## 4 Discussion
### First Part of Approach for Assessment of Quality of Motion  
A piecewise linear multi-regression was used to assess the contributions of the five parts composing the kinematic data \(y\) (WJ\({}_{x}\), WJ\({}_{y}\), WJ\({}_{z}\), \(\Delta\)WJ, COM\({}_{x}\), COM\({}_{y}\), COM\({}_{z}\), \(\Delta\)COM): (1) the \(a_{0}\) part assesses the contribution of non-monitored active muscles, passive structures/tissues, and motion of the trunk and finger, and the supination-pronation of the wrist joint; (2) four muscle parts assess the contributions of four tested muscles (\(a_{4}\): EMG\({}_{4}\), \(a_{2}\): EMG\({}_{2}\), \(a_{3}\): EMG\({}_{3}\), \(a_{1}\):EMG\({}_{1}\)). Their contributions were assessed in two different external conditions (NC and PMC) by using two different external weights to perform the motion (light (**motion1**) and heavy (**motion2**)). We found highly statistically significant results (R\({}^{2}\geq 0.75\)) between the tested muscle activities (treated as independent variables) and the kinematic data (treated as dependent variables) in tested the RS and DMD patient and depict these results in Figure 5a,b and Supplement S3 and S4. Considering the shapes of the distributions of the synergistic/antagonistic contributions at the time of the motion (statistically significant results of the multi-regression analysis), we identified three types of behavior (p.3.1) that can be interpreted in the following ways: (1) the first type means that the kinematic data \(y\) is under the direct control of the visual and/or proprioception senses and the main participation has non-monitored active muscles and passive structures/tissues, motions of the trunk and finger, or pronation/supination of the wrist joint; (2) the second type means that tested kinematic data \(y\) is not under the control of visual nor proprioception senses; (3) the third type means that within some fragments of the motion, the considered kinematic data \(y\) (related to the activity of testing muscles (\(y_{1}\), \(y_{2}\), \(y_{3}\), \(y_{4}\))) is under the direct control of visual and/or proprioception senses.  
We show that the revealed statistically significant correlations between muscle activity and: (1) the displacement of the wrist joint (\(\Delta\)W)\({}_{x}\), \(\Delta\)W)\({}_{y}\), \(\Delta\)W)\({}_{z}\), \(\Delta\)WJ) is a manifestation of the kinematics control of the tested arm motion which may be mainly controlled by the visual sense and the proprioception sense (position sense); (2) the displacement of upper-limb COM is an indication of the dynamics control of the tested arm performance which could be mainly controlled by the proprioception sense. One can argue that WJ displacement mainly occurs due to the control of visual and proprioception senses. However, the COM displacement can be mainly controlled by the proprioception sense because the visual sense cannot identify the position of the COM in space (COM is a fictitious point assessed on the base of biomechanics).  
Considering the results of the distributed participations in time (p.3.1), we revealed the following similarities between the RS and DMD patient. Both tested subjects used the third type of behavior to realize: (1) the dynamic control focused on the \(x\)-th coordinate (COM\({}_{x}\)) in **motion2** under N; (2) the kinematic control focused on the \(z\)-th coordinate (WJ\({}_{z}\)) in **motion1** under NC, **motion2** under NC, and in **motion2** under PMC.