## 2 Materials and Methods
### Location of Finger Bases  
The finger orientation design is incomplete without locating the base joints on the palmar surface. The location of the MCP joints plays a critical role in hand dimensions and the overall dexterity of the designed prosthetic hand. The locations of the finger bases on the palm surface shown in Figure 9 are obtained, as explained below. It is also noted that the location of the thumb CMC joint is discussed in this subsection. This is because the location of the MCP joints is directly dependent on the location of the thumb, and it would be impossible to design a functional prosthetic hand otherwise. The palm is considered to be a square of size 80 mm for calculation, considering anatomical references [25]. The origin \(\mathrm{G}(0,0)\) is the wrist frame of reference. The vertical lines of action for the index, middle, ring, and little fingers are considered to be equidistant with each other. Based on this consideration, the base of the middle finger \(\mathrm{O}_{3}\) is located on the edge of the palmar square. Anatomical and physiological studies of human grasping indicate that a line joining the MCP joint of the middle and little fingers is inclined at an angle of \(30^{\circ}\) with respect to the horizontal when all the fingers are fully flexed [13]. Accordingly, the little finger base \(\mathrm{O}_{5}\) is located at the intersection of the vertical and the \(30^{\circ}\) line originating from \(\mathrm{O}_{3}\). Grasping studies of the human hand indicate that the middle, ring, and little fingers converge towards the base of the thumb and index finger [13]. This convergence provides more significant interaction between the thumb and the other fingers, resulting in higher dexterity. Further, the presence of the thenar and hypothenar muscles in the human hand facilitates the interaction between the thumb and the little finger, enabling precise and coordinated movements. However, in prosthetic hands, the thumb is located more toward the base of the middle finger. This design modification is intended to have a 2-fold effect--(i) successful interaction with the little finger and (ii) the ability to perform a lateral pinch in coordination with the index finger. To simplify the location of the thumb base, the CMC joint of the thumb in the current design is assumed to be along the line of action of the middle finger. Figure 9: Location of finger bases on the palmar surface. The interaction between the thumb and little finger is considered to finalize the location of the thumb CMC joint. The kinematics during the Kapandji test for the thumb dexterity measurement show that the median line of action for the thumb and little finger are collinear during opposition motion [31]. Accordingly, in the current design, the line of action of the little finger at its maximum anatomical abducted position is considered to be the intersection with the thumb CMC joint. Hence, the thumb base O\({}_{1}\) is located on the palm surface with the line joining O\({}_{5}\) and O\({}_{1}\), inclined at 45\({}^{\circ}\) with respect to the vertical, which is its maximum abducted position [1]. The above considerations provide maximum interaction between the thumb and little finger to enhance grasp stability. The ring MCP joint O\({}_{4}\) is located at the intersection of its vertical line of action and the 30\({}^{\circ}\) line between O\({}_{3}\) and O\({}_{5}\). The location of the base of the index finger is critical since the index, middle, and thumb form the tripod grasp, which serves as the primary and most commonly used grasp. The index finger Ab/Ad joint angle was calculated and analyzed to locate its MCP joint. It was observed that the average abduction angle calculated from the experimental data for the index finger was 19.6\({}^{\circ}\). Further, the mean of the anatomical ROM for the index finger Ab/Ad joint is 20\({}^{\circ}\), which closely matches the experimental average.