## 4 Discussion
### Repeatability of the Maximal Joint Angles and the ROM  
A paired t test revealed no statistically significant difference of the mean values of the maximal joint angles and the ROM between test and retest. It supports the assumption that in a healthy population no change in maximal joint mobility occurs within the two testing days. Therefore, the precision of the presented protocol can be estimated from the obtained individual test-retest differences. It comprises the positional error of the measurement system as well as errors associated with the protocol and the measured sample. To quantify the effect size of each individual factor of the error, however, further validation would be needed. Furthermore, the alignment of the JCS seems to account for another big influence on the kinematic output and different approaches lead to different joint angles [4]. A detailed analysis of the JCS alignment applied in this paper is planned in an upcoming study. In general, good data quality can be obtained using a hand-specific camera setup. Nevertheless, as a result of the complexity of hand measurements, some data had to be excluded, which explains the different sample sizes in Figure 5.  
The presented repeatability measures (Figure 5) are specific to the applied protocol and cannot be generally transferred to other approaches but rather serve as a reference for comparisons of different protocols. The value range that can be expected if a single testing is repeated by the same rater is provided by the SEM (intra-rater), whereas the MDD reflects a measure of the difference between repeated testing by taking the measurement error of both sessions into account. Hereby, only changes bigger than the variability induced by the method can be regarded as real changes [25]. In this context, only subjects whose difference on repeated testing is at least equal to or greater than the MDD reflects a real difference at a 95% confidence interval [25]. Both SEM and MDD are independent of the variation within the measured sample [23; 24] and are therefore directly applicable to patients with various pathologies and restricted mobility. The measurement of the thumb and its repeatability have been shown to be remarkably challenging, which might be explained by the complexity of thumb kinematics based on its unique kinematic and anatomical characteristics compared to the other four digits [30]. Furthermore, the measurement of the joints of the fingers closer to the thumb tended to be more repeatable, which might possibly be explained by the bigger segment size and finger coordination due to involvement in various grip types. In a recent systematic review, it was revealed that only reduced marker sets have been used to measure the thumb during daily activities of the hand so far [5], which may, however, result in a poor representation of the complex 3D movements of the thumb. Our kinematic model allows the analysis of three DOFs for each thumb joint. With regard to functional assessments, this is a clear advantage of the presented method. Due to differences in methodology, a comparison to previous studies analyzing the repeatability of hand motion analysis protocols is limited. Our study was the only one using maximum joint angles during dynamic motion tasks to assess the test-retest difference, whereas all other studies focused on static postures defined by casts [31], metallic frames [8] or gripping of specific objects [11; 13]. On the level of individual joints, the error was slightly smaller for the wrist, MCP2 and MCP3 joints than previously reported [8; 31]. Averaged over all hand joints, the repeatability (MDD 8.8\({}^{\circ}\)/RMSE 4.7\({}^{\circ}\)) lies in a similar range as previously published (MDD 5.5-10.5\({}^{\circ}\)/RMSE 3.4) [11; 13], but with the above-mentioned difficulties for the measurement of the thumb and the small finger.