Skeletal Muscle Signature of a Champion Sprint Runner

Abstract

Human skeletal muscle contains three types of the main motor protein myosin heavy chain (MHC) and individual muscle fibers contain only one type of MHC (pure fiber types: I, IIa and IIx) or a mixture of MHC (hybrid fiber types: I/IIa and IIa/IIx). Each of these fiber types has a distinct phenotypic profile that contributes to human locomotion. The mixture of these fiber types within the muscle can have a profound impact on athletic performance. Our laboratory recently had the opportunity to examine the skeletal muscle profile of a World Record holder and World Champion sprint runner. PURPOSE: To examine single muscle fiber MHC composition (i.e. fiber type) and single muscle fiber contractile function (strength, speed and power) from this World Class sprinter. METHODS: A large number of isolated single muscle fibers from the vastus lateralis were analyzed for contractile function (n=53) and fiber type profile (n=329 fibers). RESULTS: Fiber type profile showed that 72% of the muscle contained fast motor proteins (MHC IIa and IIx). The novel finding was that the sprint champion's muscle contained 24% of the pure MHC IIx fiber type. Normal healthy human adult muscle contains less than 2% of the pure MHC IIx fiber type. Power output of the MHC IIx muscle fibers were the highest values recorded to date by a wide margin (35±4 W·l-1). Single fiber power output data from other athletic populations (swimmers and distance runners) and recreational athletes generally fall in the range of 5-10 W·l-1 and do not exceed 20 W·l-1. CONCLUSION: The unusually high abundance and high power output of the rare and powerful pure MHC IIx fiber type represent a unique muscle signature that provides a myocellular basis for the high level of athletic success achieved in sprinting events by this individual.