Development of a vibro-ultrasound method for muscle stiffness measurement in vivo

xviii, 151 p. : ill. ; 30 cm.PolyU Library Call No.: [THS] LG51 .H577P HTI 2011 WangStiffness has been proved to be a very important property of skeleton muscle and contribute significantly to muscle function. A number of methods have been developed to quantitatively assess muscle stiffness, such as indentation assessment, sonoelastography, transient elastography, supersonic shear imaging (SSI), shear-wave dispersion ultrasound vibrometry (SDUV) and magnetic resonance elastography (MRE). Each of them has unique features and limitations in in vivo studies. One major limitation of these methods is that the measurement ranges using these methods cannot exceed 50% of the maximum voluntary contraction (MVC) level. However, the information at high intensity contraction levels is more important for muscle functional assessment. Previous studies have reported that skeleton muscle stiffness is positively correlated to the non-fatiguing muscle contraction level. However, due to the limitation mentioned above, this conclusion has not been verified in a full range of isometric muscle contraction, i.e. from 0% to 100% MVC level. Furthermore, the relationship between muscle stiffness and isometric contraction level has seldom been studied systematically, with missing knowledge on how joint angle, gender and age may affect the relationship. In this study, a novel vibro-ultrasound muscle stiffness assessment system based on the measurement of shear wave velocity in tissue was developed and its feasibility was evaluated on custom-made phantoms with different stiffness using traditional indentation method. Then it was used to assess the shear modulus of vastus intermedius (VI) muscle, one of the quadriceps femoris muscles, along the direction of muscle action. The relationship between the muscle stiffness and the step isometric contraction level was studied from 0% to 100% MVC under two different knee joint angles. The differences of muscle stiffness of the subjects with different gender and age ranges were also studied at different step isometric contraction levels. Furthermore, VI muscle shear modulus values were compared with the root mean square (RMS) values of the surface electromyography (EMG) signals collected from vastus lateralis (VL) muscle. Forty healthy subjects volunteered to participate in the experiment and were divided into four groups: young males (n = 10, 29.4 ± 4.8 yr), young females (n = 10, 27.6 ± 5.0 yr), elder males (n = 10, 60.6 ± 7.6 yr) and elder females (n = 10, 56.7 ± 4.9 yr). The experiments were repeated at two different knee joint angles, 90° and 60° (0° for full extension). During the tests, MVC torque value was first determined as the highest torque of knee extensor achieved under voluntary contraction, which could be maintained for at least 5 seconds. Next, muscle stiffness measurements were performed under relaxed condition with 3 trials. Then the subjects were asked to maintain isometric contraction at different percentage of MVC levels, from 10% to 100%, for approximately 4 seconds. For each level, there were 3 trials and one-minute rest was allowed between any two measurements to avoid muscle fatigue.The Pearson's correlation coefficient (CC) between the shear modulus values measured on the 10 pieces of silicone using indentation method and those measured using vibro-ultrasound method phantoms was 0.994. The Bland-Altman plot, in which the differences between the pairs of corresponding results against their mean were plotted, demonstrated that all the plots were in the range between mean ± 2sd and this indicated that the results of these two methods have a high agreement. These results proved the feasibility of our system. The results of in vivo test showed that the stiffness of VI muscle along the direction of muscle action was positively correlated to the step isometric contraction level in a full range for both knee joint angles (both p < 0.001). The relationship between them was close to a quadratic polynomial curve (p < 0.001). It was also found that under both relaxed condition and all step isometric contraction levels, the VI muscle shear modulus measured at 90° knee joint angle was larger than that measured at 60° knee joint angle (both p < 0.001). There was no significant difference in the relaxed muscle shear modulus between different genders (p = 0.156) and age ranges (p = 0.221). However, when performing step isometric contraction, the muscle shear modulus of the male subjects was larger than that of the females (p < 0.001), and the muscle shear modulus of the young subjects was larger than that of the elderly participants (p < 0.001) at the same contraction level and knee joint angle. A high correlation was found between the muscle shear modulus values of the VI muscle and the normalized RMS values of the surface EMG of the VL muscle. In conclusion, we have successfully demonstrated that the newly developed vibro-ultrasound method can provide us a unique tool for muscle stiffness assessment in research and clinical practices. Muscle stiffness can provide complementary information of muscle mechanical properties in comparison with the traditional bioelectrical and morphological information, such as EMG and sonomyography (SMG). For the first time, we measured the muscle shear modulus change over a full range of isometric contraction, i.e. from 0% to 100% MVC. We also investigated the relationships between the muscle stiffness and the isometric contraction levels, with the effects of gender, age, and joint angle considered. The newly developed vibro-ultrasound method together with these results can help us to better understand the muscle recruitment strategies under different isometric contractions levels. Future studies are needed to further improve the reliability and measurement range of the vibro-ultrasound method, to apply our new technology in more clinical practices, and to combine it with other measurement systems.Department of Health Technology and InformaticsPh.D., Dept. of Health Technology and Informatics, The Hong Kong Polytechnic University, 201