Diverging times in movement analysis

In movement analysis, more than one measuring system is often used to record biomechanical variables. Usually, it is desired to assign the occurring events to a common time line, which can be accomplished by synchronizing data acquisition, i.e. using a pulse to trigger a sample on all systems. However, this method is not supported by every system. Alternatively, the measurements of different systems can be started by a common trigger signal with no further synchronization of their sampling clocks during acquisition. With that, two systematic errors may be introduced, namely time lag and time drift. The extents of these errors not only depend on the individual system properties, but also depend on the set of systems combined. In this study, we introduce a simple method to determine time lag and time drift for two systems including cameras and force plates. Our results show that both parameters are present and dependent on chosen sampling frequencies. We conclude that in order to avoid misinterpretation of recorded signals the identified time lag and time drift need to be taken into account for trials of all durations.