The Relationship Between Skinfold Thickness and the Amplitude and Frequency Domains of the Surface Electromyographic Signal During Cycle Ergometry

Abstract

It has been suggested that the thickness of the subcutaneous tissue layer affects time and frequency domain parameters of the surface electromyography (EMG) signal. PURPOSE: The purpose of the present study was to examine the effects of skinfold thickness on the amplitude and mean power frequency (MPF) of the EMG signal from the vastus lateralis (VL) muscle during cycle ergometry. METHODS: Twenty adults (13 men and 7 women; mean age ± SD = 23.8 ± 3.0 yr) volunteered for this study. A bipolar surface EMG electrode configuration (30 mm center-to-center distance) was placed over the vastus lateralis (VL) muscle at one-third of the distance from the lateral border of the patella to the anterior superior iliac spine. EMG amplitude and MPF values were measured during incremental cycle ergometry at 50, 75, 100, 125, and 175 W. The incremental cycle ergometry test began at 50 W and increased by 25 W every 2 min. In addition, three skinfold measurements (mm) were taken from the VL at the site of the EMG electrode placement with the average value used in subsequent analyses. Simple linear regression was used to determine the relationships for EMG amplitude and MPF versus skinfold thickness at each power output. In addition, independent t-tests were used to compare the mean values for EMG amplitude and MPF between the 10 subjects with the largest skinfold values (mean ± SD = 16.7 ± 5.2 mm) and those (n = 10) with the smallest skinfold values (mean ± SD = 6.6 ± 1.2 mm). RESULTS: The simple linear regression analyses revealed no significant (p > 0.05) correlations between skinfold thickness and EMG amplitude (r = 0. 18 – 0.40) or MPF (r = 0. 17 – 0.41) at any of the power outputs. Furthermore, there were no significant mean differences (p > 0.05) in EMG amplitude or MPF between the subjects with the largest skinfold values and those with the smallest skinfold values. CONCLUSIONS: The results of the present study indicated that skinfold thickness did not affect absolute values for EMG amplitude or MPF from the VL muscle during cycle ergometry at five power outputs. These findings suggest that there maybe differences between the results of simulation studies and measured findings from surface EMG studies. PRACTICAL APPLICATIONS: Subcutaneous tissue thickness is just one of many possible factors that can affect the amplitude and frequency domains of the EMG signal. Other factors include the conductivities of the tissue, interelectrode distance, and the number of recruited motor units. The current findings provide information regarding a methodological consideration associated with the application of surface EMG measurements.