Effect of ankle joint position on electrically evoked surface myoelectric signals of the tibialis anterior muscle.

The relationship between surface myoelectric signal variables and ankle joint angle is studied in the tibialis anterior muscle of ten normal subjects during electrical stimulation of the main muscle motor point in isometric conditions. Rectangular current pulses of 0.1 ms width and 20Hz repetition rate were applied for 10s with a monopolar technique using a stimulation/detection system providing stimulation artifact suppression. Electrically evoked responses (M-waves) were detected with a four-bar electrode placed on the skin below the lowest motor point near the tendon end of the muscle. Average rectified value, root mean square value, mean and median spectral frequency, conduction velocity, and latency of the surface signal were computed for ankle angles of 15 degrees dorsal flexion (-15 degrees), 0 degrees, 15 degrees, 30 degrees and 45 degrees of plantar flexion. All variables were considerably affected by the joint angle. Spectral variables and latency showed a statistically significant decrement as angle increased from -15 degrees to 45 degrees whereas conduction velocity and amplitude variables had a less regular behavior. The evoked response showed a latency decrement and a change of shape (mostly a widening of the second phase), as ankle joint angle increased from -15 degrees to 45 degrees. In most subjects conduction velocity appeared to be over estimated at -15 degrees and 45 degrees, a fact attributed to the small distance between the electrodes and either the tendon or the innervation zone at these two angles.(ABSTRACT TRUNCATED AT 250 WORDS)