Low-load resistance training to task failure with and without blood flow restriction: muscular functional and structural adaptations

Abstract

The application of blood flow restriction (BFR) during resistance exercise is increasingly recognized for its ability to improve rehabilitation and for its effectiveness in increasing muscle hypertrophy and strength among healthy populations. However, direct comparison of the skeletal muscle adaptations to low-load resistance exercise (LL-RE) and low-load BFR resistance exercise (LL-BFR) performed to task failure is lacking. Using a within-subject design, we examined whole muscle group and skeletal muscle adaptations to 6 wk of LL-RE and LL-BFR training to repetition failure. Muscle strength and size outcomes were similar for both types of training, despite ~33% lower total exercise volume (load × repetition) with LL-BFR than LL-RE (28,544 ± 1,771 vs. 18,949 ± 1,541 kg, P = 0.004). After training, only LL-BFR improved the average power output throughout the midportion of a voluntary muscle endurance task. Specifically, LL-BFR training sustained an 18% greater power output from baseline and resulted in a greater change from baseline than LL-RE (19 ± 3 vs. 3 ± 4 W, P = 0.008). This improvement occurred despite histological analysis revealing similar increases in capillary content of type I muscle fibers following LL-RE and LL-BFR training, which was primarily driven by increased capillary contacts (4.53 ± 0.23 before training vs. 5.33 ± 0.27 and 5.17 ± 0.25 after LL-RE and LL-BFR, respectively, both P < 0.05). Moreover, maximally supported mitochondrial respiratory capacity increased only in the LL-RE leg by 30% from baseline (P = 0.006). Overall, low-load resistance training increased indexes of muscle oxidative capacity and strength, which were not further augmented with the application of BFR. However, performance on a muscle endurance test was improved following BFR training.

Keywords: BFR resistance exercise; capillary; high-resolution respirometry; low-load; mitochondria; repetition failure.

Conflict of interest statement

No conflicts of interest, financial or otherwise, are declared by the authors.

Figures

Fig. 1.

Schematic representation of the study design. DEXA, dual-energy X-ray absorptiometry; US, ultrasound; MET, muscle endurance test; RM, repetition maximum.

Fig. 2.

Exercise training volume and muscle strength changes following 6 wk of low-load resistance exercise (LL-RE) or low-load blood flow restriction resistance exercise (LL-BFR) training. A: repetitions to failure for each set of the first training session. B: repetitions for each set of the last training session. C: daily training volume for the first and last (18th) training sessions. Cumulative exercise training volume is shown at right. D: dynamic muscle strength before (Pre) and after 2 wk (W2), 4 wk (W4), and 6 wk (Post) of training. Absolute change in each group is shown at right. RM, repetition maximum. ‡P < 0.05 vs. set 1. ωP < 0.05 vs. set 2. †P < 0.01 vs. LL-RE at similar time points. *P < 0.01 vs. Pre. Symbols represent individual values, and bars represent group means (n = 10/group).

Fig. 3.

Concentric power output during a muscle endurance test before (Pre) and after (Post) 6 wk of low-load resistance exercise (LL-RE) or low-load blood flow restriction resistance exercise (LL-BFR) training. A: absolute power output for each repetition. B: relative power output expressed as percentage of the highest power output of a single repetition. C and D: change from baseline for absolute and relative power output, respectively, which was analyzed in tertiles. Rep, repetition. Values are means ± SE (n = 10/group).

Fig. 4.

Skeletal muscle microvascular properties before (Pre) and after 6 wk of low-load resistance exercise (LL-RE) or low-load blood flow restriction resistance exercise (LL-BFR) training. A: representative images for immunohistochemical staining of serial cross sections. Top: type I [myosin heavy chain (MHC) I, blue] and type II (MHC IIa, green; MHC IIx, red) muscle fibers. Bottom: muscle fiber boarder (dystrophin; red), capillary (collagen IV, green), and nuclei (DAPI, blue). White and yellow arrows represent a capillary and a myonucleus, respectively. X depicts the same fiber within each condition for both sets of images. Scale bars = 100 μm. B–D: capillary contacts, capillary-to-fiber ratio, and capillary-to-fiber perimeter exchange (CFPE) index. *P < 0.05 vs. Pre. Symbols represent individual values, and bars represent group means (n = 9/group).

Fig. 5.

Mitochondria-located protein expression and respiratory capacity in permeabilized muscle fibers before (Pre) and after 6 wk of low-load resistance exercise (LL-RE) or low-load blood flow restriction resistance exercise (LL-BFR) training. A: representative Western blot images for each protein for 2 participants. COXIV, cytochrome c oxidase complex IV; CI–CIII and CV, complexes I–III and V; MiCK, mitochondrial creatine kinase; VDAC, voltage-dependent anion-selective channel protein; PDHE1α, pyruvate dehydrogenase E1 α-subunit. B: quantification of Western blots for proteins located within mitochondria. AU, arbitrary units. C: high-resolution respirometry illustrating maximal complex I/II-linked respiration. D: Michaelis-Menten kinetic curves and apparent Km for ADP. Jo2, O2 consumption; PM, pyruvate + malate; +D, PM + ADP; +G, PMD + glutamate; +S, PMDG + succinate; +C, PMDGS + cytochrome c; RCR, respiratory control ratio. *P < 0.01 vs. Pre. Symbols represent individual values, and bars represent group means [n = 7–10/group (Western blot data) and n = 10/group (respirometry data)].