Crosstalk in surface electromyography: Theoretical and practical estimates

The purpose of this paper is to address four aspects of surface electromyography associated with crosstalk between adjacent recording sites. The first issue that is addressed in the potential crosstalk between electrodes located on muscles with different functions: antagonist pairs, or muscles with one common and one different function (i.e. soleus/peroneus longus or soleus/ gastrocnemius). Practical functional tests are utilized to demonstrate the crosstalk between muscle pairs to be negligible. The second goal is to estimate the depth of pick-up and the crosstalk between myoelectric signals from agonist muscles using a theoretical model. The depth of pick-up was estimated to be 1.8 cm (including a 2 mm layer of skin and fat) using electrodes of 49 mm² with bipolar spacing of 2.0 cm. A cross-correlation technique is demonstrated which predicts the common signal (crosstalk) between surface electrodes with electrode-pair spacing of 1 cm around a hypothetical muscle. The predicted crosstalk using cross-correlation measures was 49% at 1 cm electrode-pair spacing dropping to 13% at 2 cm spacing and 4% at 3 cm. The third part compares these predictions with crosstalk measures from experimental recordings taken from electrode pairs spaced 2.5 cm apart around the quadriceps. At 2.5 cm spacing there was 22-24% common signal dropping to between 4-7% at 5 cm and to between 1 and 2% at 7.5 cm. The fourth and last component of this report assesses three methods to decrease the range of pick-up and thereby potential crosstalk: electrodes of smaller surface area, reduced bipolar spacing and mathematical differentiation. All three techniques reduce the common signal by varying amounts; all three techniques combined reduce the predicted crosstalk for the 1.0 cm electrode-pair spacing from 49-10.5%.