Online muscle fibre action potential conduction velocity measurements using the surface e.m.g. cross-correlation technique

To DETERMINE the time required for a signal to pass through a linear system, the cross-correlation function between the input and output signal may give the time delay information directly. The cross-correlation function will peak at the particular time shift corresponding to the time necessary for the signal to pass through the system. This technique has been used to measure the muscle fibre action potential conduction velocity using two surface e.m.g, signals (NAEIJE and ZORN, 1982a). In this study the e.m.g, signals were recorded by electrodes placed on one side of the endplate region of the muscle parallel to the direction of the muscle fibres. Several investigators have looked for alternatives to the e.m.g, cross-correlation technique (L1NDSTROM et al., 1970; LYNN, 1979) because the possibility for online measurements is hampered by the rather time consuming nature of the crosscorrelation technique, despite the fast Fourier transform algorithm. The authors mentioned above used techniques based on a considerable data reduction. With Lynn's method the e.m.g, signals are bandpass filtered and clipped. After these operations the time intervals between the corresponding zero-crossings (changes in sign) are measured. With Lindstrom's dip methods only certain dips in the e.m.g. power spectrum are used, thus ignoring the rest of the e.m.g. information. As a result of this data reduction more ' input' is needed to perform reliable measurements, and so longer sample records must be taken. For instance, Lynn used 2-5 s records, whereas for NAEIJE and ZORN (1982a), using the e.m.g, cross-correlation, 0.8 s records were sufficient. In this paper a method is described to considerably reduce the calculation time for the action potential conduction velocity by limiting the number of cross-correlation calculations as much as possible. This makes it possible to record online the action potential conduction velocity using the e.m.g, cross-correlation technique.