Stroke is the third most common cause of disability worldwide and leads to upper limb motor disease in more than half of people affected. Recent data demonstrate that upper limb rehabilitation can be pursued using techniques such as the observation of action (Action Observation Therapy - AOT) or the stimulation of limb musculature using surface electrodes (Neuromuscular Electrical Stimulation - NMES). To date, no rehabilitation studies used both the treatments (AOT-NMES) for the rehabilitation of upper limb after stroke. The goal of this clinical trial is to study the efficacy of this combined approach (AOT-NMES) in people who developed upper limb motor impairment after stroke. The main question this study aims to answer is if the rehabilitation performed using both action observation and neuromuscular stimulation has an higher efficacy than the use of AOT alone and higher than the observation of non-motor stimuli. Participants will be people with upper limb impairment after stroke and will perform 15 rehabilitation sessions (5/week, 3 weeks, 60 minutes each). Each participant will be casually included in one of following three rehabilitation groups: * Action observation associated with neuromuscular stimulation (AOT-NMES, experimental condition): they will observe upper limb movements while their arm muscles will be stimulated. After the observation phase they will try to perform the same movements with the impaired arm. * Action observation alone (AOT): subjects will observe upper limb movements and after the observation phase then they will try to execute them with the impaired arm. * Motor-neutral observation (MNO): subjects will observe non-movement videos and after the observation phase they will try to execute upper limb movements with the impaired arm. Each participant will be evaluated for motor function before and after rehabilitation treatment and researchers will compare the motion improvement between the groups to assess the efficacy of AOT-NMES over other treatments.