# A Comparative Analysis of IMUs and Optical Systems in Layup Shot Biomechanics
## 1 Introduction  
Experimental biomechanics studies commonly utilise optical motion capture technologies in controlled laboratory settings. However, a significant limitation of such studies is their confinement to small capture volumes [1] and the elimination of sport-specific environmental contexts [2]. These constraints hinder the ecological validity of biomechanical research, making it challenging to generalise findings to real-world scenarios. Inertial measurement units (IMUs) offer a promising solution to some of the main drawbacks associated with optical motion capture systems. Unlike optical systems, IMUs are portable, lightweight, and capable of capturing motion data in varied and less restrictive environments [3]. Their flexibility in terms of capture location and volume makes them particularly appealing for biomechanical studies conducted in real world environments. The portability of IMU-based systems, such as Xsens, has driven increasing interest in their use for analysing complex and dynamic movements [4]. However, despite the advantages of IMUs, optical motion capture systems (e.g., Vicon) remain the gold standard in biomechanics research, offering superior data quality and established validity and reliability [5]. Recent systematic reviews have highlighted inherent limitations of current IMUs, such as calibration error, magnetic field disturbance, and sensor bias [6]; however, their accuracy has been shown to be comparable to optical motion capture systems under certain conditions [7]. Studies on Xsens performance during walking and running demonstrated generally acceptable root mean square differences (RMSD) in joint angles: 3.2\({}^{\circ}\)\(\pm\) 1.5 for knee flexion-extension and 4.5\({}^{\circ}\)\(\pm\) 1.8 for ankle dorsi-plantarflexion joint angles. However, larger differences were noted for the hip flexion-extension joint angles (10.1\({}^{\circ}\)\(\pm\) 5.6), with abduction-adduction and external-internal joint angles performing better for the hip but worse for the knee and ankle joints. These differences likely arise from differences in participant calibration and marker placement procedures [8; 9; 10]. The validity of IMUs for upper limb kinematics was investigated in comparison to optical motion capture, predominantly in controlled, low-range motion tasks [11]. In a simulated surgical training task, where movement complexity and joint ranges of motion were considerably lower than those observed in dynamic sports performance, IMUs demonstrated accuracy within 2.9 \(\pm\) 0.9\({}^{\circ}\) for neck flexion/extension and 1.6 \(\pm\) 1.1\({}^{\circ}\) for trunk flexion/extension. However, larger discrepancies were observed in shoulder elevation (6.8 \(\pm\) 2.7\({}^{\circ}\)) and elbow flexion (8.2 \(\pm\) 2.8\({}^{\circ}\)), highlighting potential limitations when applied to higher-velocity, multi-planar movements typical of sport-specific tasks. While IMUs are increasingly used in sports biomechanics, their application in dynamic, sport-specific contexts requiring multi-planar lower and upper body tracking, like basketball, require further assessment. One key area of importance is the movement of the arms above shoulder level during a layup shot, which involves complex, high-velocity, and multi-planar motion requiring large joint ranges of motion and rotational velocities. This movement exceeds the typical range assessed in prior studies, making it essential to address the utility of IMUs for evaluating dynamic, overhead movements in basketball and other sports. Most reliability studies on IMUs have focused on simple planar movements of basic locomotor tasks [3]. In basketball, IMU applications have primarily been limited to activity classification for athlete performance monitoring rather than detailed kinematic analysis [12]. Studies on jump shots [13] have examined relatively constrained upper body movements where the shoulder and elbow remain within moderate ranges of motion compared to the more dynamic layup shot.