The purpose of this study was to examine the relationship between kinematic variables at the knee, hip, and trunk during a single-leg step-down test (SDT) and running. Twenty-five healthy subjects (12 male, 13 female) participated in the study; mean ± SD age, 32.8 ± 5.9 years; height, 173.9 ± 8.7 cm; body mass, 70.84 ± 11.3 kg; run volume, 59.5 ± 30.4 km/wk; cadence, 173.1 ± 11.5 steps/min). Dominant leg peak knee flexion was identified during the run (PKF-RUN) and used to find frontal plane knee and hip, and sagittal plane trunk angles. The same treadmill-matched knee flexion angle for the run was used to find the knee flexion angle identified during the SDT (TMKF-SDT). Knee, hip, and trunk angles were also identified at the point of the SDT where the heel made contact with the ground (HEEL-SDT). Two separate two-tailed paired samples t-tests were used to analyze the difference between the means of each test condition and Pearson Product Correlation coefficients were computed for each condition. Statistics revealed significant differences in frontal plane knee and hip angles between PKF-RUN (6.18 degrees ± 8.90) and TMKF-SDT (8.13 degrees ± 8.88), t(24) = -2.21, p = 0.037 for frontal plane knee adduction, and; PKF-RUN (11.14 degrees ± 3.22) and TMKF-SDT (6.48 degrees ± 4.53), t(24) = 6.17, p \u3c 0.0001 for frontal plane hip adduction. There were significant differences between mean PKF-RUN (6.18 degrees ± 8.90) and HEEL-SDT (16.65 degrees ± 12.60), t(24) = -6.79, p \u3c 0.0001 frontal plane knee adduction, and; PKF-RUN (11.14 degrees ± 3.22) and HEEL-SDT (17.84 degrees ± 5.63), t(24) = -6.45, p \u3c 0.0001 for frontal plane hip adduction. No significant differences were found between mean PKF-RUN (6.44 degrees ± 3.67) and TMKF-SDT (6.33 degrees ± 6.46), t(24) = 0.104, p = 0.918 sagittal plane trunk flexion. There were significant differences between mean PKF-RUN (6.44 degrees ± 3.67) and HEEL-SDT (10.32 degrees ± 10.04), t(24) = -2.19, p = 0.039 sagittal plane trunk flexion. Correlations between PKF-RUN and TMKF-SDT were strong in the knee (r = 0.88, p \u3c 0.0001, R2 = 0.768) and moderate in the hip (r = 0.57, p = 0.003, R2 = 0.325). Correlations between PKF-RUN and HEEL-SDT were strong in the knee (r = 0.80, p \u3c 0.0001, R2 = 0.634) and fair in the hip (r = 0.42, p = 0.038, R2 = 0.175). For the trunk, correlations between PKF-RUN and TMKF-SDT were moderate (r = 0.53, p = 0.006, R2 = 0.285) and correlations between PKF-RUN and HEEL-SDT were fair-to-moderate (r = 0.49, p = 0.014, R2 = 0.237). The SDT and running may not be directly relatable to one another in the knee and hip. The trunk is also not relatable to running at the bottom of the SDT. Clinicians should use caution when utilizing the SDT
