Comparison Between Eccentric vs. Concentric Muscle Actions On Hypertrophy: A Systematic Review and Meta-analysis.

da Silva, LSL, Gonçalves, LdS, Alves Campos, PH, Benjamim, CJR, Tasinafo Júnior, MF, de Lima, LCR, Bueno Júnior, CR, and Alves, CPdL. Comparison between eccentric vs. concentric muscle actions on hypertrophy: a systematic review and meta-analysis. J Strength Cond Res 39(1): 115-134, 2025-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 SPORTDiscus databases. To be eligible for inclusion, studies had to: (a) be randomized/controlled trials; (b) investigate the effects of CON vs. ECC resistance training programs in apparently healthy adults; (c) assess hypertrophy outcomes using direct imaging for cross-sectional area, muscle thickness, or muscle volume. A total of 15,778 studies were identified, and 26 (682 subjects included in the meta-analysis) met the inclusion criteria. The main findings indicated no statistical 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 (18-59 years old and ≥60 years old) and weeks of intervention duration (>8 weeks) did not reveal differences between ECC vs. CON. Subgroup analysis revealed an effect favoring the ECC for the upper limb muscles (p = 0.018), ≤8 weeks of intervention (p = 0.046), muscle thickness assessment (p = 0.0352), and isokinetic contraction (p = 0.0251). Our findings suggest similar hypertrophy between ECC and CON muscle actions in apparently healthy adults. However, it appears that the muscles of the upper limbs, shorter interventions, hypertrophy assessment method, and the contraction type may favor ECC muscle actions.