Unraveling the Effects of Expertise and Fatigue on Kinematics and Stride-to-Stride Variability in Running

Unser Körper ermöglicht es uns, ohne große Anstrengung komplexe Bewegungen auszuführen. Aufgrund der Vielzahl von Freiheitsgraden (DoF) im Muskel-Skelett-System ist unser Körper ein hochredundantes System. Für jede denkbare Bewegung gibt es daher mehrere Lösungsmöglichkeiten, welche wiederum zu einer Vielzahl an Bewegungsausführungen führen. Von außen betrachtet liegt die Vermutung nahe, dass innerhalb einer zyklischen Bewegung, wie z.B. dem Laufen, immer wieder der gleiche Bewegungsablauf ausgeführt wird. Dies führt oft zu der Annahme, dass die Beobachtung eines einzigen Laufzyklus ausreicht, um die Biomechanik des Laufens zu analysieren. Dabei werden allerdings Informationen übersehen, die in den Variationen zwischen aufeinanderfolgenden Zyklen liegen. Tatsächlich könnte eine reine Reproduktion desselben Laufzyklus unter gleichen Bedingungen zu Verletzungen führen, da immer dieselben Strukturen in demselben Maße belastet werden würden. Jedoch ist der Zustand des Läufers und seiner Randbedingungen von Laufzyklus zu Laufzyklus nicht immer identisch, daher ist eine exakte Reproduktion desselben Bewegungsmusters unwahrscheinlich. Eine mögliche Veränderung der Randbedingungen könnte das Auftreten von Ermüdung sein, welche bei Ausdauersportarten unvermeidlich ist. Die Vielzahl gleichwertiger Bewegungslösungen und die daraus resultierende Variabilität zwischen einzelnen Laufzyklen eines Läufers sind daher wertvolle Merkmale und ein wichtiges Thema für Forschungsarbeiten im Kontext der menschlichen Bewegungskoordination. Auf dem Forschungsgebiet der Bewegungsvariabilität wurden zwei vielversprechende spezifische Methoden entwickelt und auf biomechanische Daten angewendet: die Uncontrolled Manifold-Methode (UCM) und die Tolerance Noise Covariation-Methode (TNC). Die UCM hat ihren Ursprung im Forschungsfeld der motorischen Kontrolle, wohingegen die TNC aus dem Bereich des motorischen Lernens kommt. Mit Hilfe der UCM und der TNC Methoden wird analysiert, wie die Variabilität auf der Ebene der Gelenkwinkel mit der Variabilität der Zielgröße zusammenhängt. Sie wurden hauptsächlich auf eingeschränkte Bewegungen mit nur wenigen DoF angewendet und kaum zur Untersuchung von Ganzkörperbewegungen, wie z.B. des Laufens, genutzt. Bei Untersuchungen des Gehens wurde festgestellt, dass trotz Zyklus-zu-Zyklus Variabilität (SSV) auf unterschiedlichen Ebenen (z.B. Gelenkwinkel) diese so kanalisiert werden kann, dass eine Zielgröße (z.B. Körperschwerpunk, CoM) über die Zyklen hinweg annähernd konstant bleibt. Diese Arbeit erörtert auf der Basis von fünf Studien, wie sich Expertise und Ermüdung auf die Laufkinematik auswirken, indem sie nicht nur eine biomechanische Analyse der Effekte von Ermüdung auf die Lauf-Kinematik durchführt, sondern auch komplexe Methoden zur Analyse der Bewegungsvariabilität anwendet. Da diese Methoden in der internationalen sportwissenschaftlichen Forschung bisher kaum Anwendung gefunden haben, wird mit der vorliegenden Arbeit auch geprüft, ob sich die anhand von einfachen, experimentellen Paradigmen der Grundlagenforschung entwickelten Methoden, gewinnbringend auf sportwissenschaftliche Problemstellungen übertragen lassen. In der ersten Studie wurden die Auswirkungen von Expertise auf die SSV des CoM beim Laufen bei 10 und 15 km/h analysiert. Novizen zeigten bei 15 km/h eine größere Variabilität als Experten. In der zweiten Studie wurde ein klassischer biomechanischer Ansatz gewählt, um die Ermüdungsreaktionen von erfahrenen Läufern zu untersuchen. Dabei wurden Veränderungen sowohl in Raum-Zeit- und Steifigkeitsparametern, als auch in der Gelenkkinematik gefunden. Diese Ergebnisse zeigten, dass die Kinematik im ermüdeten Zustand deutlich verändert ist. Die dritte Studie erweiterte diese Erkenntnisse durch die Verwendung der UCM-Methode. Dabei wurde ein probandenspezifisches 3D-Modell für den menschlichen Körper eingeführt, um den Ganzkörper-CoM genau berechnen zu können. Es wurden geringe Veränderungen bei Ermüdung gefunden. Dies zeigte, dass erfahrene Läufer in der Lage sind, ihre CoM-Trajektorie auch in einem ermüdeten Zustand zu kontrollieren. In der vierten Studie wurden diese Ergebnisse durch die Verwendung der TNC-Methode erweitert. Es zeigte sich, dass die Variabilität des CoM sowohl in medio-lateraler als auch in vertikaler Richtung mit Ermüdung zunimmt. In der fünften Studie wurde wieder ein klassischer biomechanischer Ansatz gewählt, um die Reaktionen auf Ermüdung zu charakterisieren, dieses Mal bei Lauf-Novizen. Es wurden keine Veränderungen in den Raum-Zeit- und Steifigkeitsparametern gefunden, obwohl die Gelenkkinematik durch die Ermüdung beeinflusst wurde. Diese Ergebnisse deuten darauf hin, dass Novizen möglicherweise Strategien fehlen, um eine konstante Laufgeschwindigkeit unter Ermüdung beizubehalten. Mit dieser Studienreihe wird das Wissen über die Auswirkungen von Expertise und Ermüdung auf die Kinematik und SSV beim Laufen erweitert. Nachdem die grundsätzliche Anwendbarkeit von neuen Ansätzen, wie der UCM oder der TNC Methode, auf komplexe sportwissenschaftliche Probleme gezeigt wurde, können diese Methoden bei der Anwendung auf praxisorientierte Probleme in der Sportwissenschaft geprüft und zu verbessert werden

Stride-to-Stride Variability of the Center of Mass in Male Trained Runners After an Exhaustive Run: A Three Dimensional Movement Variability Analysis With a Subject-Specific Anthropometric Model

The motion of the human body can be described by the motion of its center of mass (CoM). Since the trajectory of the CoM is a crucial variable during running, one can assume that trained runners would try to keep their CoM trajectory constant from stride to stride. However, when exposed to fatigue, runners might have to adapt certain biomechanical parameters. The Uncontrolled Manifold approach (UCM) and the Tolerance, Noise, and Covariation (TNC) approach are used to analyze changes in movement variability while considering the overall task of keeping a certain task relevant variable constant. The purpose of this study was to investigate if and how runners adjust their CoM trajectory during a run to fatigue at a constant speed on a treadmill and how fatigue affects the variability of the CoM trajectory. Additionally, the results obtained with the TNC approach were compared to the results obtained with the UCM analysis in an earlier study on the same dataset. Therefore, two TNC analyses were conducted to assess effects of fatigue on the CoM trajectory from two viewpoints: one analyzing the CoM with respect to a lab coordinate system (PV$_{lab}$) and another one analyzing the CoM with respect to the right foot (PV$_{foot}$). Full body kinematics of 13 healthy young athletes were captured in a rested and in a fatigued state and an anthropometric model was used to calculate the CoM based on the joint angles. Variability was quantified by the coefficient of variation of the length of the position vector of the CoM and by the components Tolerance, Noise, and Covariation which were analyzed both in 3D and the projections in the vertical, anterior-posterior and medio-lateral coordinate axes. Concerning PV$_{lab}$ we found that runners increased their stride-to-stride variability in medio-lateral direction (1%). Concerning PV$_{foot}$ we found that runners lowered their CoM (4 mm) and increased their stride-to-stride variability in the absorption phase in both 3D and in the vertical direction. Although we identified statistically relevant differences between the two running states, we have to point out that the effects were small (CV ≤ 1%) and must be interpreted cautiously

Changes in Key Biomechanical Parameters according to the Expertise Level in Runners at Different Running Speeds

Running has become increasingly popular worldwide. Among runners, there exists a wide range of expertise levels. Investigating the differences between runners at two extreme levels, that is novices and experts, is crucial to understand the changes that occur as a result of multiple years of training. Vertical oscillation of center of mass (CoM), stride frequency normalized to the leg length, and duty factor, which describes the step time relative to the flight time, are key biomechan- ical parameters that have been shown to be closely related to the running economy and are used to characterize the running style. The variability characteristics of these parameters may reveal valua- ble information concerning the control of human locomotion. However, how the expertise level and running speed affect the variability of these key biomechanical parameters has not yet been inves- tigated. The aim of this study was to analyze the effects of expertise level (novice vs. expert) and running speed (10 km/h vs. 15 km/h) on these parameters and their variability. It was hypothesized that expert runners would have lower vertical oscillation of CoM, normalized stride frequency, and duty factor and show less variability in these parameters. The parameters’ variability was opera- tionalized by the coefficient of variation. The mean values and variability of these key biomechani- cal parameters according to expertise level and running speed were compared with rmANOVAs. The results showed that the experts had a lower duty factor and less variable vertical oscillation of CoM and normalized stride frequency, independently of the running speed. At a higher running speed, the variability of vertical oscillation of CoM was higher, whereas that of normalized stride frequency and duty factor did not change significantly. To the best of our knowledge, this is the first study analyzing the effects of expertise level and running speed on the variability of key biomechanical parameters

Running-Induced Fatigue Changes the Structure of Motor Variability in Novice Runners

Understanding the effects of fatigue is a central issue in the context of endurance sports. Given the popularity of running, there are numerous novices among runners. Therefore, under- standing the effects of fatigue in novice runners is an important issue. Various studies have drawn conclusions about the control of certain variables by analyzing motor variability. One variable that plays a crucial role during running is the center of mass (CoM), as it reflects the movement of the whole body in a simplified way. Therefore, the aim of this study was to analyze the effects of fatigue on the motor variability structure that stabilizes the CoM trajectory in novice runners. To do so, the uncontrolled manifold approach was applied to a 3D whole-body model using the CoM as the result variable. It was found that motor variability increased with fatigue (UCMꓕ). However, the UCMRatio did not change. This indicates that the control of the CoM decreased, whereas the stability was not affected. The decreases in control were correlated with the degree of exhaustion, as indicated by the Borg scale (during breaking and flight phase). It can be summarized that running-induced fatigue increases the step-to-step variability in novice runners and affects the control of their CoM

Biofidelity Corridors for Sternum Kinematics in Low-Speed Side Impacts

Objective: Field data show that side impact car crashes have become responsible for a greater proportion of the fatal crashes compared to frontal crashes, which suggests that the protection gained in frontal impact has not been matched in side impact. One of the reasons is the lack of understanding of the torso injury mechanisms in side impact. In particular, the deformation of the rib cage and how it affects the mechanical loading of the individual ribs have yet to be established. Therefore, the objective of this study was to characterize the ribcage deformation in side impacts by describing the kinematics of the sternum relative to the spine. Methods: The 3D kinematics of the 1st and of the 5th or 6th thoracic vertebrae and of the sternum were obtained for three Post Mortem Human Subjects (PMHS) impacted laterally by a rigid wall traveling at 15 km/h. The experimental data were processed to express the kinematics of the sternum relative to the spine throughout the impact event. Methods were developed to interpolate the kinematics of the vertebrae for which experimental data were not available. Results: The kinematics of the sternocostal junction for ribs 1 to 6 as well as the orientation of the sternum were expressed in the vertebra coordinate systems defined for each upper thoracic vertebra (T1 to T6). Corridors were designed for the motion of the sternum relative to each vertebra. In the experiments, the sternum moved upward for all rib levels (1 to 6), and away from the spine with an amplitude that increased with the decreasing rib level (from rib 1 to rib 6). None of the differences observed in the kinematics could be correlated to the occurrence of rib fractures. Conclusions: This study provides both qualitative and quantitative information for the ribcage skeletal kinematics in side impact. This data set provides the information required to better evaluate computational models of the thorax for side impact simulations. The corridors developed in this study provide new biofidelity targets for the impact response of the ribcage. This study contributes to augmenting the state of knowledge of the human chest deformation in side impact to better characterize the rib fracture mechanisms.The analysis performed in this study has been funded by and carried out in association with SAFER - Vehicle and Traffic Safety Centre at Chalmers University of Technology, Sweden. D. Subit thanks European Union for its financial support through the Marie Curie International Incoming Fellowship (FP7-PEOPLE-2013-IIF, project BioAge # 622905). F. M ¨ ohler gratefully acknowledges the financial support from the Franco-German University through the Franco-German double degree program between ENSAM and KIT. The views expressed in this article are those of the authors and do not necessarily represent the views of the funding bodies

Influence of Controlled Stomatognathic Motor Activity on Sway, Control and Stability of the Center of Mass During Dynamic Steady-State Balance—An Uncontrolled Manifold Analysis

Multiple sensory signals from visual, somatosensory and vestibular systems are used for human postural control. To maintain postural stability, the central nervous system keeps the center of mass (CoM) within the base of support. The influence of the stomatognathic motor system on postural control has been established under static conditions, but it has not yet been investigated during dynamic steady-state balance. The purpose of the study was to investigate the effects of controlled stomatognathic motor activity on the control and stability of the CoM during dynamic steady-state balance. A total of 48 physically active and healthy adults were assigned to three groups with different stomatognathic motor conditions: jaw clenching, tongue pressing and habitual stomatognathic behavior. Dynamic steady-state balance was assessed using an oscillating platform and the kinematic data were collected with a 3D motion capturing system. The path length (PL) of the 3D CoM trajectory was used for quantifying CoM sway. Temporal dynamics of the CoM movement was assessed with a detrended fluctuation analysis (DFA). An uncontrolled manifold (UCM) analysis was applied to assess the stability and control of the CoM with a subject-specific anthropometric 3D model. The statistical analysis revealed that the groups did not differ significantly in PL, DFA scaling exponents or UCM parameters. The results indicated that deliberate jaw clenching or tongue pressing did not seem to affect the sway, control or stability of the CoM on an oscillating platform significantly. Because of the task-specificity of balance, further research investigating the effects of stomatognathic motor activities on dynamic steady-state balance with different movement tasks are needed. Additionally, further analysis by use of muscle synergies or co-contractions may reveal effects on the level of muscles, which were not visible on the level of kinematics. This study can contribute to the understanding of postural control mechanisms, particularly in relation to stomatognathic motor activities and under dynamic conditions

IDENTIFICATION OF FATIGUE-RELATED KINEMATIC CHANGES IN ELITE RUNNERS USING A SUPPORT VECTOR MACHINE APPROACH

Understanding the kinematic changes underlying fatigue is essential in running biomechanics. The aim of this study was to identify fatigue-related kinematic changes in elite runners using a support vector machine approach. Full-body kinematics of thirteen trained runners were recorded in a non-fatigued and a fatigued state during treadmill running at their individual fatigue-speed. A support vector machine was trained and used to identify kinematic differences between the non-fatigued and fatigued state based on principal component scores. Strides during non-fatigued and fatigued running could be separated with 99.4% classification accuracy. Four upper limb (two shoulder and two elbow), four lower limb (one ankle, two knee and one hip) and two trunk (one thoracic and one lumbar spine) principal component scores were identified as most discriminative kinematic features between non-fatigued and fatigued running. The findings of the study suggest the feasibility of a support vector machine approach to identify subtle fatigue-related kinematic changes in elite runners