Entropy measures of back muscles EMG for subjects with and without pain

We have previously reported that the time-dependent entropy $S(t) $ calculated from electromyography time series of low back muscles exhibit plateau-like behavior for intermediate times [$50 \, \mbox{ms} < t < 0.5 \, \mbox{s}$]. We proposed that the plateau value can be used to characterize the sEMG signal of subjects with low back pain [J. Rehab. Res. Dev. {\bf 44}, 599 (2007)]. We report results of a larger study, and compare the entropies for the left -and right thoracic and left- and right lumbar muscles. We also compare entropies from muscles before and after physical therapy intervention.