Mary Moffroid

Trunk extensor EMG-torque relationship

The integrated surface electromyogram (IEMG) of the lumbar erectores spinae and the torque generated were simultaneously recorded from 27 healthy subjects in the standing posture while they pulled isometrically against resistance provided by a harness around the shoulders. The IEMG – torque ratio (efficiency of electrical activity, or EEA) was used to characterize each subject. Individual recordings showed evidence of nonlinearity of the IEMG–torque relationship in that a statistically better fit to experimental recordings was obtained by using two straight lines with a breakpoint between them. However, with repeated testing, the gradients of these two lines were more variable than the slope of the single straight line fitted to the entire recording. The slope of the best fit line (EEA) was less for recordings made during torque decrease than for increasing-torque recordings. This also showed as a “hysteresis” pattern in the recordings. The coefficient of variability (within subjects) of the EEA was greater in day-to-day testing (24%) than with repeated pulls at the same testing session (14%). This was similar to variability of the maximum generated torque. About 25% of the variability between subjects was found to be due to anthropometric differences. The residual variability of the relationship would limit the accuracy of IEMG as a measure of muscular effort under changing torque conditions. However, the EEA may be useful for characterizing muscle performance, especially when maximum effort cannot be achieved.

EMG to torque relationship in rectus abdominis muscle. Results with repeated testing.

Rectus abdominis muscles of young healthy female volunteers were studied to determine both the maximum strength in isometric flexion and the relation between the surface electromyogram (RMS EMG) and torque. Both the upper and lower portions of the abdominal muscle were studied during graded Increase and decrease of torque. Repeated testing was performed over 6 weeks. The form of the relation between torque and EMG was better described by a quadratic than by a linear regression relationship, but with considerable variability about the best-fit line. The torque-increasing and torque-decreasing parts of each test were different and were analyzed separately. After 6 weeks of repeated testing, maximum voluntary isometric flexion torque Increased 16.8% (P < 0.01). There was a decrease in the ratio of electromyographic activity to torque production, which was statistically significant in the torque-Increasing recordings. These changes in maximum torque and in the EMG-torque relation were attributed to learning through a test-retest effect, rather than to a true change in muscle characteristics. These findings, which show changes in normal subjects undergoing repeated testing, do not support the reliability of isometric strength measurements, or measurements based on RMS EMG recordings, for quantifying abdominal muscle function in patients with back pain, or those undergoing strengthening exercises.

A time-frequency approach to evaluate electromyographic recordings

An attempt has been made to develop and test a new electromyographic (EMG) signal processing technique that may prove useful for dynamic muscle contractions. The technique described was accomplished by implementing a smoothed version of the Wigner-Ville distribution (WVD) in software. The WVD can display the frequency content of a signal as a function of time, thus utilizing all available information contained in the EMG signal. Although the EMG signal can often be considered quasi-stationary, there is important information which is transient and may only be distinguished by the WVD. The WVD was evaluated with the use of some experimental EMG data. Specifically, the soleus and medial gastrocnemius muscles were studied under the specific conditions of involuntary recruitment.<<ETX>>

Computer solutions to identify EMG latency of automatic postural reactions

A study undertaken to determine the usefulness of a computer algorithm for assessing one aspect of balance control, the latency of the muscles reaction to an unexpected perturbation, is discussed. The procedures, instrumentation, data collection methods, visual determination methods and computer algorithms used are described. The results of visual and computer determinations of latency are presented.<<ETX>>

Computer Solutions To Identify Emg Latency Of Postural Reactions

We have devised and tested a computerized method for identifying the onset of the EMG reflex response to postural perturbations delivered by an electronically-controlled hydraulic platform. Perturbations lasted 2.0 sec total. Surface EMG activity from relevant muscles was recorded and digitally processed to RMS. An algorithm based on a multiple of resting average RMS-EMG plus standard deviation, and expected time windows for minimal contractions and event times was developed. The algorithm was evaluated by comparisons to observers independent determinations of EMG latency.