Adaptations of the human skeletal muscle: understanding exercise-induced sarcoplasmic hypertrophy

Abstract

Muscle hypertrophy induced by resistance training (RT) can be defined as the increase in cross-sectional area (CSA) muscle, due to the increase in extracellular content and/or increase in intracellular content (i.e., increase in muscle fiber CSA), due to the addition of myofibrils in the intracellular space and increased sarcoplasmic volume. The myofibrillar content makes up most of the muscle fiber. Some estimates suggest that myofibrils occupy <85% of the intracellular space. Sarcoplasm comprises <15% of the intracellular space. Therefore, it is plausible to suggest that if an individual exhibits a 20% increase in muscle fiber CSA, 17% of this increase was due to the addition of myofibrillar proteins and 3% sarcoplasmic content. Although this model of hypertrophy is reasonable, little is known about the effects of RT on alterations in these different contents of muscle fibers that accompany muscle fiber hypertrophy. Recent studies suggest that unaccustomed RT (i.e., different from that usually performed) may promote a greater increase in sarcoplasmic hypertrophy compared to that usually performed. In other words, there would be a proportionally greater increase in the volume of sarcoplasm (e.g., intracellular fluids, sarcoplasmic proteins and enzymes, glycogen, volume of mitochondria, sarcoplasmic reticulum, etc.) than in myofibrillar proteins (myosin, actin, troponin and tropomyosin, proteins of the sarcomere Z line, etc.). However, this hypothesis still needs to be confirmed. To test this hypothesis, the present project will consist of two phases. Phase 1 has already been carried out and received regular assistance from FAPESP (# 2017 / 04299-1). In this phase, 20 well-trained young men underwent two RT protocols for 8 weeks. One of the participants' legs was submitted to a standard RT protocol (CON) (i.e., always with the same volume [number of sets], intensity [target zone of maximum repetitions], type of contraction [eccentric or concentric contractions] and pause [rest interval between sets and exercises]), which the participants were already accustomed to accomplish. The other leg, performed a varied RT protocol (VAR), which was different at each session (i.e., greater number of maximum repetitions, greater number of sets, emphasis on the eccentric phase and greater rest intervals between sets) and, therefore, provided an unaccustomed stimulus to skeletal muscle. Before and after the RT protocols, images of the vastus lateralis were acquired by ultrasound for analysis of the muscle CSA and muscle biopsies for analysis of the CSA of type I and II fibers and myofibrillar and sarcoplasmic content. As a result of phase 1, although the VAR protocol introduced a greater variation in the stimulus imposed on the exercised muscle, the increase in the CSA of the vastus lateralis muscle and type I and II fibers was similar to the CON protocol. However, it is not possible to state that the same has occurred with regard to changes in sarcoplasmic and myofibrillary contents. Therefore, in phase 2 of this project, which will be carried out at Auburn University (USA) and will aim to investigate the effects of the VAR and CON protocols on the following parameters related to sarcoplasmic hypertrophy: 1) the abundance of the actin protein by cross section of the fibers; 2) the relative abundances of myofibrillar proteins per milligram of muscle tissue (myosin, actin, troponin and tropomyosin); 3) content of sarcoplasmic proteins; 4) space between the myofibrils occupied by the sarcoplasm; 5) muscle edema. (AU)