I was in Karate class regularly and started having great difficulty doing anything with my left leg. Kicks and such were nearly impossible and I couldn't jump with that leg anymore. I didn't think much of it at the time. Although I would leave class with a limp, if I drank Gatorade and ate a banana, rested well, I felt 'fine' but there was always something about my leg that made me feel like it was weak. I couldn't place it and simply thought that since it was my left side (I'm right-handed) it was just normal weak-side behavior.
After a few months of that, I got to where I could not progress in Karate. I took a break and went to see a trainer to get my muscles stronger.
After about 9 months to a year working with her, we both noticed some improvement to my balance but no major strength differences and I still had the fatigue / limp issue after exercise.