Reproducibility and adaptation of the EMG responses of the lower leg following perturbations of upright stance.

The electromyographic (EMG) response of gastrocnemius, soleus and anterior tibialis muscles to a backwardly directed perturbation of stance was recorded in 12 normal subjects using surface electrodes and studied with regard to its reproducibility (test-retest reliability coefficients, variability coefficients) and to adaptational effects. (1) Reproducibility was shown to be uniformly high and can be interpreted as an index for the high intraindividual constancy of the results. (2) Adaptational effects have been found and should be circumvented, either by pre-adapting the subjects to the motor task, or by restriction of the period of measurement. (3) Variation of the position of the electrodes produced only small effects. The impedance of the surface electrodes was not critical if kept below 5 k omega. EMG investigations with surface electrodes during stance and perturbations of stance provide highly reliable results with respect to intraindividual changes but interindividual variability of the results clearly marks the limits of this method. The interindividual variability observed with surface electrodes is of the same order as that reported in the literature for inserted needle recording.