Proliferation of myogenic stem cells in human skeletal muscle in response to low-load resistance training with blood flow restriction

Abstract

Low-load resistance training with blood flow restriction has been shown to elicit substantial increases in muscle mass and muscle strength; however, the effect on myogenic stem cells (MSCs) and myonuclei number remains unexplored. Ten male subjects (22.8 ± 2.3 years)performed four sets of knee extensor exercise (20% 1RM) to concentric failure during bloodflow restriction (BFR) of the proximal thigh (100 mmHg), while eight work-matched controls(21.9 ± 3.0 years) trained without BFR (control, CON). Twenty-three training sessions were performed within 19 days. Maximal isometric knee extensor strength (MVC) was examined pre- and post-training, while muscle biopsies were obtained at baseline (Pre), after 8 days intervention(Mid8) and 3 (Post3) and 10 days (Post10) post training to examine changes in myofibre area (MFA), MSC and myonuclei number. MVC increased by 7.1% (Post5) and 10.6% (Post12)(P <0.001) with BFR training, while type I and II MFA increased by 38% (Mid8), 35 – 37%(Post3) and 31 – 32% (Post10) (P <0.001). MSCs per myofibre increased with BFR training from 0.10 ± 0.01 (Pre) to 0.38 ± 0.02 (Mid8), 0.36 ± 0.04 (Post3) and 0.25 ± 0.02 (Post10) (P <0.001). Likewise, myonuclei per myofibre increased from 2.49 ± 0.07 (Pre) to 3.30 ± 0.22(Mid8), 3.20 ± 0.16 (Post3) and 3.11 ± 0.11 (Post10), (P<0.01). Although MFA increased in CON at Mid8, it returned to baseline at Post3. No changes in MSC or myonuclei number were observed in CON. This study is the first to show that short-term low-load resistance exercise performed with partial blood flow restriction leads to marked proliferation of myogenic stem cells and resulting myonuclei addition in human skeletal muscle, which is accompanied by substantial myofibre hypertrophy.

Figures

Figure 1. Representative immunohistochemical staining (BFR, Mid8) containing Pax7 (red), laminin (green) and MHC I/Dapi (blue)

Arrows denote Dapi+/Pax7+ myogenic stem cells and stars mark myonuclei (scale bar = 50 μm).

Figure 2. Maximal isometric knee extensor muscle strength at baseline (Pre), and 5 and 12 days after cessation of training (Post5 and Post12)

Pre to Post differences: *P < 0.001. Mid5 to Post12 difference: †P < 0.05. Values are means ± SEM; BFR: n = 10; C: n = 8.

Figure 3. Myofibre cross sectional area at baseline (Pre), 8 days into the training intervention (Mid8), and 3 and 10 days after cessation of training (Post3 and Post10)

A, BFR and CON response in type slow myofibres; B, BFR and CON response in type fast myofibres. Pre to Mid8/Post differences: *P < 0.001. Pre to Mid8 difference: **P < 0.01. Between group difference: †P < 0.05. Values are means ± SEM; BFR: n = 10 at Mid8 n = 9; C: n = 7.

Figure 4. Pax7 positive cells at baseline (Pre), 8 days into the training intervention (Mid8), and 3 and 10 days after cessation of training (Post3 and Post10)

A and B, Pax7+ myogenic stem cells per myofibre, type I and II myofibres (group response). C and D, Pax7+ myogenic stem cells per myofibre, type I and II myofibres (individual response). E and F, Pax7+ myogenic stem cells per myofibre cross sectional area (mm2) (group response). G and H, Pax7+ myogenic stem cells to myonuclei ratio (group response). Pre to Mid8/Post differences: *P < 0.001. Mid8/Post3 to Post12 differences: ‡P < 0.01. Between group difference: †P < 0.05. Values are means ± SEM; BFR: n = 10 at Mid8 n = 9; C: n = 7.

Figure 5. Myonuclei number at baseline (Pre), 8 days into the training intervention (Mid8), and 3 and 10 days after cessation of training (Post3 and Post10)

A and B, myonuclei number per myofibre, type I and II myofibres. C and D, cross sectional area (μm2) per myonuclei, type I and II myofibres. Pre to Mid8/Post differences: *P < 0.001, **P < 0.01. Mid8 to Post10 difference: ‡P < 0.05. Between group difference: †P < 0.05. Values are means ± SEM; BFR: n = 10 at Mid8 n = 9; C: n = 7.