## 5 Kinematic Assessment of Movement Performance
### Inertial Measurement Unit--IMU
#### 5.1.3 IMU Installation into Sports Equipment  
In the sports engineering field, IMU devices, thanks to their very small dimensions and weight, are broadly applied inside sports equipment because they can detect athletes'motor behaviors with a limited impact on them [153]. In this category of performance analysis, the sports federations and the world of sports equipment manufacturers focused on the sensorization of both apparel and equipment that athletes utilize. For example, several studies used gyroscopes placed inside shoes that focus on the foot's position and the direction of applied forces to the foot or ankle. The main target of these studies is understanding the correct running style and enhancing the running formation and performance from both athletes' and shoe manufacturers' points of view. Among others, refs. [154, 155] presented some results showing that the athlete can benefit from the proper foot orientation with better support surface in both locomotion and running with consequent improvements in injury prevention. Other studies that address the movement analysis of sprint running to improve performances focus on assessing the hip flexion and foot contact phase pattern by measurements obtained with six DoF IMUs placed on the dorsal surface of the foot or right above the ankle [156, 157, 158]. Moreover, in those sports that require an external object or equipment such as, for example, a stick in hockey, baseball, or golf, a racket in tennis or ping-pong, and a paddle in some water sports, the coaches can receive real-time feedback on kinematic aspects of the athletes' performance by mounting inside such equipment IMU sensors and proper acquisition systems able to analyze the action of the player and their skills during the use of the equipment itself [159, 160, 161]. For example, in [162, 163], attaching an IMU device to a golf stick, a kayaking paddle, or an oar, the coach can attain some kinematic variables useful for the gesture assessment, such as angular velocity and acceleration for the golf stick by analyzing the swing while hitting the ball and stroke duration, symmetry, and frequency in kayaking or rowing. In the e-Kayak system (shown in Figure 6), the IMU is placed on the boat on the rear of the seat, and it is synchronized with the force sensors mounted inside the shaft and footrest, respectively. Such a DAQ system can analyze the source of the propulsion (the force on the paddle) together with forward acceleration and roll and pitch angles of the skull (the effects of the paddling action on the boat), assessing, in this way, both the dynamic and kinematic performance of the paddler in driving the boat and how he is making propulsion. At the same time, by exploiting the measures of the forces applied by the legs on the footrest and the force signal on the paddle, the system can assess the effectiveness of the paddling technique. In the new IMU technologies and abilities [164, 165], waterproof sensors employ electrical connections in the apparel yarn by textile fabric electrodes. An interesting example is the application of this technique in a swim bralette top used to monitor some health aspects and physical activities [166]. This technology will overse  
Figure 6: Bird view vision of the boat seat: (**a**) the waterproof primary node (E-Kayak system by APlab [17]) is installed (**b**) to acquire kinematic data from the IMU and GNSS. underwater usage caused by the imbalance due to having a massive external device that increases the drag value, albeit with the restriction to having real-time monitoring of the performances due to the impossibility of carrying out radio transmissions in water. IMU incorporating gyroscopes has been considered a practical alternative to 3D Optical MoCap systems for motion assessment in tennis. These two systems were compared in the evaluation of the angular velocity in the various tennis strokes.