The effect of skinfold on frequency of human muscle mechanomyogram.

The aim of the study was to assess the effect of skinfold thickness on median and peak frequency of mechanomyographic (MMG) signal in relation to subject's age, gender and force during voluntary contraction of elbow flexor and extensor muscles. Seventy-nine healthy subjects participated in the study: 22 young females (age 20.1+/-1.1 years), 22 young males (age 23.4+/-1.1 years), 17 elderly females (age 64.9+/-5.1 years), and 18 elderly males (age 67.4+/-6.2 years). Three identical MMG probes were used to record MMG signals from above the triceps brachii (TB), biceps brachii (BB), and brachioradialis (BR) muscles simultaneously with the force signal. The results showed that the tissue between the muscle and the skin surface has a major contribution to the median and a minor contribution to the peak MMG frequencies independent of subjects' age (with force having more than 2 folds a smaller effect). During antagonistic function of the main elbow flexors and extensors, there is a decreasing effect of skinfold thickness and an increasing effect of force on the MMG frequency, and the relative contribution of both factors to the MMG signal is age related, especially in the TB and BR muscles. The BR muscle differs from the TB and BB muscles in regard to the effects of skinfold thickness and force on the MMG frequency, as well as in the effect of age on the relationship between the MMG frequency and skinfold thickness and force. The effect of age on the relative contribution of skinfolds and force to MMG frequency is specific for muscle and its function. It was concluded that studies that report MMG frequency with different values of skinfold thickness cannot be easily compared, especially when maximally activated prime movers are tested. A use of force and skinfold thickness as covariates is recommended when an MMG frequency is analyzed in subjects differing in the skinfold thickness.