# Modification of Hand Muscular Synergies in Stroke Patients After Robot-Aided Rehabilitation
## 1 Introduction
After a stroke, the human brain puts in place a reorganization of healthy tissue, favoring a progressive recovery of functions of the injured part of the CNS. This neural plasticity is more evident in the acute and sub-acute phase and tends to decrease in the chronic phase, as described in [20]. Over the years, the increased incidence of this pathology has pushed researchers to investigate rehabilitation solutions able to provide an adequate level of recovery and the restoration of motor function. The introduction of robotic platforms for rehabilitation should favor the progressive recovery of muscle patterns. However, it is necessary to ensure that the use of the robot does not introduce disturbing factors in the muscle patterns [21]. Few studies have been devoted to exploring the impact of stroke in motor control, to analyze how, and how much, robot-aided rehabilitation allows improving hand control and manipulation [15; 18; 22], also in combination with conventional treatment or other, such as transcranial direct current stimulation [23]. Different outcomes of robot-aided rehabilitation treatment were observed, on the basis of the time elapsed since neurological damage. For acute patients, researchers observed a reduction in the number of synergies compared to the number of synergies extracted from the healthy limb [24]. Moreover, patients in the acute phase show a reduction in the number of synergies on the healthy side before a robot-aided rehabilitation treatment. This result should be interpreted, taking into account that this category of patients demonstrates a tendency for splitting and merging of muscle synergies [15; 25]. In chronic stroke patients, the structure of muscle synergies involved in movement appears to remain unaltered, but the modulation of such synergies is often compromised [15; 26]. Furthermore, the patient with impaired motor functions tends to show an adaptation of muscle synergies to typical characteristics of movement (such as type of movement, speed, compensation of asymmetry due to neurological damage) [15; 25; 26]. In such a context, the human hand represents one of the most complex anatomical districts of the human body and, thanks to more than 20 degrees of freedom, allows performing gripping and manipulation tasks. Some studies have shown that muscle synergies represent a tool for a complete description of grip control and that they can be adopted as a predictive tool to generate new hand postures [2]. An accurate analysis of muscular and postural synergies was carried out in [27]. This study showed that healthy subjects who performed gripping tasks in different configurations show a statistical overlapping among muscle synergies. This result is not trivial, as it allows us to demonstrate not only the existence of muscle synergies, but also the role they play in terms of control of the different grips [28]. Moreover, recent studies confirm that stroke subjects with mild and moderate impairment show altered upper limb muscle patterns when compared with healthy subjects during the performance of hand-reaching tasks in different directions of the space [29]. To the best of our knowledge, there are no studies analyzing the motor patterns of the hand of subjects affected by stroke after a robot-aided rehabilitation treatment, although it represents a tool of fundamental importance for performing activities of daily living (ADLs), and stroke can significantly alter the mobilization of this part of the body. Muscle synergies are useful to recognize alterations during the execution of motor tasks, since they allow highlighting the contribution of the single components that constitute complex movements in chronic stroke patients [3]. Muscular synergies of the upper limb have been adopted to evaluate possible benefits of robot-aided rehabilitation in post-stroke patients. Furthermore, it has been demonstrated a similarity between muscle synergies of affected and unaffected limbs. Similar studies have not been performed on the hand, yet.