Long-distance runners generally have a remarkably high proportion of slow type I fibers in their lower muscle groups. However, the transformation of type II fast fibers to slow type I fibers as a result of exercise has not been demonstrated clearly. We report the analysis of muscle type composition on m. vastus lateralis from six endurance athletes through the expression of fast, slow, and developmental myosin isoforms, and alpha-actinin-3 (ACTN3) protein. Only one among the marathon runners presented evident type I fiber predominance, and surprisingly, a second athlete showed a deficiency of ACTN3. The deficiency of ACTN3 in the muscle tissue of endurance athletes confirmed the redundancy of this protein for muscle function, even in muscles that are highly required.