Interrelationships between muscle structure, muscle strength, and running economy

Purpose: The present study was designed to investigate possible differences in running economy (RE) among elite middle-distance runners by examining muscle structure and maximal isometric force (MVC). Methods: Ten young male runners ran at six different running speeds. During the running bouts, respiratory gases, and blood lactate were measured. Muscle biopsies were obtained from the vastus lateralis muscle for analyzing fiber type distribution, muscle fiber area, myosin heavy chain (MHC) composition, activities of a number of metabolic enzymes (citrate synthase, lactate dehydrogenase, phosphofruktokinase, and 3-hydroxyacyl-CoA-dehydrogenase), and titin isoforms. Results: Energy expenditure (EE) increased linearly up to the speed of 6.0 m(.)s(-1). The relative distribution of the MHC isoforms was MHC I: 67.0%, MHC IIA: 31.5%, and MHC IIX: 1.5%. The present results demonstrated that higher the area of Type 11 fibers, higher the MVC (r = 0.59, P < 0.05). The amount of MHC 11 correlated inversely with EE when running close to the competition speed (r = -0.61, P < 0.05). Enzyme activities did not correlate significantly with either RE or EE. Titin analysis revealed that a faster-mobility titin band was observed in all subjects, whereas a lower-mobility titin band was observed only in the most economical runner. Conclusion: Differences in RE among homogeneous group of middle-distance runners were observed at various running speeds. This may partly be explained by differences in muscle fiber distribution, MHC composition, and titin isoforms.