Cycling exercise-induced myofiber transitions in skeletal muscle depend on basal fiber type distribution

Abstract

The link between specific changes in myofiber type proportions and modulation of training in human skeletal muscle has yet to be unraveled. We investigated whether a defined increase in training volume induces a corresponding change of myofiber shifting in human skeletal muscle with distinct basal myofiber distribution. Twenty-one male cyclists (Age 26 ± 4 years) with different performance levels were exposed to increased cycling training volume with reduced power output for 3 months. Biopsies were taken from vastus lateralis muscle PRE-POST and the proportions of type I, IIa, IIx and IIc myofibers were determined. Total training time did not correlate to the degree of fiber type shifting of any type. In the entire sample of subjects, the proportion of type I myofibers tended to increase (P = 0.14) while IIa fibers decreased significantly (P < 0.05). Subgroups of subjects possessing higher (HPS) and lower proportions (LPS) of type I myofibers at baseline showed a distinct pattern in changing myofiber distribution. Subjects in HPS offered no change in myofiber proportions of any type. In contrast, subjects in LPS showed marked increases in type I (P = 0.06) and a significant reduction in IIa myofibers (P = 0.01). An inverse correlation between baseline proportion of type I and IIa myofibers and its change was observed. We conclude that individual myofiber composition constitutes a modulating factor for exercise-induced changes in its distribution. This might be influenced by altered demands of myofiber recruitment in relation to the intensity of muscle contraction but also by its relative abundance in contracting muscle.

PKM2 Determines Myofiber Hypertrophy In Vitro and Increases in Response to Resistance Exercise in Human Skeletal Muscle

“…The cross-sections were mounted on Vectabond (Vector Laboratories, Burlingame, CA, USA) coated slides and stored in −80 °C until further analysis. To determine the different fiber types in each rat skeletal muscle, images were captured from adenosinetriphosphatase (ATPase) stained cross-sections following the methods described in [ 64 , 65 ]. Digital pictures of cross-sections were captured using a 20× objective (DMRB microscope, Leica, Wetzlar, Germany) to finally differentiate between the four fiber types (type I, type IIA, type IIX, and type IIB [ 7 ].…”

A data‐driven methodology reveals novel myofiber clusters in older human muscles

“…Thus, alterations in muscle function are reflected by changes in myofiber type composition. Fiber classification is, therefore, regarded the key for understanding processes affecting muscle biology, physiology, and pathophysiology . For instance, muscle degeneration is often characterized by atrophy of especially fast‐glycolytic myofibers, while insulin resistance is associated with shifts from slow‐oxidative to fast‐glycolytic myofibers .…”

Enhanced muscular oxygen extraction in athletes exaggerates hypoxemia during exercise in hypoxia

“…It is well established that O 2 extraction capacity is higher in oxidative type I muscle fibers than in the more glycolytic type II fibers (16). In addition, consistent endurance exercise training, apart from producing type II to type I transitions (5,17,18), markedly upregulates oxidative energy turnover in either fiber type (13,17). Thus it has been extensively documented that aerobic training increases mitochondrial volume density and oxidative enzyme activity.…”