## 1 Introduction  
Optimization of any sport skill requires re-organization of limbs coordination responsible for governing the movement performance [1]. This type of motor learning can be enhanced through a number of methods that are utilized in research and application settings alike. In general, verbal feedback of coaches in real-time is commonly used as a preliminary means of instilling motor learning [1; 2]. Due to the rapidity and complexity of some sport skills as well as invisibility of some parameters (e.g., force), the real-time feedback of coaches is often a subjective guess based on experience. For increasing the reliability of feedback in training, biomechanical means are used to supplement the verbal instructions [3; 4; 5; 6]. The hammer throw is such a sport skill that needs a combination of a coach's experience and biomechanical feedback in elite sport training to facilitate motor learning and optimize outcomes.  
Men's hammer throw has been part of Olympics track and field competitions since 1900, but unlike other events, the hammer throw has not seen a new world record since 1986 [7]. This standstill may be caused by the lack of scientifically based training. While extensive 3D motion analysis technologies do supply highly trustworthy information for human motor skill quantification [8; 9; 10; 11], due to their drawbacks, the analysis and feedback has traditionally occurred offline after completion of a given testing session, i.e., it is post-measurement feedback, rather than real-time [11; 12; 13; 14]. The drawbacks of a 3D motion capture system include complicated operation, high cost, long calibration and setup procedures, time-consuming course on data collection, processing, analysis as well as the movement constrains induced by dozens of capture markers attached on a subject's body [15]. These drawbacks have hindered the use of 3D motion capture systems in sport training practice. As a consequence, research has been initiated to develop real-time biomechanical feedback devices for hammer throw training, beginning with wire-tension measurement [12].  
Most recently, a pilot study [16] using 3D motion capture technology (Figure 1) found that the timely displacements of hip and wrist may be used to reveal the upper and lower limbs' coordination when analyzing hammer throw. The pilot study has shown that the timely change of vertical displacements of hip and wrist are closely related to the turning speed, the ratio of one-leg/two-leg support (power generation), and hammer velocity change during the skill performance. Therefore, obtaining the dynamic distance data of these two anatomical landmarks would be vital for real-time feedback training.  
The results of the two studies would suggest that a combination of the wire-tension measurement and the dynamic vertical-displacements of hip and wrist could have great potential for substituting 3D motion capture technology in the skill analysis of the hammer throw. Since real-time wire-tension is developed (i.e., already wearable) [12], developing wearables for tracking hip and wrist movements would realize the real-time biomechanical feedback learning/training in the hammer throw. Encouraged by the results of the studies, we are aiming at developing a practical wearable device for pursuing real-time training. This paper will highlight our approach of using the inertial measurement unit (IMUs) as a practical approach for the development of a wearable system for biomechanical feedback training in the hammer throw.