## 1 Introduction  
Muscle stiffness is one of the mechanical properties of skeletal muscle that is recognized as a marker of muscle performance and joint stability during the operation of functional movements [1, 2]. The flexor digitorum superficialis (FDS) muscle is an important muscle involved in finger movement and grip strength [3, 4]. Alterations in the stiffness of the FDS muscle have been noticed in neuromuscular pathology [5, 6, 7]. Moreover, this parameter can be used for examining the process of muscle recovery from hand injuries [8, 9, 10, 11] or adaptation owning to exercise [12, 13, 14]. Therefore, exploring FDS muscle stiffness would provide key insight into clinical assessment [15, 12] and the design of assistive technologies and strategies for hand rehabilitation [16, 17]. Ultrasound shear wave elastography (SWE) has been developed for the direct measurement of the mechanical properties of the muscle. This approach is able to offer a non-invasive and reliable tool for characterizing muscle stiffness [18, 19, 20]. Shear waves are generated into the tissue by acoustic radiation force, and then the velocity of wave propagation is estimated and used for elasticity calculation [19, 21, 22, 23]. Most previous studies have emphasized measuring the stiffness of the musculotendinous structures of the lower extremities in the context of sports medicine [24; 25; 26; 27] and certain diseases [28; 29; 30; 31]. On the other hand, a few research studies applying the method to the upper limb muscles have been reported [32; 33; 34; 35]. However, assessing the stiffness of the muscle contributing to hand function and dexterity is currently less well-established [36; 37; 38]. Moreover, flexor digitorum superficialis (FDS) muscle stiffness has never been investigated within quantitative SWE [7]. In the meantime, most studies have underlined reporting the normative data of resting muscle stiffness, and the quantification of muscle stiffness during muscle contraction has been constrained [18; 19; 39; 40; 41; 42]. However, there are some limitations of SWE for clinical use, including its high-cost and technical skill requirement [23; 43; 44]. Recently, MyotonPRO (Myoton AS, Estonia) has been proposed as a portable, non-invasive, painless, and reliable tool for measuring the mechanical properties of the muscle [45; 46]. It is a handheld digital palpation device equipped with a muscle stiffness measurement function. Once the measurement has been performed, the damped natural oscillation of the biological tissue is recorded as a form of the acceleration signal. Subsequently, the simultaneous computation of the related parameters is evaluated [47; 48]. Previous studies have revealed excellent intra- and inter-tester reliability of the device for measuring the mechanical properties of the muscle. Therefore, MyotonPRO is a feasible alternative tool for quantifying muscle stiffness in different conditions [49; 50]. However, only a few studies have reported using MyotonPRO to evaluate the upper limb muscles [47; 51; 52]. Currently, studies assessing how the stiffness of the muscle contributes to hand control using this device have not been reported. However, there is a deficiency of studies that have revealed the capability and effectiveness of MyotonPRO for measuring hand muscle stiffness compared to SWE. Therefore, it remains to be seen whether MyotonPRO and SWE can be used to evaluate FDS muscle stiffness and what the relationship between the muscle stiffness obtained from the different approaches is. Therefore, the primary purpose of this study was to assess the capability and effectiveness of MyotonPRO and SWE for FDS muscle stiffness characterization. The second purpose was to investigate the correlation between the muscle stiffness achieved from the two different approaches in order to confirm that MyotonPRO could be used as a complementary tool for muscle stiffness measurement instead of SWE.