Frequent Manipulation of Resistance Training Variables Promotes Myofibrillar Spacing Changes in Resistance-Trained Individuals

Abstract

We sought to determine if manipulating resistance training (RT) variables differentially altered the expression of select sarcoplasmic and myofibril proteins as well as myofibrillar spacing in myofibers. Resistance-trained men (n = 20; 26 ± 3 years old) trained for 8 weeks where a randomized leg performed either a standard (CON) or variable RT protocol (VAR: manipulation of load, volume, muscle action, and rest intervals at each RT session). A pre-training (PRE) vastus lateralis biopsy was obtained from a randomized single leg, and biopsies were obtained from both legs 96 h following the last training bout. The sarcoplasmic protein pool was assayed for proteins involved in energy metabolism, and the myofibril protein pool was assayed for relative myosin heavy chain (MHC) and actin protein abundances. Sections were also histologically analyzed to obtain myofibril spacing characteristics. VAR resulted in ~12% greater volume load (VL) compared to CON (p < 0.001). The mean fiber cross-sectional area increased following both RT protocols [CON: 14.6% (775.5 μm2), p = 0.006; VAR: 13.9% (743.2 μm2), p = 0.01 vs. PRE for both], but without significant differences between protocols (p = 0.79). Neither RT protocol affected a majority of assayed proteins related to energy metabolism, but both training protocols increased hexokinase 2 protein levels and decreased a mitochondrial beta-oxidation marker (VLCAD protein; p < 0.05). Citrate synthase activity levels increased with CON RT (p < 0.05), but not VAR RT. The relative abundance of MHC (summed isoforms) decreased with both training protocols (p < 0.05). However, the relative abundance of actin protein (summed isoforms) decreased with VAR only (13.5 and 9.0%, respectively; p < 0.05). A decrease in percent area occupied by myofibrils was observed from PRE to VAR (−4.87%; p = 0.048), but not for the CON (4.53%; p = 0.979). In contrast, there was an increase in percent area occupied by non-contractile space from PRE to VAR (10.14%; p = 0.048), but not PRE to CON (0.72%; p = 0.979). In conclusion, while both RT protocols increased muscle fiber hypertrophy, a higher volume-load where RT variables were frequently manipulated increased non-contractile spacing in resistance-trained individuals.

Effects of low-load resistance training with blood flow restriction on muscle fiber myofibrillar and extracellular area

“…We hypothesized that there would be a decrease in myofibrillar area and an increase in non-myofibrillar area with type I and II fCSA increases following LL-BFR. In contrast, we hypothesized that HL-RT would promote conventional hypertrophy, consistent with findings from previous studies ( Fox et al, 2021 ; Ruple et al, 2021 ; Vann et al, 2022 ). Finally, we hypothesized that extracellular area would be greater following LL-BFR.…”

“…Conventional hypertrophy, or the proportional increase in myofibrillar area with an increase in fCSA, has recently been challenged by studies that demonstrated sarcoplasmic expansion after some resistance training protocols. For example, Fox et al (2021) compared traditional HL-RT with a protocol with frequent manipulation of resistance training variables (e.g., load, sets, muscle action, and rest). In contrast to what was observed after HL-RT, the protocol with variation in exercise stimulus induced an increase in the non-myofibrillar area after 16 resistance training sessions.…”

“…This staining procedure facilitated the distinction of the sarcolemma (visualized through the FITC filter) and myofibrils (observed via the Texas Red filter). Multiple 20× images section were obtained and the quantification of myofibril area per fiber was conducted utilizing ImageJ software developed by the National Institutes of Health (NIH) according to procedures previously carried out in our laboratory ( Fox et al, 2021 ; Ruple et al, 2021 ; Michel et al, 2023 ). In brief, the image scale in ImageJ was set to 0.451 μm per pixel.…”

“…Additionally, an increase in myofibrillar number, but not size, has been shown to accompanies the increases in fCSA with weeks of resistance training ( Jorgenson et al, 2023 ). However, select evidence suggests that non-myofibrillar area expansion may predominate during muscle fiber hypertrophy in resistance training protocols that diverge from traditional methods (e.g., an unconventional or very high-volume training paradigms) ( Haun et al, 2019 ; Fox et al, 2021 ; Vann et al, 2022 ). Despite extensive research into these intracellular changes in recent years, little is known regarding how LL-BFR affects these features in muscle fibers.…”

Proteolytic markers associated with a gain and loss of leg muscle mass with resistance training followed by high‐intensity interval training

“…Briefly, serial sections were allowed to air dry for ∼1.5-2 h at room temperature followed by fixation in chilled acetone (−20 objective. According to previous reports, the quantification of myofibril area per fibre was performed using ImageJ software (Fox et al, 2021;Ruple et al, 2021). Briefly, the relative scale within ImageJ was set such that 1 pixel was equivalent to 0.451 μm, images were then split into RGB channels, and the red, phalloidin-containing channel was converted to greyscale.…”