## 4 Discussion
Incorporating tactile feedback or other sensory cues could improve the motor control and interaction experience, making the rehabilitation process more effective and realistic. Differences in kinematic synergies are observed between real and virtual BBT in both groups (healthy participants and patients) across all synergies. Notably, while the virtual test leads to greater thumb abduction in all participants, it induces increased thumb and PIP joint flexion in patients, contrary to the behaviour seen in healthy participants with fingers more relaxed and extended. This increased motor control difficulty in the virtual BBT, as indicated by trajectory parameters, likely results in greater difficulty grasping the cubes, requiring different thumb and finger flexion synergies, likely due to the absence of the tactile feedback that is present in the real BBT. When analysing the groups separately based on whether the test is real or virtual, the real BBT shows minimal range differences due to health conditions. However, in the virtual BBT, patients adopt a more flexed PIP position. This increased flexion may reflect greater motor control challenges for patients due to the absence of tactile feedback during the virtual test or may be attributed to the common hand-closing tendency observed in SCI patients, commonly associated with spasticity or impaired neuromuscular control [27, 28]. As an innovative contribution, this study analyses the temporal evolution of kinematic synergies during a movement cycle in the BBT, identifying phases where consistent patterns emerge. Figure 4 highlights patterns for healthy participants across all kinematic synergies, except for _Thumb_F(t)_, with notable differences depending on whether the task involves RBBT or VBBT. The inexistence of a clear pattern in thumb flexion might be due to higher variability in thumb synergies in healthy patients during manipulability [24]. The figure also includes a frame-by-frame analysis. These global differences in kinematic synergies due to the test condition are reflected at each frame, except for _MCPs_F(t)_ during cube release. The temporal evolution of kinematic synergies for patients is also presented in the figure, but no clear temporal patterns emerge in their case. It is evident that the temporal evolutions of patients differ markedly from healthy individuals, particularly in VBBT for _Thumb_F(t)_ and _PIPs_F(t)_, especially during the initial transport phase and while grasping the cube. This difference may stem from the fact that, in patients with SCI, spasticity or a tendency toward a closed-hand posture is common due to muscle weakness in the extensors [29]. This forces patients to compensate by using greater thumb flexion and PIP joint movement to achieve and maintain a grip on the cube. This behaviour may be exacerbated by the lack of tactile feedback, leading to less precise movements, which affects patients more significantly. Figure 5 also highlights temporal patterns for flexion-extension kinematic synergies for both RBBT and VBBT, which are different depending on health condition. Cycle-averaged sEMG parameters derived from signal waveforms (_NZC_ and _EWL_) revealed that healthy participants exhibited lower values during VBBT compared to RBBT across some parameters, likely due to the reduced sensory-motor demands and altered interaction dynamics in the virtual environment. In contrast, patients demonstrated higher values in many parameters, particularly in _EWL_ across all sensors and in all _NZC_ except for wrist flexion and radial deviation, digit flexion, and thumb movement. This could stem from impaired coordination and reliance on excessive muscle activity to stabilize movements when grasping a virtual cube in the absence of sensory feedback. Patients show smaller EWL values in RBBT but higher EWL values in VBBT across most sensors, while NZC values in patients are lower for some sensors in RBBT but higher for others in VBBT.