RESUMEN
We sought to determine whether early increases in cross-sectional area (CSA) of different muscles composing the quadriceps with low-load resistance training with blood flow restriction (LL-BFR) were mainly driven by muscle hypertrophy or by edema-induced swelling. We also compared these changes to those promoted by high-load resistance training (HL-RT). In a randomized within-subject design, fifteen healthy, untrained men were submitted to magnetic resonance imaging (MRI) for CSA and edema-induced muscle swelling assessment (fast spin echo inversion recovery, FSE-STIR). MRI was performed in LL-BFR and HL-RT at baseline (W0) and after 3 weeks (W3), with a further measure after 6 weeks (W6) for HL-RT. Participants were also assessed at these time points for indirect muscle damage markers (range of motion, ROM; muscle soreness, SOR). CSA significantly increased for all the quadriceps muscles, for both LL-BFR and HL-RT at W3 (all P < .05) compared to W0. However, FSE-STIR was elevated at W3 for all the quadriceps muscles only for HL-RT (all P < .0001), not LL-BFR (all P > .05). Significant increases and decreases were shown in SOR and ROM, respectively, for HL-RT in W3 compared to W0 (both P < .05), while these changes were mitigated at W6 compared to W0 (both P > .05). No significant changes in SOR or ROM were demonstrated for LL-BFR across the study. Early increases in CSA with LL-BFR seem to occur without the presence of muscle edema, whereas initial gains obtained by HL-RT were influenced by muscle edema, in addition to muscle hypertrophy.