## 1 Introduction  
Shoulder impairment is a common complication after stroke, persisting in almost 60% of patients [1] at 12 weeks after stroke [2]. It is characterized by reduced glenohumeral motion, spasticity, subluxation, and somatosensory impairments in the paretic arm, contralateral to the brain lesion [3]. The non-paretic arm, ipsilateral to the cerebral lesion, has also been demonstrated to show strength and motor control impairments [4]. Abnormal active shoulder complex movement causes loss of function, limitation in activities of daily living [5], and a major issue for rehabilitation. Poor scapulothoracic positioning and altered scapulohumeral motion are considered risk factors in the development of shoulder dysfunction and pain after stroke [3].  
Shoulder kinematics is the result of a complex motion that involves the movement of the glenohumeral joint and the movement of the scapula with respect to the thorax [3, 5]. The coupling of these two movements during arm elevation has a precise ratio, described as the scapulohumeral rhythm (SHR).  
Three-dimensional shoulder complex motion has been studied in patients with chronic stroke using different clinical and instrumental assessment measures. While clinical scoresare affected by subjectivity and inaccuracy in approaching diagnosis and are very time-consuming [6; 7], instrumental measures provide objective measures. Specifically, wearable sensors, easy to use in a clinical setting, are gaining a certain popularity [6]. Other studies have addressed the instrumental assessment of shoulders using non-wearable devices such as optoelectronic systems, ultrasound-based systems, and electromagnetic tracking systems. These systems typically provide reliable measures, but they cannot be used for routine clinical assessment [3; 6]. Most of the studies that have explored shoulder kinematics in post-stroke patients are based on instrumental 3D motion analysis [8; 9; 10; 11; 12; 13; 14; 15].  
The available studies based on the use of wearable devices in stroke patients [16] are mainly devoted to exercise monitoring (also remote) during rehabilitation [16; 17; 18; 19] or the estimation of clinical scores for quality of movement (e.g., the Wolf Motor Function Test [17; 20], Fugl-Meyer Assessment test [7; 21; 22], and Constant-Murley score [23]).  
Several variables have also been explored, including shoulder motion coordination, smoothness, the presence of compensatory movements, speed, acceleration, and the amplitude of ROM [6; 24]. However, many problems exist in these studies due to the variability in the instrument type, location of sensors, and motor task explored [16; 24; 25; 26]. Only one study provides reference values for stroke shoulder flexion and abduction using a wearable device to monitor the effects of rehabilitative training [22]. Since effective rehabilitative treatment requires a good understanding of the mechanisms underlying shoulder impairment, a reliable quantitative assessment is relevant for clinical decision making and outcome measurement.  
To evaluate the feasibility of measuring 3D shoulder complex kinematics (gleno-humeral and scapulothoracic motion) in a clinical setting and during active flexion and abduction of the paretic arm in individuals with stroke, a validated inertial system was used. A post-stroke group of patients was compared with healthy controls for paretic and non-paretic upper limb motion. The kinematics obtained were then correlated with clinical-functional measurements.