The duration of the eccentric phase may affect resistance training (RT) adaptations, with narrative reviews yielding equivocal conclusions. Thus, five databases were searched following Cochrane and PRISMA-guidelines, to compare RT outcomes following shorter (SEG) and longer (LEG) eccentric phase durations. A prospectively registered (https://osf.io/s6uqn/) random-effects multi-level meta-analysis with robust variance estimation was performed for strength, hypertrophy and countermovement jump height (CMJ), with results interpreted relative to a region of practical equivalence. Evidence quality was assessed using the Cochrane RoB 2 and GRADE criteria. Nine studies involving 166 participants (46% trained, 77% male) were included. SEG enhanced CMJ by a practically worthwhile degree with moderate certainty (Hedge`s g = -0.73 [90% CI = -1.34, -0.12; 90% PI = -1.34, -0.12]), while uncertain estimates were observed for maximal strength (g = 0.25 [90% CI = -0.17, 0.67; 90% PI = -0.86, 1.36]) and muscle hypertrophy (g = 0.05 [90% CI = -0.22, 0.33; 90% PI = -0.54, 0.64]). LEG led to practically equivalent or enhanced strength gains, compared to SEG, in trained participants (g = 0.33 [90% CI = 0.07, 0.60; 90% PI = 0.07, 0.60]) and volume-load matched trials (g = 0.25 [90% CI = 0.04, 0.45; 90% PI = 0.04, 0.45]) with moderate certainty in subgroup analyses. Therefore, SEG enhance CMJ, while LEG may cause similar or higher strength increases in trained participants and volume-load matched conditions. However, more research is needed.
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