Change in intramuscular inorganic phosphate during multiple sets of blood flow-restricted low-intensity exercise

Muscular blood flow reduction (BFR) during multiple sets of low-intensity exercise training has been shown to elicit muscle hypertrophy and strength gain. Several hypotheses have been proposed to explain the hypertrophic adaptations to low-intensity BFR exercise, which include muscle fatigue with metabolic stress. However, the change in intramuscular inorganic phosphate (Pi, an index of muscle fatigue) during multiple sets of low-intensity exercise with BFR is poorly understood. Eight men performed four sets of unilateral plantar flexion exercise (20% 1-RM) on a (31)P-magnetic resonance spectroscopy. Each subject wore a cuff (5-cm wide) on the most proximal portion of the thigh; the cuff was inflated during the exercise session at three different pressures [0 mmHg as the control ± (CON), 180 mmHg as moderate restriction (BFR-M) and 230 mmHg as high restriction (BFR-H)]. During the first and second exercise sets, the increase in Pi was higher (P<0·05) with BFR-H than with BFR-M and CON. On the other hand, the decrease in Pi was lower with BFR-H than with CON during the second and third rest periods between sets. As a result, the Pi concentration increased progressively (P<0·05) with BFR-H, while the Pi was relatively constant with BFR-M and CON during the exercise session. Our results suggest that intramuscular Pi accumulation during multiple sets of low-intensity exercise can be produced only by a high level of BFR, but not by moderate reduction. The Pi accumulation was associated both with exercise and with the rest period between sets.