can Cause Severe Weakness In The Legs After Recovering From Flu?

12 days ago I got the flu. A little nausea, but mainly diarrhea for about five days. I was also a little lightheaded some of the time, but felt completely exhausted the whole time. I would walk 20 feet and feel so tired I thought I could collapse and my legs felt like I’d done a 2 hour leg workout. All symptoms disappeared after about 5 days except my legs feeling like I’d done the 2 hour workout, which persists still. Just standing up makes them feel exhausted. I’ve been working out for years and concentrating on legs for the past 8 -10 months. I do weighted backpacking, step-ups and many other exercises preparing for mountain backpacking, just turned 47 and haven’t had any problems or issues with my legs before. Jeff