Abstract
In their daily lives, people perform various movements, which are caused by muscle contraction. The contraction accompanies changes in the muscle's shape and biomechanical properties. Measurement of biomechanical properties of muscle in vivo is therefore an important subject. The authors have developed an impact force measurement system for biometric use. The purpose of this study was to measure biomechanical properties of muscle in vivo from the skin surface, using that system. Before we began measuring muscle, we used experimental models to examine how far from the surface mechanical properties were detectable. It was confirmed that the detectable area reached to a depth of around 20mm and that biomechanical properties of muscle should be measured from the skin surface. The loading period of an impact force, which had been found to be related to stiffness, was also defined beforehand. First, the relation between muscle contraction and its biomechanical properties was examined. The wave heights and loading periods were calculated from impact forces as parameters to express biomechanical properties. The contraction of an antebrachial flexor was induced by making subjects grasp a hand dynamometer. Impact forces and surface electromyogram (EMG) were measured at four graduations. Integrated EMG (IEMG) was calculated to confirm the activities of muscle fiber. We found that the wave heights increased and the loading periods decreased as IEMGs increased. These changes show the muscle gets harder, which agrees with tactile hardness. Both parameters had a tendency to be saturated according to IEMGs. It was consequently confirmed that there was a significant relation between muscle contraction and biomechanical properties measured from the skin surface. Next, the relation between muscle fatigue and its mechanical properties was also examined. Muscle fatigue was caused by applying a light load to an antebrachial flexor for a specified period. Impact forces and EMG were measured, and medium frequency (MF) of the EMG power spectra was calculated as a criterion of muscle fatigue. Together with fatigue, wave heights increased and loading periods decreased. These changes show the antebrachial flexor getting harder as muscle fatigue proceeds. The parameter shifts depending on contraction averaged +0.0768N and -6.04ms, respectively. On the other hand, those depending on muscle fatigue were +0.1247N and -2.75ms, respectively. Moreover, an adequate correlation between MFs and both parameters were observed, the correlation with loading periods being strong. This clearly demonstrated that muscle hardening caused by muscle fatigue is measurable from the skin surface.
