Comparison of acoustic and electrical signals from erectores spinae muscles.

Comparison was made between simultaneous recordings of the electromyogram (EMG) and acoustic myogram (AMG) signals and the torque produced by the lumbar erectores spinae muscles of three healthy subjects while they performed isometric contractions. Repeatability of each signal was studied. The AMG had a relationship to the torque which was more quadratic than linear, whereas that of the EMG was more linear. The coefficient of variability of residuals about a "best-fit" line was 8.55% for torque-increasing EMG recordings and 19.2% for torque-increasing AMG recordings. At the same torque, the EMG was on average about 10% greater for increasing-torque recordings than for decreasing-torque parts of the recordings, but the magnitude of the AMG signal was about 10% less. With repeated testing on the same day, the coefficient of variability of the signal/torque relationship (torque-increasing) was 9% for EMG and 23% for AMG signals. With long-term testing over 2 weeks these coefficients were 23 and 39%, respectively. Correlation analysis showed that the two signal/torque relationships varied independently of each other.