Relationship between stability and variability of the core in dynamic reaching tasks

Core stability is important to many functional and athletic tasks. Motion variability has been proposed as a measure to characterize core stability. Based on motor learning theories, the current study hypothesized  that variability and stability of core movements show a U-shaped relationship and further investigated whether functional range of motion (“mobility”) or speed of motion affect this relationship.  Twenty-four healthy subjects performed 20 cycles of two different unilateral hand reaching tasks for both the left and right hand under stable and unstable conditions. Reach targets were positioned to trigger large core movements. Specifically, the anterior target was positioned midsagittal at arm length´s distance and hip height. Two posterior targets (60 degrees posterior to neutral stance frontal plane) on both the left and right side were high (at arm length distance and height with accrued 10 cm) and low (arm length distance at hip height). Kinematic data were recorded and three-dimensional angles between pelvis and thorax (core) were calculated. Pearson correlation coefficients and paired T-tests were calculated to assess variability, mobility and speed of the core movements. A parabolic function was fitted to the variability data and the quality of the fit was assessed by calculating adjusted R-squared values.  In the sagittal plane, variability could be modeled with a U-shaped distribution; in the other planes of motion this was the case in 2 of 4 reaching tests. In two tests, movement speed changed between the stable and unstable conditions. Mobility did not appear to affect variability in the stable condition, but some correlations were observed in the unstable condition. The relationship between mobility and variability, and the change in variability were task-specific

Quantitative downhill skiing technique analysis according to ski instruction curricula: A proof-of-concept study applying principal component analysis on wearable sensor data

$\textit{Downhill skiing technique}$ represents 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 techniques $\textit{parallel}$ and $\textit{carving turns}$ were 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

A discussion of the Muscle Tuning and the Preferred Movement Path concepts – comment on Nigg et al.

Nigg and colleagues propose two new paradigms, the Muscle Tuning and the Preferred Movement Path concepts. The purpose of this commentary is to discuss plausibility and challenges of these two concepts. Both concepts are highly plausible from a mechanical point of view and they also go in line with every-day observations. The main challenges for the muscle tuning paradigm are that (a) this mechanism is only one of several mechanisms in how the body adapts to impacts, and (b) it is very difficult to develop testable predictions from this paradigm since the mechanical (vibrational) properties of the leg are highly subject-specific and complex. The main open questions regarding the preferred movement path paradigm relate to (a) its integration with the concepts for movement variability, and (b) to the circumstances under which the preferred movement path might change

DOES A SKIER’S POSITION ON THE SKI AFFECT THE RESULTS OF GLIDING TESTS USED TO ASSESS SKI-SNOW FRICTION?

Ski manufacturers and ski racers test the gliding performance of skis by standardized gliding tests on straight runs. These test procedures have the advantage that they resemble the real situation in ski racing, but they have the disadvantage that the tester might influence the test result by influencing air drag or ski-snow friction. In this study we analysed if the position of the skier on the ski, which determines the force transfer from the skier onto the ski, affects the results of gliding tests. Three professional ski testers were asked to perform five different tasks: glide with the skis as flat as possible, stabilize your motion by edging the skis, glide in neutral, in forward, or in backward leaning position. The results show that edging on the one hand stabilizes the gliding motion, on the other hand, it significantly increases gliding time. The position of the skier in direction of the ski axis did not affect the skiers’ gliding times, which contradicts a common opinion of many ski racing experts

Functional Relevance of the Small Muscles Crossing the Ankle Joint

It has been suggested that increasing muscle strength could help reducing the frequency of running injuries and that a top-down approach using an increase in hip muscle strength will result in a reduced range of movement and reduced external moments at the knee and ankle level. This paper suggests, that a bottom-up approach using an increase of strength of the small muscles crossing the ankle joint, should reduce movement and loading at the ankle, knee and hip. This bottom-up approach is discussed in detail in this paper from a conceptional point of view. The ankle joint has two relatively “large” extrinsic muscles and seven relatively small extrinsic muscles. The large muscles have large levers for plantar-dorsi flexion but small levers for pro-supination. In the absence of strong small muscles the large muscles are loaded substantially when providing balancing with respect to pro-supination. Specifically, the Achilles tendon will be loaded in this situation asymmetrically with high local stresses. Furthermore, a mechanical model with springs shows that (a) the amplitude of the displacement with the strong small springs is smaller and (b) that the loading in the joints of the springs is substantially smaller for the model with the strong small springs. Additionally, strong and active small muscles crossing the ankle joint provide stability for the ankle joint (base). If they are weak, forces in the ankle, knee and hip joint increase substantially due to multiple co-contractions at the joints. Finally, movement transfer between foot and tibia is high for movements induced from the bottom and small for movements induced from the top. Based on these considerations one should speculate that the bottom-up approach may be substantially more effective in preventing running injuries than the top down approach. Various possible strategies to strengthen the small muscles of the ankle joint are presented

EFFECTS OF AN 8-WEEK KNEE INJURY PREVENTION PROGRAM AND TECHNIQUE MODIFICATION TRAINING ON CHANGE-OF-DIRECTION PERFORMANCE

The purpose of this analysis was to determine whether an 8-week knee injury prevention program with an additional focus on change-of-direction (COD) technique training results in improved COD performance compared to a control training group with a focus on linear sprint training. Although both groups showed indicators for superior performance during a 135-degree COD, such as a more effective reorientation of the body, the COD technique modification component was ineffective in improving overall COD completion time or ground contact times. Follow-up analyses will show whether the COD group adopted a safer COD movement strategy following training, e.g. by reducing the knee valgus loading

Identifying differences in gait adaptability across various speeds using movement synergy analysis

Introduction The aim of this study was to identify movement synergies during normal-walking that can differentiate healthy adults in terms of gait adaptability at various speeds. To this end, the association between movement synergies and lower-limb coordination variability or Deviation Phase (DP) was investigated. This study also investigated the moderating effect of movement synergies on the relationship between DP and the smoothness of arm-swing motion (NJI). Method A principal component analysis of whole-body marker trajectories from normal-walking treadmill trials at 0.8m/s, 1.2m/s and 1.6m/s was undertaken. Both DP and NJI were derived from approx. 8 minutes of perturbed-walking treadmill trials. Principal movement components, PMk, were derived and the RMS of the 2nd-order differentiation of these PMk (PAkRMS) were included as independent variables representing the magnitude of neuromuscular control in each PMk. Each PAkRMS were input into maximal linear mixed-effects models against DP and (DP x NJI) respectively. A stepwise elimination of terms and comparison of models using Anova identified optimal models for both aims. Results The principal movement related to the push-off mechanism of gait (PA4RMS) was identified as an optimal model and demonstrated a significant negative effect on DP however this effect may differ considerably across walking-speeds. An optimal model for describing the variance in (DP x NJI) included a fixed-effect of PA6RMS representing Right—Left side weight transfer was identified. Interpretation The hypotheses that individuals who exhibited greater control on specific kinematic synergies would exhibit variations during perturbed walking was substantiated. Supporting evidence for the role of movement synergies during the double-support phase of gait in proactively correcting balance was presented as well as the potential for this approach in targeted rehabilitation. The potential influence of leg dominance on gait adaptability was also discussed. Future studies should investigate further the role of walking-speed and leg dominance on movement synergies and look to generalize these findings to patient populations

DETERMINATION OF TURNING PARAMETERS IN CARVED SKIING AND APPLICATION TO A NUMERICAL SKI-BINDING MODEL

Skiing has regained in popularity after the introduction of the caNing technique. The biomechanics of caNing have been investigated in numerous studies. However, a comprehensive study of the behaviour of the ski/binding system taking into account the interactions between athlete, skiing equipment, and snow is still missing. In a first phase of the current study, the forces acting between skier and ski equipment and the evolution of the edging angle during caNing were determined using video analysis and force measurements. Next, the passive snow resistance to a penetrating ski was determined using two specially designed tools. Finally, the determined quantities seNed as boundary conditions for a finite-element simulation of the ski/binding system in the caNing situation. Calculated ski shapes were compared against measured turn radii and good agreement was found. The implemented model is intended to help in the development of improved ski equipment. As such, it can for example be used to study the effect of different skier's actions on the equipment behaviour