The Xsens Link motion capture suit has become a popular tool in investigating 3D running kinematics based on wearable inertial measurement units outside of the laboratory. In this study, we investigated the reliability of Xsens-based lower extremity joint angles during unconstrained running on stable (asphalt) and unstable (woodchip) surfaces within and between five different testing days in a group of 17 recreational runners (8 female, 9 male). Specifically, we determined the within-day and between-day intraclass correlation coefficients (ICCs) and minimal detectable changes (MDCs) with respect to discrete ankle, knee, and hip joint angles. When comparing runs within the same day, the investigated Xsens-based joint angles generally showed good to excellent reliability (median ICCs > 0.9). Between-day reliability was generally lower than the within-day estimates: Initial hip, knee, and ankle angles in the sagittal plane showed good reliability (median ICCs > 0.88), while ankle and hip angles in the frontal plane showed only poor to moderate reliability (median ICCs 0.38–0.83). The results were largely unaffected by the surface. In conclusion, within-day adaptations in lower-extremity running kinematics can be captured with the Xsens Link system. Our data on between-day reliability suggest caution when trying to capture longitudinal adaptations, specifically for ankle and hip joint angles in the frontal plane.

Markerless motion capture systems, such as Theia3D, offer a promising alternative to traditional marker-based systems (e.g., Vicon) not only for dynamic motion analysis, but also potentially for postural control research. The purpose of this study is to simultaneously derive and then compare principal movements (PMs) from Theia3D and Vicon data collected from 13 volunteers during eyes-open and eyes-closed bipedal quiet stance using principal component analysis (PCA). Agreement between systems was assessed by computing correlation coefficients for corresponding PM time series and evaluating their sensitivity to visual condition changes. Theia3D and Vicon produced highly similar PM structures, with the first two PMs-representing anterior-posterior ankle-strategy and medio-lateral sway-exhibiting near-perfect correlation (r = 99.8 % and r = 99.0 %) and explaining a comparable portion of the entire postural variance (PM1: 74 % vs. 76 %; PM2: 9.4 % vs. 9.9 % for Vicon and Theia3D results, respectively). The first seven PMs, which collectively accounted for 97 % of postural variance, showed strong agreement (r = 0.814 to 0.928) despite fundamental differences in marker placement and tracking methodology. Correlation declined in higher-order PMs, suggesting they capture distinct aspects of postural movements or system-specific noise. Both systems demonstrated comparable sensitivity to postural control modulations induced by visual deprivation. Notably, Theia3D exhibited higher noise levels, particularly in higher-order PMs, indicating that pre-processing with filtering should be considered before PCA. Overall, these findings confirm that Theia3D provides highly comparable results to Vicon for postural PMs, reinforcing its potential as a valid and efficient alternative for postural control research using PCA-based approaches.

Downhill skiing techniquerepresents the complex coordinative movement patterns needed to control skiing motion. While scientific understanding of skiing technique is still incomplete, not least due to challenges in objectively measuring it, practitioners such as ski instructors have developed sophisticated and comprehensive descriptions of skiing technique. The current paper describes a 3-step proof-of-concept study introducing a technology platform for quantifying skiing technique that utilizes the practitioners’ expert knowledge. The approach utilizes an inertial measurement unit system (Xsens™) and presents a motion analysis algorithm based on the Principal Movement (PM) concept. In step 1, certified ski instructors skied specified technique elements according to technique variations described in ski instruction curricula. The obtained data was used to establish a PM-coordinate system for skiing movements. In step 2, the techniquesparallelandcarving turnswere compared. Step 3 presents a case study where the technique analysis methodology is applied to advise an individual skier on potential technique improvements. All objectives of the study were met, proving the suitability of the proposed technology for scientific and applied technique evaluations of downhill skiing. The underlying conceptual approach - utilizing expert knowledge and skills to generate tailored variability in motion data (step 1) that then dominate the orientation of the PMs, which, in turn, can serve as measures for technique elements of interest - could be applied in many other sports or for other applications in human movement analyses.

In view of the frequent use of the portable balance board (Tymo^®, Tyromotion, Graz, Austria) in neurorehabilitation, the objective of this study is to assess its posturographic validity and reliability in comparison to a floor-embedded force platform (AMTI, Watertown, MA, USA), as well as its clinical validity in stroke patients. To evaluate posturographic validity, thirty healthy participants were tested in four different measurement conditions (M1–M4: standing with/without a balance pad, eyes open/closed) while simultaneously measuring with both devices placed on top of each other. Clinical validity is obtained by correlating stroke patients’ Tymo^® outcome variables (path length and ellipse) of M1–M4 with Berg Balance Scale (BBS) results. Reliability is determined by correlating repeated measurements. The Center of pressure (COP) paths of both force plates show a very strong correlation. The repeated measurement reliability of both force plates is comparable. Tymo^® outcome variables exhibit moderate to strong correlation with the BBS and demonstrated repeated measurement reliability analogous to healthy participants. Therefore, the Tymo^® device demonstrates measurement accuracy comparable to a gold-standard device while offering the advantages of portability and user-friendly software. Tymo^® is suitable for assessing balance in stroke patients, though protocol adaptations and averaging repeated measurements are recommended to enhance feasibility and reliability.

Quantifying movement coordination in cross‐country (XC) skiing, specifically the technique with its elemental forms, is challenging. Particularly, this applies when trying to establish a bidirectional transfer between scientific theory and practical experts' knowledge as expressed, for example, in ski instruction curricula. The objective of this study was to translate 14 curricula‐informed distinct elements of the V2 ski‐skating technique (horizontal and vertical posture, lateral tilt, head position, upper body rotation, arm swing, shoulder abduction, elbow flexion, hand and leg distance, plantar flexion, ski set‐down, leg push‐off, and gliding phase) into plausible, valid and applicable measures to make the technique training process more quantifiable and scientifically grounded. Inertial measurement unit (IMU) data of 10 highly experienced XC skiers who demonstrated the technique elements by two extreme forms each (e.g., anterior versus posterior positioning for the horizontal posture) were recorded. Element‐specific principal component analyses (PCAs)—driven by the variance produced by the technique extremes—resulted in movement components that express quantifiable measures of the underlying technique elements. Ten measures were found to be sensitive in distinguishing between the inputted extreme variations using statistical parametric mapping (SPM), whereas for four elements the SPM did not detect differences (lateral tilt, plantar flexion, ski set‐down, and leg push‐off). Applicability of the established technique measures was determined based on quantifying individual techniques through them. The study introduces a novel approach to quantitatively assess V2 ski‐skating technique, which might help to enhance technique feedback and bridge the communication gap that often exists between practitioners and scientists.

Introduction & Purpose “Running technique is an important component of running economy and performance” (Folland et al., 2017). While economy and performance can directly be measured, no common measuring method exists for running technique. The aims of this study were twofold: (I) develop practice-informed running technique measures based on technique characterizations used by coaches (i.e. distinct variations of body segment movements instead of discrete kinematics); (II) investigate, how running technique changes with running speed. Methods Two independent kinematic data sets were recorded on a treadmill using a retro-reflective marker setup (“Plugin-Gait”) and a 10-camera system (Vicon Motion Systems Ltd., Oxford, UK). First, 20 experienced runners (age 25 ± 2 y; 10 females, 10 males) were tasked to vary their running according to 14 technique elements (vertical, horizontal, hip and upper body movement, foot strike, track width, ankle rotation, leg swing, back posture, elbow flexion/extension, arm swing amplitude and direction, gaze direction, cadence) into two opposing extreme forms per element. Measures for the 14 running technique elements were developed using principal component analyses (one PCA per two opposing technique element variations) and selecting principal components (PCs; Federolf, 2016) that aligned with the represented technique element. Second, an additional 19 experienced runners (age 30 ± 9 y, 3 females, 16 males) ran at different speeds (10-17 km/h with 1 km/h incremental steps) without any technique instructions. Their data were projected onto the selected PCs, resulting in averaged PC score waveforms of the different speeds along each technique element. Waveforms were analysed using statistical parametric mapping (SPM) with a repeated measures ANOVA design (Pataky et al., 2013). Results For all 14 technique elements, sections of significant differences between at least two different speeds were found within the cycle-normalized PC score waveforms. Figure 1 exemplifies the results for the foot track width, where the lateral range of motion of the feet decreased with speed. Post-hoc analyses revealed further differences, such as less vertical and greater horizontal movement, as well as smaller leg swing and greater arm swing amplitude at 17 km/h compared to 10 km/h. Discussion Firstly, practice-informed and data-driven running technique measures could be developed (I). Secondly, the measures could be applied on a group of runners and plausible changes in their technique due to running speed were identified (II). So far, the speed-dependent technique adaptations only reflect the specific techniques on a treadmill for a predominately male reference group, but the presented concept can be transferred to other runners and settings. Conclusion Running technique varies with speed, and the developed technique measures facilitate comparisons of these differences using practically relevant technique elements. Since the movement adaptations associated with running speed are directly quantifiable and visualizable, technique models for different speed ranges might be developed by researchers and then utilized by runners to train and assess their technique in alignment with these models. References Folland, J. P., Allen, S. J., Black, M. I., Handsaker, J. C., & Forrester, S. E. (2017). Running technique is an important component of running economy and performance. Medicine & Science in Sports & Exercise, 49(7), 1412-1423. https://doi.org/10.1249%2FMSS.0000000000001245 Federolf, P. A. (2016). A novel approach to study human posture control: “Principal movements” obtained from a principal component analysis of kinematic marker data. Journal of Biomechanics, 49(3), 364-370. https://doi.org/10.1016/j.jbiomech.2015.12.030 Pataky, T. C., Robinson, M. A., & Vanrenterghem, J. (2013). Vector field statistical analysis of kinematic and force trajectories. Journal of Biomechanics, 46(14), 2394-2401. https://doi.org/10.1016/j.jbiomech.2013.07.031