# A Comparative Analysis of IMUs and Optical Systems in Layup Shot Biomechanics
## 4 Discussion
### Between-System Analysis
This highlights the inherent challenges in accurately capturing shoulder kinematics during dynamic, sport-specific tasks. Similarly, despite comparable hip joint angle waveforms between the systems, high error rates (Table 1) and the presence of significant difference clusters in the SPM plots questioned Xsens accuracy. In a recent systematic review by [PERSON] et al. [3], the validity of hip kinematics ranged from fair to excellent, with RMSD values spanning 0.2\({}^{\circ}\) to 9.3\({}^{\circ}\) and correlation coefficients (r) between 0.53 and 1.00. Movements involving hip rotation exhibited the greatest inaccuracies, with RMSD reaching up to 11.8\({}^{\circ}\) and correlations dropping as low as 0.35. The discrepancies presented in this work may be attributed to the unique demands of this layup shot task, which differ significantly from the predominantly uni-planar movements typically studied in running and walking datasets [24; 25; 43; 44; 45; 46; 47]. Unlike the consistent, uni-planar patterns of walking and running, the layup shot demands dynamic, multi-planar hip movements, including frequent transitions between flexion, abduction, and rotation to adapt to task-specific constraints. Rapid accelerations, decelerations, and directional changes add variability, causing noise and algorithmic drift in IMU data. These factors increase cross-talk between planes (e.g., [41]), reducing accuracy in capturing all planal motions of the hip and further highlighting the challenges of applying IMUs to complex sport-specific tasks. Flexion-extension joint angles of the smaller joints (i.e., wrist and ankle) have demonstrated relatively low RMSD values (Table 1). This can be confirmed by the SPM trajectory lacking any difference signifying clusters for the wrist joint and by the general waveform agreement for the ankle joint flexion-extension angle. However, wrist ulnar-radial deviation and ankle external-internal joint angles also displayed low RMSD results (Table 1), but this was not reflected congruently across participants' SPM trajectory ensemble plots. This may be due to the smaller ranges of motion, which have amplified levels of noise compared to flexion-extension joint angles [48], with some noise potentially resulting from between-plane cross-talk when one plane motion is incorrectly detected as the motion of another [41]. This has been a common phenomenon reported with studies featuring multi-planal complex movements in which the accuracy of inertial sensors can be affected by high segment linear accelerations [3]. Similar disagreements have been noted by previous sensor vs. marker studies for the ankle external-internal angle [47; 48], but the discrepancy in the agreement of the ankle rotational angle between the systems may also be due to the foot sensor fixation method being prone to impact, as statistically significance difference has been detected roughly--but not exceptional to--around ground contact points (i.e., toe-off and landing). Efforts to secure the foot sensor placement can be made through tightening the shoe strap or taping the sensor to reduce impact effect. Despite the flexion-extension knee and elbow joint angles displaying relatively high errors (Table 1) and the presence of supra-threshold clusters across the joint angle trajectories, the two waveforms remain morphologically similar, except for low deviations at random time points that do not constitute a marked anomaly, according to the trajectory plots. The observed SPM clusters between the systems may partly be attributed to differences in their event detection methods. Although both systems are consistent, slight synchronization delays, potentially up to \(\leq\)4 data points, might occur between the two waveforms. This could result in misalignment, particularly at time points where rapid changes in joint angles take place.