## 4 Discussion  
The purpose of the present study was to assess the capability and effectiveness of MyotonPRO and SWE for the characterization of the flexor digitorum superficialis (FDS), which is the key muscle that contributes to the movement of the hand, especially the grasping task. The mechanical properties of skeletal muscle, including passive and active contraction properties, are essential for performing the function of the muscle. Therefore, FDS passive and active muscle stiffness was measured using ultrasound shear wave elastography (SWE) and MyotonPRO, both at rest (0%MVC) and during muscle contraction (20%, 40%, 60% and 80%MVC), respectively. The muscle stiffness values of males and females were compared in order to achieve key insight into the optimal value of the hand muscle in healthy subjects at different conditions of muscle activation. In addition, a correlation between the muscle stiffness value achieved from two different approaches was investigated to identify whether MyotonPRO can be used for assessing muscle stiffness compared to SWE. The flexor digitorum superficialis (FDS) muscle is considered to be one of the key muscles that controls the movement of the hand. It is the largest extrinsic muscle of the hand, located in the anterior compartment of the forearm. Although this muscle forms as the intermediate muscle layer between the superficial and deep muscle groups of the forearm, its muscle belly is close to the surface of the forearm. Therefore, the activation level of this muscle is relatively straightforward to record and analyse [53; 67]. Although some previous studies have emphasized the measurement of muscle stiffness in the upper extremities using ultrasound shear wave elastography [41; 68; 69; 70], the evaluation of the stiffness of the muscle of the hand is currently less well established [1; 36; 37; 71]. Therefore, the task in the present study has advantages over previous studies owing to the focus on the evaluation of the muscle stiffness of the FDS muscle. From the measurement of muscle stiffness using SWE and MyotonPRO, the results of this study showed that the muscle stiffness of the FDS muscle increased when the intensity of muscle contraction was increased from 0%MVC to 80%MVC. Previous studies state that the magnitude of the mechanical changes with contraction is positively and linearly correlated with muscle force [20; 70]. The increment in the elasticity during muscle activation is possible because of the rising number of attached cross-bridges of actin and myosin filaments during the cross-bridges cycle [41; 72]. The present study found that there were significant differences (\(p<0.05\)) between the mean Young's modulus of males and females only at the resting condition of the muscle. However, there were no significant differences in the muscle elasticity during muscle contraction, at 20%, 40%, 60% or 80%MVC. SWE measurement relies on the principle \begin{table}
\begin{tabular}{l l l l l} \hline \hline  & **Young’s Modulus** & **Dynamic Muscle** & **R** & **\(p\)-Value** \\  & **(kPa):** & **Stiffness (N/m):** & **R** & **\(p\)-Value** \\ \hline
0\%MVC & 31.50 \(\pm\) 8.30 & 328.46 \(\pm\) 45.33 & 0.489 & 0.013 \({}^{\star}\) \\
20\%MVC & 51.76 \(\pm\) 16.02 & 405.60 \(\pm\) 64.61 & 0.366 & 0.072 \\
40\%MVC & 66.44 \(\pm\) 16.17 & 458.22 \(\pm\) 121.72 & 0.479 & 0.015 \({}^{\star}\) \\
60\%MVC & 82.90 \(\pm\) 19.05 & 508.82 \(\pm\) 89.96 & 0.330 & 0.107 \\
80\%MVC & 103.07 \(\pm\) 18.77 & 548.56 \(\pm\) 100.17 & 0.243 & 0.242 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Correlation coefficients (R) of Young’s modulus and dynamic muscle stiffness of in different intensity of muscle contraction (\(n=25\)). that soft tissues are purely elastic, incompressible and isotropic.