Background: Running biomechanics is considered an important determinant of running economy (RE). However, studies examining associations between running biomechanics and RE report inconsistent findings. Objective: The aim of this systematic review was to determine associations between running biomechanics and RE and explore potential causes of inconsistency. Methods: Three databases were searched and monitored up to April 2023. Observational studies were included if they (i) examined associations between running biomechanics and RE, or (ii) compared running biomechanics between groups differing in RE, or (iii) compared RE between groups differing in running biomechanics during level, constant-speed, and submaximal running in healthy humans (18-65 years). Risk of bias was assessed using a modified tool for observational studies and considered in the results interpretation using GRADE. Meta-analyses were performed when two or more studies reported on the same outcome. Meta-regressions were used to explore heterogeneity with speed, coefficient of variation of height, mass, and age as continuous outcomes, and standardization of running shoes, oxygen versus energetic cost, and correction for resting oxygen or energy cost as categorical outcomes. Results: Fifty-one studies (n = 1115 participants) were included. Most spatiotemporal outcomes showed trivial and non-significant associations with RE: contact time r = - 0.02 (95% confidence interval [CI] - 0.15 to 0.12); flight time r = 0.11 (- 0.09 to 0.32); stride time r = 0.01 (- 0.8 to 0.50); duty factor r = - 0.06 (- 0.18 to 0.06); stride length r = 0.12 (- 0.15 to 0.38), and swing time r = 0.12 (- 0.13 to 0.36). A higher cadence showed a small significant association with a lower oxygen/energy cost (r = - 0.20 [- 0.35 to - 0.05]). A smaller vertical displacement and higher vertical and leg stiffness showed significant moderate associations with lower oxygen/energy cost (r = 0.35, - 0.31, - 0.28, respectively). Ankle, knee, and hip angles at initial contact, midstance or toe-off as well as their range of motion, peak vertical ground reaction force, mechanical work variables, and electromyographic activation were not significantly associated with RE, although potentially relevant trends were observed for some outcomes. Conclusions: Running biomechanics can explain 4-12% of the between-individual variation in RE when considered in isolation, with this magnitude potentially increasing when combining different variables. Implications for athletes, coaches, wearable technology, and researchers are discussed in the review. Key Points: Among spatiotemporal variables, ground contact time, flight time, and duty factor showed trivial and non-significant associations with running economy, while a higher step frequency was weakly associated with a better running economy. Lower vertical displacement and higher vertical and leg stiffness were moderately associated with better running economy, while joint angles at specific instances of the gait cycle, joint angle range of motion, peak vertical ground reaction force, mechanical work variables, and electromyographic muscle activation showed non-significant and often trivial associations with running economy. Nevertheless, some non-significant trends of at least a small magnitude were observed for some outcomes (e.g., co-contraction duration, joint angles at toe-off). Overall, our findings show that biomechanical variables can explain 4-12% of the between-individual variance in running economy when considered in isolation, with this magnitude potentially increasing when combining different variables.
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