J Appl Physiol. 2009 Feb 12. Intramuscular Metabolism during Low-Intensity Resistance Exercise with Blood Flow Restriction.Suga T, Okita K, Morita N, Yokota T, Hirabayashi K, Horiuchi M, Takada S, Takahashi T, Omokawa M, Kinugawa S, Tsutsui H. Although recent studies have reported that low-intensity resistance training with blood flow restriction could stress the muscle effectively and provide rapid muscle hypertrophy and strength gain equivalent to those of high-intensity resistance training, the exact mechanism and its generality have not yet been clarified. We investigated the intramuscular metabolism during low-intensity resistance exercise with blood flow restriction and compared it with that of high-intensity and low-intensity resistance exercises without blood flow restriction using (31)P-magnetic resonance spectroscopy. Twenty-six healthy subjects (22 +/- 4 yr) participated and performed unilateral plantar flexion (30 times/min) for 2 min. Protocols were as follows: low-intensity exercise (L) using a load of 20% of one-repetition maximum (1 RM), L with blood flow restriction (LR), and high-intensity exercise using 65% 1 RM (H). Intramuscular phosphocreatine (PCr) and diprotonated phosphate (H2PO4(-)) levels and intramuscular pH at rest and during exercise were obtained. We found that the PCr depletion, H2PO4(-) increase and the intramuscular pH decrease during LR were significantly greater than those in L (p<0.001); however, those in LR were significantly lower than those in H (p<0.001). The recruitment of fast-twitch fiber evaluated by inorganic phosphate splitting occurred in only 31% of the subjects in LR, compared to 70% in H. In conclusion, the metabolic stress in skeletal muscle during low-intensity resistance exercise was significantly increased by applying blood flow restriction, but did not generally reach that during high-intensity resistance exercise. This new method of resistance training needs to be examined for optimization of the protocol to reach equivalence with high-intensity resistance training. Key words: Resistance training, Skeletal muscle, Energetic metabolism, Blood flow restriction.