Mental fatigue reduces resistance exercise volume by moderate effect in healthy adults

The recent increase in randomized controlled trials evaluating the effects of mental fatigue (MF) on RE warrants an update of the available meta-analytical evidence. This study presents the results of a comprehensive systematic review and meta-analysis, including GRADE qualification, examining the effects of MF on RE across different subgroups. We included only randomized controlled trials involving healthy human participants, a high-demand cognitive task requiring cognitive effort (e.g., Stroop test), a low-demand or passive control condition (e.g., watching documentaries), and a resistance exercise performance task assessing volume. A total of 11 studies reported 14 comparisons involving more than 205 participants, providing moderate-level evidence. The random-effects meta-analysis revealed a significant mean negative effect of cognitive effort on RE volume (g = -0.39 and p < 0.01). Subgroup analyses suggested that multijoint exercises may be more susceptible to impairment (g = -0.45 and p < 0.01) than single-joint exercises (g = -0.20 and p = 0.09). The magnitude of impairment appeared larger at moderate-intensity loads (60%-79% 1RM and g = -0.56) relative to low-intensity (g = -0.40) and bodyweight conditions (g = -0.25). Similarly, the effect appeared greater in high training volume conditions (g = -0.54) compared to moderate volume (g = -0.37). Overall, although this review confirms the negative impact of MF on RE volume, the findings should be interpreted with caution due to low-quality evidence.