## 4 Discussion
### Comparison Fmi vs. FM2  
Two reduced marker concepts for the fingers, which both allow the analysis of one DOF in the PIP and DIP joints, and two DOFs in the MCP joints, were compared comprising the same mathematical procedure for the calculation of the joint angles. Flexion-extension movements are of greatest clinical importance and represent the major motion plane to be assessed [12]. From this point of view, a reduced marker set is suitable to measure the most relevant joint angles, even though rotations around other axes might occur. Overall, the measurements of the ROM with two markers per segment (FM2) have proven to be more repeatable than with one marker on the joint (FM1). Still, in four joints,  
Figure 5: SEM (left scale) and the MDD (right scale) for the maximal angles and ROM. AA = addiction (Max1)-abduction (Max2); PS = pronation (Max1)-supination (Max2); all other: flexion (Max1)-extension (Max2). Data sets included \(n\) = 20; exceptions for MCP1: \(n\) = 19, MCP4: \(n\) = 19, MCP5: \(n\) = 18, DIP5: \(n\) = 19, TMC: \(n\) = 19, and radioulnar PS: \(n\) = 16.  
namely PIP2, PIP3, DIP2, DIP4, the SEM value of FM1 was smaller than the SEM of FM2. These four joints correspond to the joints with the lowest SEM values when comparing within FM1. In addition to the lower average SEM, however, the SEM values of FM2 are generally more similar among all joints (SD 0.7\({}^{\circ}\)), whereas the SEM values of FM1 show a larger dispersion (SD 1.9\({}^{\circ}\)). Skin movement artefacts affect measurement quality, especially when markers are located on the joint heads [10], a region known to be highly susceptible to skin displacement. The displacement of the skin also depends on the joint angle [27], leading to an ambiguous identification of the marker position on the joint in FM1. Therefore, the placement of markers on the segments (FM2) seemed to be clearer and more straight forward. Interestingly, it was observed in a previous study that the accuracy of finger flexion angles with FM1 could benefit from skin movements during flexion [28], whereas the effects of systematic soft tissue displacement [27; 28] for joint angles of FM2 are still unknown. As shown in this study, the choice of marker positioning, considering skin movement in particular, has a big impact on the repeatability of the resulting kinematics. A limitation of this study is seen in that the comparison between the two finger marker sets was carried out on two different groups and group sizes of healthy volunteers. The resulting variation needs to be considered when interpreting these results and further investigation on the difference between these two marker sets is still needed.  
Complementary to a previous study, showing that joint angles derived from FM2 are more similar to FM3 compared to measurements with FM1 [10], we found that FM2 outperforms FM1 in ROM repeatability. Hence, our results strongly support the recommendation to prefer two markers per segment (FM2) over FM1, if a reduced marker concept has to be applied. It represents a good compromise between the visibility of the markers and accuracy and it has already been successfully applied in measurements of more complex functional tasks [29].