Coefficient of cross correlation and the time domain correspondence

Time histories of neuromuscular and mechanical variables of human motion ar e often compared by using discrete timing events (onset, offset, time to pe ak, zero crossing, etc). The determination of these discrete timing points is often subjective and their interpretation can cause confusion when attem pting to compare patterns. In this technical note, cross correlation and th e 95% confidence interval of its maximum value are proposed as an objective means of pattern recognition and comparison. EMG patterns of cycling at di fferent cadences were used as an example to demonstrate the effectiveness o f this cross correlation method in identification of changes between condit ions. Using a standard method of threshold identification, different onset and offset values can be found by using different thresholds, and the seque nce of the offset timings between conditions can change. This is a clear in dication of the inherent subjectivity with these discrete timing methods. I n contrast, calculation of cross correlation for incremental phase shifts p ermits the identification of a maximal value that is an objective measure o f the actual phase shifting between the two time series. Further, calculati on of the 95% confidence interval allows one to determine whether the phase shifting is statistically significant. The application of this method is n ot limited to EMG pattern comparison, and can also be applied to other time histories such as kinematic and kinetic parameters of human motion. (C) 19 99 Elsevier Science Ltd. All rights reserved.