Abstract—                                                                       !                            "    #$ %$ $    "%     "     "            &              $   $   I. INTRODUCTION UMAN beings are always in touch with their surrounding environment in all their life spans. The outer surface of the human body is mainly composed of soft tissues and most of the interaction with the surrounding environment is done by these tissues. That is why understanding and classifying the mechanical response of soft biological tissues is a fundamental problem in biomechanics. Soft tissue usually exhibits nonhomogeneity, anisotropy, non&linearity, and viscoelasticity. Some difficulties are encountered in characterizing the mechanical response of viscoelastic materials [1]. Soft tissues are relatively compliant at low strain rates and become dramatically stiffer at high strain rates. There is a continuing need to develop methods that reveal the complete set of anisotropic material properties. There are several procedures and instruments in mechanical engineering for material testing. For work on solid biomaterials, the “Universal Testing Machine” is the most useful general purpose machine. On the other hand, studying soft biological tissue on these machines can be inadequate. Usually adaptation of mechanical engineering tests has been used for soft tissue research. Most of these tests are very specialized but have not been standardized [1] [2]. Manuscript received April 22, 2009. S. Aritan is with the Biomechanics Research Group, School of Sports Science and Technology, Hacettepe University, Beytepe, Ankara, Turkey. (phone: +90&312&297&6893; fax: +90&312&299&2167; e&mail: serdar.aritan@ hacettepe.edu.tr). S. O. Oyadiji is with the Dynamics and Aeroelasticity Research Group, School of Mechanical, Aerospace and Civil Engineering, University of Manchester, Manchester, UK (e&mail: s.o.oyadiji@manchester.ac.uk). R. M. Bartlett is with the School of Physical Education, University of Otago, Dunadin, New Zealand (e&mail: roger.bartlett@otago.ac.nz). Although, there is numerous research based on in vitro studies, there are few in vivo studies. In vivo experiments are performed on a living human body. This type of experiment gives the most accurate information about the mechanical behavior of the tissue [3]. In order to observe the mechanical properties of soft tissue, it is necessary to design a special machine. Therefore in vivo mechanical properties of muscular bulk tissue have not yet been investigated sufficiently. A muscle is ordinarily thought of as an active system; its prime function is to generate force. Muscles are composite materials, composed of stiff strong fibers, plates or particles in a relatively complaint matrix. Like many soft composites, muscles change their mechanical properties as part of their normal functioning. Muscle is stiffened by contraction and softened by relaxation. The object of this study was to design an instrument to apply uniform controllable pressure on the upper arm and obtain the bulk modulus of the upper arm under relaxed and controlled contraction that was 25% of the maximum voluntary contraction (MVC). II. METHODS A. Test Machine Design and Instrumentation A new testing machine was designed to generate a continually increasing compression on the upper arm. A schematic representation of the compression test design is shown in fig. 1. The movement of cylinders (Fig.1. (13)) was transmitted to the upper arm with a squeezing chamber (9). Reference [4] shows a detailed explanation of the squeezing chamber. This device is a cuff that applies controllable compression on a 47 mm wide band of the upper arm. A seat belt was preferred as the compression band for many reasons. Firstly, its mechanical properties that have not shown any time dependent behavior. Secondly, it is flexible enough to cover the upper arm without causing discomfort and finally it has an optimum width and a texture that eliminating the rough edges of conventional belts that are likely to cause pain due to friction and cut into the skin. The controlled deformation of the arm was achieved by using pneumatic cylinders, a constant speed of the cylinders was achieved by using a linear stepper motor (10). In order to prevent the linear stepper motor stalling, a voltage&to& pressure converter (2) and a feedback control unit were added to regulate the air pressure with respect to the load. A schematic diagram of the pneumatic system can be seen in the fig. 2. Stress&strain response of the upper arm was collected by using a load cell (5). The analog signal from the load cell was digitized by using a 12&bit analog&to digital    ’ (  ’ )  Serdar Arıtan, Member, IEEE, S. Olutunde Oyadiji and Roger M. Bartlett H 5259 31st Annual International Conference of the IEEE EMBS Minneapolis, Minnesota, USA, September 2-6, 2009 978-1-4244-3296-7/09/$25.00 ©2009 IEEE