## 2 Materials and Methods
### Testing Protocol  
In our research, there were eight measurements to be measured, including six measurements for the lower limb and joint strengths (the foot forces with the movements of hip flexion or extension, knee flexion or extension and ankle plantar flexion or dorsiflexion) and two measurements for the upper limb and joint strengths (the hand pushing forces with specific postures in the elbow and shoulder joints). The participants were instructed to push the FS device as much as possible for five trials in each measurement using their dominant leg and hand. Each participant practiced each measurement before data were formally collected. The FS was reset to zero before each trial. Participants were required to attend the Motion Analysis Laboratory, Tayside Orthopedics and Rehabilitation Technology Center, Ninewells Hospital and Medical School, University of Dundee for one single testing session. Each session lasted approximately two hours, and the participants supplied their own sportswear (shorts and t-shirt) and were barefoot. Upper-limb dominance was determined by asking volunteers to write their name and sign a consent form while lower-limb dominance was determined by asking the volunteer to kick a ball three times. Then, anthropometric measurements of height and weight were measured and recorded. In measurements of lower limb and hip joint strength, participants were asked to stand in a comfortable posture and to push the FS using their heels or toes. The FS was attached to a wall, and participants applied force forwards by their toes or backwards by the heels, according to participant requirement of height. When pushing the FS, participants were required to keep the lower limbs as straight as possible. Participants were asked to push the FS using their heel backwards from a standing position with hip and knee extended to measure the toe forces when the hip joint moved in flexion, and the participant applied the force as much as possible (Figure 3 left). Participants were then asked to push the FS using their heel backwards from a standing position to measure the heel forces when the hip joint moved in extension (Figure 3 right). It should be noted that these measuring ways are different from the traditional ways that use isokinetic dynamometers, and it is also recognized that the lower/upper limbs and other joints also take part in the movements and make some degree of contribution to the readings. \begin{table}
\begin{tabular}{c c c c} \hline
**No.** & **FS (N)** & **FP (N)** & **FS-FP (N)** \\ \hline
1 & 477.4 & 476.9 & 0.50 \\
2 & 643.2 & 642.8 & 0.40 \\
3 & 664.7 & 661.2 & 3.50 \\
4 & 670.8 & 671 & \(-\)0.20 \\
5 & 695.7 & 695.9 & \(-\)0.20 \\
6 & 813.6 & 814 & \(-\)0.40 \\
7 & 876.8 & 876.4 & 0.40 \\
8 & 932.4 & 931.6 & 0.80 \\
9 & 1016.5 & 1014 & 2.50 \\ \hline mean & 754.57 & 753.76 & 0.81 \\ SD & & & 1.32 \\ \hline \end{tabular}
\end{table} 
Table 4: Repeatability analysis of the force sensor. For measuring of knee muscle strengths, participants were asked to push the FS device forwards using their forefoot from a seated position with their foot on the wooden step, maintaining 90\({}^{\circ}\) in the knee and hip joints and the ankle joint on the natural position (Figure 4). In addition, participants were asked to push the FS, positioned under the chair, using their heel backwards in the knee joint at 90\({}^{\circ}\) and in the ankle joint at natural position (Figure 4). Participants were then instructed to push the FS from a seated position with 90\({}^{\circ}\) at the knee and hip joints and neutral position of the ankle joint downwards from motor electric adjustable height multifunction. As participants had different leg lengths, they were required to maintain the knee at 90\({}^{\circ}\) and to push FS in plantar flexion and dorsiflexion (Figure 5).