Six Weeks of Low-Load Blood Flow Restricted and High-Load Resistance Exercise Training Produce Similar Increases in Cumulative Myofibrillar Protein Synthesis and Ribosomal Biogenesis in Healthy Males

Abstract

Purpose: High-load resistance exercise contributes to maintenance of muscle mass, muscle protein quality, and contractile function by stimulation of muscle protein synthesis (MPS), hypertrophy, and strength gains. However, high loading may not be feasible in several clinical populations. Low-load blood flow restricted resistance exercise (BFRRE) may provide an alternative approach. However, the long-term protein synthetic response to BFRRE is unknown and the myocellular adaptations to prolonged BFRRE are not well described. Methods: To investigate this, 34 healthy young subjects were randomized to 6 weeks of low-load BFRRE, HLRE, or non-exercise control (CON). Deuterium oxide (D₂O) was orally administered throughout the intervention period. Muscle biopsies from m. vastus lateralis were collected before and after the 6-week intervention period to assess long-term myofibrillar MPS and RNA synthesis as well as muscle fiber-type-specific cross-sectional area (CSA), satellite cell content, and myonuclei content. Muscle biopsies were also collected in the immediate hours following single-bout exercise to assess signaling for muscle protein degradation. Isometric and dynamic quadriceps muscle strength was evaluated before and after the intervention. Results: Myofibrillar MPS was higher in BFRRE (1.34%/day, p < 0.01) and HLRE (1.12%/day, p < 0.05) compared to CON (0.96%/day) with no significant differences between exercise groups. Muscle RNA synthesis was higher in BFRRE (0.65%/day, p < 0.001) and HLRE (0.55%/day, p < 0.01) compared to CON (0.38%/day) and both training groups increased RNA content, indicating ribosomal biogenesis in response to exercise. BFRRE and HLRE both activated muscle degradation signaling. Muscle strength increased 6-10% in BFRRE (p < 0.05) and 13-23% in HLRE (p < 0.01). Dynamic muscle strength increased to a greater extent in HLRE (p < 0.05). No changes in type I and type II muscle fiber-type-specific CSA, satellite cell content, or myonuclei content were observed. Conclusions: These results demonstrate that BFRRE increases long-term muscle protein turnover, ribosomal biogenesis, and muscle strength to a similar degree as HLRE. These findings emphasize the potential application of low-load BFRRE to stimulate muscle protein turnover and increase muscle function in clinical populations where high loading is untenable.

Keywords: deuterium oxide; ischemic resistance training; myofibrillar protein synthesis; remodeling; ribosomal biogenesis.

Figure 1

Study overview. Pre-test was conducted 2 weeks prior to the 6-week training period. MVC maximal voluntary contraction; 1-RM one-repetition maximum; Ex, exercise; C, control.

Figure 2

Muscle fiber cross-sectional area of type I fibers (A) and type II fibers (B) at baseline (pre) and 4 days after cessation of training (post). Overall effects are given in the upper left corner of graphs of (A,B).

Figure 3

Myofibrillar protein (A) and RNA (B) synthesis rates (%/day) during the intervention period. Correlation between myofibrillar FSR and RNA FSR (C). Data are presented as mean ± SD in (A,B). In (C), data are presented as individual values and a linear regression line (solid) with 95% CI (dashed). Overall group effect is given in the upper left corner of graphs of (A) and (B). R square and significance is given in the upper left corner of (C). ^#p < 0.05, ^##p < 0.01, and ^###p < 0.001 different from CON.

Figure 4

Total RNA content at baseline (pre) and 4 days after cessation of training (post) (A). Correlation between RNA synthesis and change in total RNA content. Data are presented as mean ± SD in (A). In (B), data are presented as individual values and a linear regression line (solid) with 95% CI (dashed). Overall effects are given in the upper left corner of graphs (A). R square and significance is given in the upper left corner of (B). ^*p < 0.05 different from pre within group; ^#p < 0.01 different from CON.

Figure 5

Phosphorylation of ULK1 at Ser⁵⁵⁵ (A), the ratio of LC3B2 to LC3B1 (B) protein expression, and (C) phosphorylation of FoxO3a at Ser²⁵³ immediately (0 h) and 3 h (3 h) after acute exercise. Data are presented as mean ± SD. Overall effects are given in the upper left corner of graphs. ^*p < 0.05, ^**p < 0.01, and ^***p < 0.001 different from pre within group; (*)p < 0.1 tendency toward difference from pre within group; ^§p < 0.05 different from BFRRE within time-point; ^#p < 0.05 and ^##p < 0.01 different from CON within time-point. Representative blots are shown in (D).

Figure 6

Number of satellite cells at baseline (pre) and 4 days after cessation of training (post) expressed relative to number of fibers (A,B) and fiber CSA (C,D). Data are presented as mean ± SD. Overall effects are given in the upper left corner of graphs.

Figure 7

Myonuclei at baseline (pre) and 4 days after cessation of training (post) expressed relative to number of fibers (A,B) and as myonuclear domain (C,D). Data are presented as mean ± SD. Overall effects are given in the upper left corner of graphs.

Figure 8

Isometric muscle strength (A) and dynamic muscle strength (B) at baseline (pre), 4 days after cessation of training (post 4), and 14 days after cessation of training (post 14). Data are presented as mean ± SD. Overall effects are given in the upper left corner of graphs. ^*p < 0.05 and ^**p < 0.01 different from pre within group; ^§p < 0.05 different from BFRRE within time-point; (#) p < 0.1 tendency toward difference to CON within time-point.

Figure 9

Muscle fiber cross-sectional area (A), satellite cells per fiber (C), satellite cells per mm² (D), myonuclei per fiber (E), and myonuclear domain (F) at baseline (pre) and 4 days after cessation of training (post) in non-responders (n = 14) and responders (n = 9) with regards to muscle fiber hypertrophy. RNA synthesis in non-responders and responders (B). Data are presented as individual values as well as mean ± SD. Overall effects are given in the upper left corner of graphs. (*)p < 0.1 tendency toward difference from pre within cluster. ^*p < 0.05, ^**p < 0.01, and ^***p < 0.001 different from pre within cluster; (#)p < 0.1 tendency toward difference to non-responders within time-point.