Muscle fiber cross-sectional area is increased after two weeks of twice daily KAATSU-resistance training

Low-intensity (20% of 1-RM) resistance training combined with restriction of muscular venous blood flow (KAATSU training) produces muscle hypertrophy and strength gain (Takarada et al., 2000; 2002). Muscle CSA is increased to a similar degree as traditional high-intensity (HIT, 80% of 1-RM) resistance training when KAATSU training is performed at a similar training volume and frequency (Takarada et al., 2000). Recently, Abe et al. (2005) found that magnetic resonance imaging (MRI)measured thigh muscle volume and circulating insulin-like growth factor-1 (IGF-1) increased after two weeks of high frequency (twice a day) KAATSU training. In that study, thigh muscle size increased as much as previously reported (Jones and Rutherford 1987; Young et al. 1983) in HIT of 3-4 months (~8%). However, it is unclear whether this skeletal muscle hypertrophy is attributed to fiber hypertrophy or any other extracellular tissue/liquid gain. During KAATSU training exercise, both the venous outflow from and arterial inflow to the exercising muscle is considerably suppressed. The resulting hypoxic and acidic intramuscular environment may induce additional muscle fiber recruitment of both small (type I fibers) and large motor units (type II fibers) in order to maintain a given level of force (Takarada et al. 2000). Previous studies (Moritani et al. 1992; Sundberg 1994) have suggested that the type II muscle fibers might be recruited preferentially during exercise when blood flow is suppressed. Therefore we hypothesized that fiber hypertrophy would be larger in the type II fibers than in the type I fibers following KAATSU training. In the present study, we examined the effects of KAATSU training on muscle fiber cross-sectional area (CSA) using a muscle biopsy sample.