Due to propagation of action potentials, phase angles between surface EMG (SEMG) channels are proportional to the frequency for superficial muscles (Schack et al. 2000). For deep muscles,

e.g. m. erector spinae, the behaviour of SEMG signals is more complex. Propagating action potentials appear very rarely (Farina et al. JEK 2003), and the cross covariance function of bipolar SEMG channels shows correlation peaks with slow spatial sign reversal but without time shifts. This behaviour can be explained by means of a model of deep muscle fibres overlapping in fibre direction (Grassme et al. 2005). The spatial sign reversal of correlation peaks is composed of two components with similar behaviour. 1. Far fields from the fibre-tendon transitions (Stegeman et al. 1997) cause temporally sharp peaks with a longer range of spatial sign reversal. 2. The finiteness of the (deep) muscle fibres itself causes temporally smooth peaks with a shorter range of spatial sign reversal. If the EMG signal contribution of one segment of a chain of short, deep muscles is suppressed (inactive) in the model, there appear specific characteristics in the behaviour of the cross covariance function (asymmetric spatial sign reversal). But such characteristics are hardly to detect if further EMG activities are present.