Eccentric muscle actions do not promote superior hypertrophy compared with concentric actions

Different physiological mechanisms of sarcomere activity during eccentric (ECC) and concentric (CON) muscle actions led to investigations into muscle hypertrophy outcomes, but conclusions remain elusive. We aimed to investigate the effects of ECC vs. CON muscle actions on muscle hypertrophy in apparently healthy adults through a systematic review with meta-analysis. The searches were conducted on EMBASE, MedLine, Cochrane Library, Web of Science, Scopus, and SPORTSDiscus databases. Eligibility criteria: blinded and unblinded randomized controlled trials that investigated the effects of CON versus ECC resistance training programs in apparently healthy adults (≥18 years old). These studies should have performed hypertrophy measurements in outcomes using cross-sectional area, muscle thickness, or muscle volume assessed by imaging methods. 15778 studies were identified, and twenty-six (682 participants included in the meta-analysis) achieved the inclusion criteria. The main findings indicate that: there was no difference between ECC vs. CON on hypertrophy measurements (0.285 [95%CI: -0.131 to 0.701]; p= 0.179; I2: 84.4%; GRADE: Very low). Subgroup meta-analysis analyzing possible hypertrophy outcome moderators as age (18 to 59 years old and ≥60 years old) and weeks of intervention duration (≤8 weeks and >8 weeks) did not reveal differences between ECC vs. CON. Sub-group analysis revealed a very limited but significant effect for ECC on the upper limb muscles (1.44 [95%CI: 0.179 to 2692]; p= 0.025; I2: 91%; GRADE: Low). Our findings suggest there was no evidence of a difference in hypertrophy between ECC and CON muscle action in apparently healthy adults.