Estimation of average muscle fiber conduction velocity from simulated surface EMG in pinnate muscles

The aim of this simulation study was to assess the bias in estimating muscle fiber conduction velocity (CV) from surface electromyographic (EMG) signals in muscles with one and two pinnation angles. The volume conductor was a layered medium simulating anisotropic muscle tissue and isotropic homogeneous subcutaneous tissue. The muscle tissue was homogeneous for one pinnation angle and inhomogeneous for bipinnate muscles (two fiber directions). Interference EMG signals were obtained by simulating recruitment thresholds and discharge patterns of a set of 100 and 200 motor units for the pinnate and bipinnate muscle, respectively (15° pinnation angel in both cases). Without subcutaneous layer and muscle fibers with CV 4 m/s, average CV estimates from the pinnate (bipinnate) muscle were 4.81 ± 0.18 m/s (4.80 ± 0.18 m/s) for bipolar, 4.71 ± 0.19 m/s (4.71 ± 0.12 m/s) for double differential, and 4.78 ± 0.16 m/s (4.79 ± 0.15 m/s) for Laplacian recordings. When subcutaneous layer was added (thickness 1 mm) in the same conditions, estimated CV values were 4.93 ± 0.25 m/s (5.16 ± 0.41 m/s), 4.70 ± 0.21 m/s (4.83 ± 0.33 m/s), and 4.89 ± 0.21 m/s (4.99 ± 0.39 m/s), for the three recording systems, respectively. The main factor biasing CV estimates was the propagation of action potentials in the two directions which influenced the recording due to the scatter of the projection of end-plate and tendon locations along the fiber direction, as a consequence of pinnation. The same problem arises in muscles with the line of innervation zone locations not perpendicular to fiber direction. These results indicate an important limitation in reliability of CV estimates from the interference EMG when the innervation zone and tendon locations are not distributed perpendicular to fiber direction.