# A Comparative Analysis of IMUs and Optical Systems in Layup Shot Biomechanics
## 4 Discussion
### Between-System Analysis  
Shoulder flexion-extension joint angles demonstrated consistently high between-system RMSD levels across participants (Table 1). Although the time-series trajectories indicated waveform agreement throughout the task, occasional underestimation by Xsens was observed, particularly around ball release. This finding aligns with previous research, which suggests that IMUs generally provide acceptable accuracy for upper body kinematics but may underestimate joint angles, especially as the magnitude of movement increases [11]. These discrepancies at higher joint angles may explain the observed underestimation near ball release, where the demands on shoulder joint mobility are likely elevated. A closer examination of the discrepancies highlights several contributing factors. First, IMU-based systems, by nature, tend to underestimate joint angles at larger ranges of motion while overestimating at smaller angles [11, 32]. Bland-Altman analyses in previous studies have confirmed this trend, particularly for shoulder elevation, where the largest errors are typically observed [11]. These previous findings are consistent with the results in the current study, where the underestimation near ball release coincides with elevated shoulder flexion demands. Additionally, the soft tissue artifact (STA) inherent to all non-invasive motion analysis techniques, including optical systems, introduces error into the reference data, further complicating the comparison between systems [33]. Thus, the observed RMSD values may partly reflect STA effects, which reduce the precision of both measurement systems in capturing true joint motion. Furthermore, the complexity of shoulder kinematics, characterised by multi-planar motion and interdependence between segmental axes, adds another layer of challenge for IMUs. Notably, while studies have investigated specific upper-limb criteria, such as thescapulohumeral rhythm and repeatability of segment axes [34; 35], the influence of these factors on task-specific performance in dynamic, sport-representative environments remains underexplored. The observed high RMSD values may, therefore, reflect a combination of methodological limitations, soft tissue-induced artifacts, and the inherent challenges of capturing highly dynamic, multi-planar movements with portable systems like IMUs. This study also confirms previous studies suggesting the presence of high between-systems RMSD values for the abduction-adduction and external-internal joint angles for the complex motions of the mobile and multiplanar shoulder joint [3] (Table 1). Together with the high between-participant variability of the SPM plots (Figure 7), the validity of Xsens to detect these motions accurately for the layup cannot be confirmed. This finding is in line with the limited number of studies that have incorporated the shoulder joint for complex tasks (multi-planal motion), suggesting the validity of IMUs' detection of motion accuracy is highly variable [24; 36; 37; 38]. The shoulder joint is very mobile [39], and its biomechanical modelling requires unique mathematical representation [40]. The shoulder joint, due to its high degrees of freedom, is expected to perform multi-planar motions simultaneously. In the context of a basketball layup, while the extension-to-flexion movement of the arm is consistent throughout the task, motions in the abduction-adduction and external-internal joint angles can vary, displaying one or a combination of directions depending on player preference or task-specific constraints, such as defensive pressure. Total joint angles are derived from the summation of individual rotational components across anatomical planes. However, this process introduces the potential for cross-talk or noise, whereby inaccuracies in the measurement or alignment of rotations in one plane may erroneously influence or be attributed to rotations in another, compromising the accuracy of the resulting total angle [41; 42].