Researchers in one study said muscles require amino acids to fully recover from new or strenuous exercise. In the study, 17 untrained men performed four sets of a single-leg resistance exercise. The men pressed, flexed, and then focused on extending the leg using 120 percent of the maximum load they could flex.Immediately after exercise and continuing for 14 days, the men took 35 grams of whey protein isolate three times per day, or a carbohydrate placebo. One day after exercise, men in the placebo group had lost 21 percent of their isometric strength compared to 12 percent for whey protein. Strength returned to normal for the placebo group in 14 days and in four days for whey protein.