Myofiber hypertrophy adaptations following 6 weeks of low-load resistance training with blood flow restriction in untrained males and females

Abstract

The effects of low-load resistance training with blood flow restriction (BFR) on hypertrophy of type I/II myofibers remains unclear, especially in females. The purpose of the present study is to examine changes in type I/II myofiber cross-sectional area (fCSA) and muscle CSA (mCSA) of the vastus lateralis (VL) from before (Pre) to after (Post) 6 wk of high-load resistance training (HL; n = 15, 8 females) and low-load resistance training with BFR (n = 16, 8 females). Mixed-effects models were used to analyze fCSA with group (HL, BFR), sex (M, F), fiber type (I, II), and time (Pre, Post) included as factors. mCSA increased from pre- to posttraining (P < 0.001, d = 0.91) and was greater in males compared with females (P < 0.001, d = 2.26). Type II fCSA increased pre- to post-HL (P < 0.05, d = 0.46) and was greater in males compared with females (P < 0.05, d = 0.78). There were no significant increases in fCSA pre- to post-BFR for either fiber type or sex. Cohen's d, however, revealed moderate effect sizes in type I and II fCSA for males (d = 0.59 and 0.67), although this did not hold true for females (d = 0.29 and 0.34). Conversely, the increase in type II fCSA was greater for females than for males after HL. In conclusion, low-load resistance training with BFR may not promote myofiber hypertrophy to the level of HL resistance training, and similar responses were generally observed for males and females. In contrast, comparable effect sizes for mCSA and 1-repetition maximum (1RM) between groups suggest that BFR could play a role in a resistance training program.NEW & NOTEWORTHY This is the first study, to our knowledge, to examine myofiber hypertrophy from low-load resistance training with blood flow restriction (BFR) in females. Although this type of training did not result in myofiber hypertrophy, there were comparable increases in muscle cross-sectional area compared with high-load resistance training. These findings possibly highlight that males and females respond in a similar manner to high-load resistance training and low-load resistance training with BFR.

Keywords: blood flow restriction; fiber cross-sectional area; myofiber hypertrophy; vastus lateralis.

Graphical abstract

Figure 1.

An overview of the study timeline. Participants in the high-load (HL) and blood flow restriction (BFR) groups completed experimental testing before (Pre) and after (Post) a 6-wk resistance training (RT) protocol. Dark arrows indicate what activities were performed during each visit. 1RM, 1-repetition maximum; AMRAP, as many repetitions as possible; F, females; M, males.

Figure 2.

Images from MyoVision software used to calculate fiber cross-sectional area (fCSA) for type I and II muscle fibers of the vastus lateralis before (Pre) and after (Post) 6 wk of resistance training. Fibers shown are from representative male and female participants that either participated in low-load resistance training with blood flow restriction (BFR; A–D) or high-load resistance training (HL; E–H). Digital images were captured with a fluorescent microscope (Nikon Instruments, Melville, NY States) using a ×10 objective lens. This staining method allows for the detection of type I fiber blue cell bodies (detected by the FITC filter) and type II fiber black cell bodies (unlabeled). Dystrophin cytoplasmic protein is stained in bright green. The length of the white scale bar in A is 100 pixels at a pixel conversion ratio of 0.493 pixels/µm.

Figure 3.

Overview of the statistical analyses performed to understand how differences in sex and resistance training method affected hypertrophy of type I and II muscle fibers. In M1–M4, the asterisks in the interaction terms indicate that all lower-order interactions and main effects are also included in the model. BCA, bias corrected and accelerated; BFR, blood flow restriction group; BIC, Bayesian information criterion; CI, confidence interval; fCSA, fiber cross-sectional area; HL, high-load resistance training group; $R_{\mathrm{m}}^2$, marginal R²; $R_{\mathrm{c}}^2$, conditional R².

Figure 4.

Boxplots for load (A), repetitions (B), and volume load (C) presented for the first (red), second (green), and third (blue) sets for each week of the resistance training protocol. Data within each week represent all 3 resistance training sessions performed. Data are displayed for members of the high load group (HL; n = 15, 8 female) or the low load with blood flow restriction group (BFR; n = 15, 8 female); n represents the number of individuals/participants.

Figure 5.

Boxplots and raincloud plots for fiber cross-sectional area (fCSA) data of male (M) and female (F) participants of the high load (HL, n = 15, 8 female; A and C) and low load with blood flow restriction (BFR, n = 14, 8 female; B and D) groups before (orange) and after (yellow) resistance training; n represents the number of individuals/participants. *Statistically significant pairwise comparison at the 0.05 confidence level as revealed by bias-corrected and accelerated bootstrapped confidence intervals of pairwise comparisons using 10,000 cases. fCSA of type II fibers significantly increased from pretraining to posttraining for members of the HL group when collapsed across sex.

Figure 6.

Boxplots and raincloud plots for type I (A) and type II (B) fiber cross-sectional area (fCSA) data of male (blue) and female (green) participants of the high load (HL; n = 15, 8 female) and low load with blood flow restriction (BFR, n = 14, 8 female) groups; n represents the number of individuals/participants. *Statistically significant pairwise comparison at the 0.05 confidence level as revealed by bias-corrected and accelerated bootstrapped confidence intervals of pairwise comparisons using 10,000 cases. fCSA of type II fibers was higher in males compared with females when collapsed across times and groups.

Figure 7.

Percent change values plotted for the vastus lateralis muscle cross-sectional area (mCSA) and one-repetition maximum strength (1RM) against percent changes in both type I and II fiber cross-sectional area (fCSA) following 6 wk of either high-load resistance training (HL, triangles) or low-load resistance training with blood flow restriction (BFR, circles). Data are shown for males (blue) and females (green). Regression lines are presented with 95% confidence intervals. Two additional data points are shown in C and D (HL, n = 15, 8 female; BFR, n = 14, 8 female) compared to A and B (HL, n = 14, 7 female; BFR, n = 13, 8 female); n represents the number of individuals/participants.