## 1 Introduction  
Researchers and practitioners in kinesiology frequently utilize physical tests to assess human strength, power, and fatigue--key indicators of physical health and functional mobility in daily activities. Among these, the countermovement jump (CMJ) is a widely adopted performance test across multiple disciplines [1, 2]. It serves as a reliable measure of lower-extremity stretch-shortening cycle (SSC) function and mechanical power [3, 4], a method for monitoring neuromuscular fatigue [5], and a benchmark for tracking post-injury rehabilitation progress [6, 7]. Given the widespread application of CMJ testing, advancements in measurement technology are critical to improving its practicality, accuracy, and validity for researchers, clinicians, and sports practitioners. Numerous devices are available for measuring CMJ performance, with force platform dynamometry and optical motion capture regarded as criterion tools in laboratory settings [8, 9, 10]. These systems provide highly accurate kinematic and kinetic data, enabling detailed analyses of joint rotations and ground reaction forces (GRFs). However, their utility is constrained by high costs, the need for specialized training, and limited practicality outside laboratory environments. Both force platforms and motion capture systems are bulky, require reliable power sources, and often depend on desktop-based interfaces and complex data acquisition setups. Additionally, their typically small collection volumes restrict the range of movement tasks they can evaluate. These limitations hinder their application in real-world settings, particularly for capturing valid data on CMJ mechanics such as vertical displacement of the whole-body center of mass (WBCoM). Several practical technologies for measuring CMJ performance are widely used in the field, offering advantages over force platforms and optical motion capture [11]. Devices like the Vertec (Jump USA, Sunnyvale, CA, USA), contact mats, and optoelectronic systems address concerns of portability, affordability, and ease of use. While these tools are effective for estimating CMJ height outside the lab, they do not directly measure vertical displacement of the WBCoM. Contact mats and optoelectronic devices rely on the flight time method to estimate jump height. Although flight time can provide accurate and valid measurements for repeated trials with the same participant, caution is advised when comparing data across individuals. For instance, variations in posture during takeoff and landing may significantly overestimate CMJ height [12]. The development of microelectromechanical systems sensors has made inertial measurement units (IMUs) widely accessible for analyzing human movement. IMUs are compact electronic devices containing tri-axial accelerometers, gyroscopes, and magnetometers, which measure linear acceleration, angular velocity, and heading relative to magnetic north. By fusing data from these components, IMUs can estimate 2D or 3D linear acceleration and angular velocity relative to a global reference frame. Compared to force platforms and optical motion capture systems, IMUs are smaller, more portable, and cost-effective, offering a practical solution for collecting lab-quality data in field settings. The use of IMUs for simple applications, such as measuring peak medicine ball throwing speed, has demonstrated strong validity when compared to criterion methods (\(r=0.93\)-\(0.99\)) [13]. Positioning a centrally located IMU within a medicine ball is relatively straightforward compared to the complexity of using IMUs to measure biomechanical metrics of human motion. Human movement involves a high degree of complexity, with numerous degrees of freedom across multiple major articulating diarthrodial joints, making accurate motion capture and analysis significantly more challenging. A majority of studies have investigated the validity of estimating CMJ JH using a single IMU, reporting errors ranging from 3% to 12% when compared to force plates or optical motion capture systems [14, 15, 16, 17, 18, 19].