Background Comparison of between-limb loading asymmetry during running is often used to assess injury risk or return to run criteria. With the increased use of inertial measurement units (IMUs) in clinical and consumer contexts, it is important to determine normative values for impact asymmetry. For some metrics, such as strength, asymmetries of less than 10% are considered normal, but normal values of impact asymmetries and whether they vary depending on the runner’s sex, running speed, or footwear is unknown.

Objective The purpose of this investigation was to describe the magnitude of asymmetries of peak positive vertical accelerations (PPA) during running among healthy runners and to determine the influence of sex, speed, and footwear.

Design Cross-sectional.

Setting Lab-based.

Patients (or Participants) Healthy recreational runners.

Interventions (or Assessment of Risk Factors) Healthy runners between the ages of 18 and 60 ran in standardized footwear (minimalist, maximalist, and neutral) at three speeds (self-selected, +10%, -10%) while an insole-embedded IMU collected acceleration data at 500 Hz.

Main Outcome Measurements Asymmetries were calculated for PPA. A univariate ANOVA with mixed effects assessed the influence of sex, speed, and footwear.

Results Seventeen runners (8 female) were included. The mean asymmetry across all trials was 16.0 ± 23.5%, with an SEM of 2.6%. No significant interactions occurred with footwear or speed, but there was a significant difference between sexes (13.1%; p = .013). However, the effect sizes were very small based on Z-score comparison (-0.325 ≤ z ≤ 0.232) and unlikely to be meaningful.

Conclusions The magnitude of asymmetry varies considerably across individuals, but does not seem to be substantially affected by sex, speed, or footwear. Future studies should include a larger sample size and detailed running history variables in an effort to determine population norms and effect of training- and injury-related variables on impact asymmetry.