# Toward Personalized Vibrotactile Support When Learning Motor Skills
## 3 Vibrotactile Feedback in Motor Skills Training Systems  
Vibrotactile feedback can be provided at key moments through diverse tactile actuators (i.e., tactors) that can apply resistance, force, vibration, etc. By adjusting the density and frequency of the tactors, realistic surface contours can be created such as roughness, bumpiness, stickiness and so on [14]. The goal can be either consolidating or correcting a movement. For instance, if the user has performed the movement accurately, she can receive a soft vibration as a reward; in turn, if she performs the movement in an incorrect way, the system can produce some force so that the user perceives resistance to that movement and thus, has to try in a different direction or way. Existingreviews show that vibrotactile devices are an appropriate means of providing immediate feedback for motor learning and augmentation [12]. By providing vibrotactile non-invasive stimulations to the surface of the skin, vibrotactile feedback does not hinder other tasks (e.g., speaking, seeing and hearing) and receives a high level of user's acceptance [15] without the need to shift the visual attention [13]. In a previous work [9], seventeen systems that supported motor skills learning have been reviewed. Those six [16; 17; 18; 19; 20; 21] that provide vibrotactile feedback are compiled in Table 1, analyzing the information that is _monitored_ (sensors used, processing approach, measures computed), how it is _compared_ to detect errors (user, movement and context data used to diagnose errors) and what _correction_ support (actuators used, feedback trigger and delivery) is provided. In addition, it provides basic details of the system such as the body part involved and the kind of activity carried out, and discusses the personalization issues considered in the system. The detail of the descriptions synthetized in Table 1 depends on the information available in the papers analyzed. The systems analyzed in Table 1 have been developed as a general solution (e.g., [16; 20]) or to train motor skills in specific contexts, such as karate [17], tai-chi [18], snowboarding [19], and violin playing [21]. Most of the systems focus on a specific part of the body (arm and/or hands) as in [16; 17; 18; 20]. The torso is also considered in [21]; [19] focuses on different parts across the body. Regarding the monitoring, in most of the systems, movement was sensed with optical tracking techniques that detect markers placed on users' body, using motion capture systems either with Vicon cameras [16; 18] or infrared devices [17; 20]. These techniques limit the data collection to the room where the tracking infrastructure is prepared. Nonetheless, there are two systems (by coincidence the same that consider several body parts) that collect movement data with wearable inertial sensing units worn directly on the body (ad hoc combination of sensors in [19] and a commercial inertial motion capture suit in [21]), without the need of any infrastructure in the room. In turn, the processing approach is carried out in most systems by the software provided by the motion capture system when used [16; 17; 20; 21]. However, there are two systems that have implemented their own algorithms (a gesture recognition system with probabilistic neural networks and finite state machines in [18] and an empirical algorithm in [19]). Measures specified are either angles and positions [16; 17; 20] or forces [19]. The systems compare the current movement of the user with a reference, which usually is the movement carried out by another person with expertise in the performance of the movement [16; 17; 18; 19; 20]. Only [21] computes the ideal user movement and compares the performance against that reference. In any case, for the error diagnosis, systems carry out some movement modelling, with ad hoc algorithms that depend on the domain and kind of movement. User features are never modeled, although in the ideal individual path, [18] considers user's build, the way in which she is holding the violin and even the teacher's pedagogy.