It is important to understand the day-to-day variability that is attributed to repositioning of markers especially when assessing a treatment effect or response over time. While previous studies have reported reliability of waveform patterns, none have assessed the repeatability of discrete points such as peak angles, velocities and angular excursions which are often used when making statistical and clinical comparisons. The purpose of this study was to compare the within- and between-day variability of discrete kinematic, kinetic, and ground reaction force (GRF) data collected during running. Comparisons for 20 recreational runners were evaluated for within- and between-day reliability of discrete 3D kinematic, kinetic, and GRF variables. The results indicated that within-day compal-isons were more reliable than between-day. Joint angular velocity and angular excursion values were more reliable betweendays as compared to absolute peak angle measures and may be more useful in interpreting changes in treatment over time. Betweenday kinematic and kinetic sagittal plane values were more reliable than secondary plane values. Reliability of GRF data was greater than kinematic and kinetic data for between-day comparisons.
