The Skeletal Muscle Metabolome Reflects Resistance Exercise-induced Skeletal Muscle Adaptation

Abstract

Skeletal muscle contains lipids inside or outside of the muscle cells, namely intramyocellular and extramyocellular lipids (IMCL and EMCL, respectively). Accumulation of excessive IMCL becomes a cause of insulin resistance; on the other hand, IMCL is important substrate for the endurance exercise such as cycling. This bipolar relationship is called "athlete paradox". Muscle lipids increase with aging; however, the relationship of IMCL and EMCL with muscle endurance and strength performances in older individuals is not well known. PURPOSE: The purpose of this study was to examine the relationships between skeletal muscle lipids and muscle functional performances in older individuals. METHODS: Twenty-three older men and women (age, 71.1 ± 4.1years) participated in this study. Skeletal muscle lipids, e.g., IMCL and EMCL, were measured in the vastus lateralis by using chemical shift spectrum taken proton magnetic resonance spectroscopy ( 1 H-MRS). We measured isometric knee extension peak torque with maximal voluntary contraction (MVC). Based on the MVC, time to knee extension task failure during intermittent 50% MVC was measured as muscle endurance performance. This test was performed as follows: participants increased from 0 to 50% MVC in 5 s, maintained 50% MVC for 15 s and decreased from 50 to 0% MVC in 5s. They repeated this task as far as possible until unable to maintain 45% MVC for a period of 5 s. RESULTS: There was no relationship between muscle lipids and maximal knee extension torque (IMCL, r s = -0.08, P = 0.69; EMCL, r s = -0.22, P = 0.32). Time to knee extension task failure was significantly correlated with IMCL (r s = -0.43, P < 0.05), but not with EMCL (r s = 0.20, P = 0.36). CONCLUSIONS: These findings indicate that muscle lipids are associated with muscle endurance performance but not maximal strength in older individuals.