Electromyographic recordings of paraspinal muscles: variations related to subcutaneous tissue thickness

The aim of this study was to assess the effect on EMG amplitude measures of variations in the thickness of underlying tissue between surface electrodes and the active muscle. 20 normal subjects with different amounts of subcutaneous tissue performed comparable constant force contractions for a 45-second period, during which paraspinal EMG recordings were taken. Three measures of subcutaneous tissue thickness were obtained from each subject: Body Mass Index, total body fat as calculated by Durnin's formula, and skinfold thickness at the recording sites. The results show that (i) the greater the thickness of subcutaneous tissue between the surface recording site and the contracting muscles, the lower the recorded electromyographic activity, and that (ii) up to 81.2% of the variance in the EMG measures can be explained by variation in the amount of subcutaneous tissue. These findings support the view that the absolute level of surface-recorded EMG cannot simply be taken at face value. The amplitude of the signal will be affected by, for example, the amount of body fat.