Test-retest reliability of two-dimensional video analysis during running

Objectives: To examine test-retest reliability of two-dimensional measured frontal and sagittal plane kinematics during running, and to determine how many steps to include to reach and maintain a stable mean. Design: Reliability study. Setting: Research laboratory. Participants: Twenty-one recreational runners. Main outcome measures: Lateral trunk position, contralateral pelvic drop, femoral adduction, hip adduction, knee flexion and ankle dorsiflexion during midstance, and foot and tibia inclination at initial contact were measured with two-dimensional video analysis during running for 10 consecutive steps for both legs. All participants were tested twice one week apart. A sequential estimation method was used to determine the number of steps needed to reach a stable mean. Intraclass correlation coefficients (ICC) and smallest detectable differences (SDD) were calculated. Results: The minimal number of steps was 6.3 ± 0.3. Lateral trunk position, femoral adduction and foot inclination showed excellent reliability (ICC 0.90–0.99; SDD 1.3°–2.3°). Tibia inclination and ankle dorsiflexion showed good to excellent reliability (ICC 0.73–0.92; SDD 2.2°–4.8°). Hip adduction and knee flexion showed good reliability (ICC 0.82–0.89; SDD 2.3°–3.8°). Contralateral pelvic drop showed moderate to good reliability (ICC 0.59–0.77; SDD 2.7°–2.8°). Conclusion: Two-dimensional video analysis is reliable to assess running kinematics on different days. The mean of at least 7 steps should be included.