High-frequency blood flow-restricted resistance exercise results in acute and prolonged cellular stress more pronounced in type I than in type II fibers.

Myocellular stress with high-frequency blood flow-restricted resistance exercise (BFRRE) was investigated by measures of heat shock protein (HSP) responses, glycogen content, and inflammatory markers. Thirteen participants [age: 24 ± 2 yr (means ± SD), 9 males] completed two 5-day blocks of seven BFRRE sessions, separated by 10 days. Four sets of unilateral knee extensions to failure at 20% of one-repetition maximum (1RM) were performed. Muscle samples obtained before, 1 h after the first session in the first and second block ( and ), after three sessions (), during the "rest week," and at 3 () and 10 days postintervention () were analyzed for HSP70, αB-crystallin, glycogen [periodic acid-Schiff (PAS) staining], mRNAs, miRNAs, and CD68 (macrophages) and CD66b (neutrophils) cell numbers. αB-crystallin translocated from the cytosolic to the cytoskeletal fraction after and ( < 0.05) and immunostaining revealed larger responses in type I than in type II fibers (, 225 ± 184% vs. 92 ± 81%, respectively, = 0.001). HSP70 was increased in the cytoskeletal fraction at and , and immunostaining intensities were more elevated in type I than in type II fibers at (206 ± 84% vs. 72 ± 112%, respectively, <0.001), during the rest week (98 ± 66% vs. 42 ± 79%, < 0.001), and at (115 ± 82% vs. 28 ± 78%, = 0.003). Glycogen content was reduced in both fiber types, but most pronounced in type I, which did not recover until the rest week (-15% to 29%, ≤ 0.001). Intramuscular macrophage numbers were increased by ∼65% postintervention, but no changes were observed in muscle neutrophils. We conclude that high-frequency BFRRE with sets performed till failure stresses both fiber types, with type I fibers being most affected. BFRRE has been reported to preferentially stress type I muscle fibers, as evidenced by HSP responses. We extend these findings by showing that the HSP responses occur in both fiber types but more so in type I fibers and that they can still be induced after a short-term training period. Furthermore, the reductions in glycogen content of type I fibers after strenuous frequent BFRRE in unaccustomed subjects can be prolonged (≥5 days), probably due to microdamage.