Influence of Skinfold Thickness in Mechanomyography Features Eddy Krueger 1 , Eduardo M. Scheeren 3 , Guilherme N. Nogueira-Neto 2,3 , Eduardo Borba Neves 1 , Vera Lúcia S. N. Button 2 and Percy Nohama 1,2,3 1 Rehabilitation Engineering Laboratory/CPGEI/PPGEB, Federal Technological University of Paraná (UTFPR), Curitiba-PR, Brazil 2 Department of Biomedical Engineering/FEEC/CEB, University of Campinas (UNICAMP), Campinas-SP, Brazil 3 Rehabilitation Engineering Laboratory/PPGTS, Pontifical Catholic University of Paraná (PUCPR), Curitiba-PR, Brazil Abstract—The assessment and physiological register of muscular tissue can be done through mechanomyography (MMG). The oscillation of muscular displacement is acquired on the skin surface, insomuch that the skinfold thickness can influence in MMG response. The aim of this study is verify the influence of different skinfold thickness on MMG features responses. Triaxial MMG was used over the rectus femoris muscle belly of ten volunteers during maximal voluntary contraction. MMG spectral and temporal analyses were made and specific features (root mean square (RMS), Integral (Int), mean frequency (MF), zero-crossing (ZC), and μ3) were correlated with skinfold thickness. Moderate and high negative correlation occurred to MMG mean frequency in axes X ( = - 0.57) and Y ( = -0.75), respectively. As the fat tissue behaves like a low-pass filter, i.e. the thicker his skinfold the shorter its bandwidth; therefore, the skinfold thicknesses result in lower frequency responses. So, hereafter these results may be applied to calibrate MMG responses as biofeedback systems in, for instance, neuroprostheses. Keywords— Skinfold Thickness, Temporal Analysis, Spectral Analysis, Mechanomyography. I. INTRODUCTION Mechanomyography (MMG) is a noninvasive technique that can be applied with accelerometer sensors [1, 2] and may be used to assess muscular condition during isometric [3] and dynamic [4] contractions. This technique can be also used for monitoring muscle feedback as, for instance, of volunteers lower limbs under electrical stimulation [5]. MMG signals are viable as biofeedback in the control of myoelectrical prostheses [6, 7], neuroprostheses [8] and to support physical therapy sessions [9]. However, as MMG register the oscillation [10] of muscular tissue, until this vibration be detected on the skin surface [2,3], the fat tissue can attenuate MMG spectral and temporal characteristics [11] compromising the signal acquisition and processing. The aim of this study is verify the influence of different skinfold thickness on MMG features responses. II. METHODS A. Subjects This investigation was performed according to principles of the Declaration of Helsinki and was approved by Pontifical Catholic University of Paraná’s (PUCPR) Human Research Ethics Committee under register n. 2416/08. Ten healthy volunteers participated in this study. During the period of tests, the volunteers did not use any drug that could change their motor condition. Anthropometric data such as weight, height, body mass index (BMI) and the quadriceps skinfold were collected. The volunteers were divided in two groups: group I (N= 10) with skinfold below 10 mm and group II (N= 2) with skinfold above 30 mm. B. Sensors and Data Acquisition The developed MMG instrumentation used Freescale MMA7260Q MEMS triaxial accelerometers with sensitivity equal to 800 mV/G at 1.5 G (G: gravitational acceleration). Electronic circuits allowed 10x amplification and 4-40 Hz Butterworth third order filtering. A LabVIEW™ program was coded to acquire and display MMG signals and to compute the features. The data were saved into European Data Format (EDF) files. The acquisition system contained a DT300 series Data Translation™ board working at 1 kHz sampling rate. The signal modulus was computed from the three individual MMG sensor axes. A load cell (100 kg, 2.0±0.1 mV/V) was used to acquire the quadriceps torque information. C. Research design The volunteers performed a standard muscular and warm- up stretching before the experimental protocol. They were seated on a bench with the hip and knee angles set to 70º [12]. After trichotomy and skin cleaning, superficial MMG sensors were positioned over the belly of rectus femoris muscle and attached with double-sided tape. The anterior ankle joint was positioned in a brace (foam coated) positioned in 60° of flexion from total knee extension (0 o ) as shows Fig. 1. From three initial knee flexion repetitions, the M. Long (Ed.): World Congress on Medical Physics and Biomedical Engineering, IFMBE Proceedings 39, pp. 2030–2033, 2012. www.springerlink.com