## 2 Materials and Methods
### Procedures  
The location of the flexor digitorum superficialis (FDS) muscle was based on the anatomical landmarks and palpation. Participants were seated upright comfortably while their arms rested on the table. The elbow was bent approximately 90\({}^{\circ}\) of flexion, the forearm was supinated, the hand was palm up, and the wrist and fingers were relaxed. The area between the tendon of the palmaris longus and the flexor carpi ulnaris was palpated while the forearm was relaxed. Subsequently, the participant was asked to flex the middle and ring fingers with maximum force, and the researcher determined where the maximum displacement occurred on the muscle belly; normally, this will be at the midpoint of the distance between the medial epicondyle of the humerus and styloid process of the ulna. Finally, this area was marked by a non-permanent marker as a rectangular shape, approximately a dimension of 1 cm \(\times\) 4.5 cm (width \(\times\) length), to use as the marked point for transducer placement during ultrasound scanning (Figure 1A).  
For the next step, individual maximum grip force was evaluated using a digital grip dynamometer (CAMRY-EH101, Zhongshan Camry Electronic Co, Ltd., Zhongshan, Guangdong, China) to achieve the maximum voluntary contraction of the FDS muscle (100%MVC), which was used for further determining the different intensities of maximum voluntary contraction (%MVC). Once the 100%MVC was obtained, %MVC was estimated and defined as 0%, 20%, 40%, 60% and 80%MVC. Subsequently, the participant was given time to practice the required task of data acquisition. They were asked to grasp the grip dynamometer until the force reached the target value of all different %MVC and they could maintain the target force level for a while.  
Once the skill to perform different levels of grasping was achieved, SWE measurement began. The participant was in the aforementioned position and relaxed their forearm and hand. The transducer coated with ultrasound gel was positioned on a custom holder to maintain a fixed position and then placed over the marked point with firm and minimal contact pressure. The alignment of the probe was adjusted to be perpendicular to the skin surface and parallel to the direction of the muscle fibres (Figure 1B). For data acquisition, the structural image of the dominant FDS muscle in five different %MVC was acquired. First, scanning began to obtain the structure image of the muscle in a resting condition (0%MVC). The participant was then instructed to grasp the grip dynamometer with different force levels of 20%, 40%, 60% and 80%MVC randomly, to avoid any potential source of bias. Each level of grasping was maintained for 10 s. The procedure was repeated in two trials for each subject with two minutes of rest period between force levels holds and trials. The average value from these two trials was used for further data analysis.  
Figure 1: Experimental set up; (**A**) FDS muscle landmark determination, (**B**) Setup for SWE measurement and (**C**) Set up for MyotonPRO measurement.  
Once the SWE measurement was completed, the participant received a 10 min rest to achieve the baseline relaxation state of the muscle before continuing with the MyotonPRO measurement. The participant was asked to be in the same position; the centre of the ultrasound scanning landmark was marked as cross-shaped for use as the location of the measurement. The tip of the device's probe was placed perpendicular to the marked site (Figure 1C). Subsequently, the participant was instructed to grasp the grip dynamometer with different force levels of 20%, 40%, 60% and 80%MVC randomly in order to avoid any potential source of bias. Each level of grasping was maintained for 10 s. The procedure was repeated in two trials for each subject with two minutes of rest period between force levels holds and trials. The average value from these two trials was used for further data analysis.