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In this paper we examined how runners with different initial foot strike pattern (FSP) develop their pattern over increasing speeds. The foot strike index (FSI) of 47 runners [66% initially rearfoot strikers (RFS)] was measured in six speeds (2.5–5.0 ms−1), with the hypotheses that the FSI would increase (i.e., move toward the fore of the foot) in RFS strikers, but remain similar in mid- or forefoot strikers (MFS) runners. The majority of runners (77%) maintained their original FSP by increasing speed. However, we detected a significant (16.8%) decrease in the FSI in the MFS group as a function of running speed, showing changes in the running strategy, despite the absence of a shift from one FSP to another. Further, while both groups showed a decrease in contact times, we found a group by speed interaction (p < 0.001) and specifically that this decrease was lower in the MFS group with increasing running speeds. This could have implications in the metabolic energy consumption for MFS-runners, typically measured at low speeds for the assessment of running economy.

The use of motorized treadmills as convenient tools for the study of locomotion has been in vogue for many decades. However, despite the widespread presence of these devices in many scientific and clinical environments, a full consensus on their validity to faithfully substitute free overground locomotion is still missing. Specifically, little information is available on whether and how the neural control of movement is affected when humans walk and run on a treadmill as compared to overground. Here, we made use of linear and non-linear analysis tools to extract information from electromyographic recordings during walking and running overground, and on an instrumented treadmill. We extracted synergistic activation patterns from the muscles of the lower limb via non-negative matrix factorization. We then investigated how the motor modules (or time-invariant muscle weightings) were used in the two locomotion environments. Subsequently, we examined the timing of motor primitives (or time-dependent coefficients of muscle synergies) by calculating their duration, the time of main activation, and their Hurst exponent, a non-linear metric derived from fractal analysis. We found that motor modules were not influenced by the locomotion environment, while motor primitives were overall more regular in treadmill than in overground locomotion, with the main activity of the primitive for propulsion shifted earlier in time. Our results suggest that the spatial and sensory constraints imposed by the treadmill environment might have forced the central nervous system to adopt a different neural control strategy than that used for free overground locomotion, a data-driven indication that treadmills could induce perturbations to the neural control of locomotion.

Die Bewegungs-kontrollstrategien kontextabhängig und abhängig von unterschiedlichen Kriterien ausgewählt werden. Einerseits ist die Stabilität in den Bewegungszuständen wie der Fortbewegung ausschlaggebend für die ungestörte Ausführung bestimmter Handlungen und erfordert eine effektive Steuerung durch das zentrale Nervensystem. Andererseits wird die Bewegungsstrategieauswahl durch das zentrale Nervensystem dadurch bestimmt, dass die Energiekosten minimiert werden soll. Beide Konzepte (d.h. die Aufrechterhaltung der Stabilität und die Energiekostenminimierung) spielen eine fundamentale Rolle bei der Frage, warum sich Menschen so bewegen, wie sie es tun. Unklar ist dabei allerdings, auf welche Weise das zentrale Nervensystem beide Prinzipien gegeneinander gewichtet. In den letzten 20 Jahren haben uns wissenschaftliche Konzepte wie die Chaostheorie oder die Theorie komplexer Systeme eine neue Herangehensweise an diese Fragen ermöglicht. Diese Arbeit untersucht die dynamische Stabilität menschlicher Fortbewegung mit Hilfe des Konzepts der Ljapunowanalyse. Als erstes wird eine methodologische Untersuchung der Verlässlichkeit des maximalen Ljapunowexponenten beim Gehen und Laufen durchgeführt (Kapitel 2). Danach wird verglichen zwischen dem Laufen unter normalen Umständen und dem darauffolgenden Laufen ohne Schuhe, wobei letzteres eine Abnahme der Stabilität nach dem Übergang zu den neuen Umständen zur Folge hat (Kapitel 3). In der letzten Untersuchung wurde ein unterschiedlich langes Training zur Verbesserung der Laufenergetik durchgeführt, in einer Gruppe nur über einen kurzen und in einer anderen Gruppe über einen etwas längeren Zeitraum (Kapitel 4). Die Ergebnisse zeigen, dass Bewegungskontrollfehler für die Energiekosten beim Laufen eine Rolle spielen können, und legen somit eine flexible Priorisierung der Bewegungskontrolle nahe., Motor control strategies are chosen in a context dependent manner, based on different criteria. On the one hand stability in dynamic conditions such as locomotion, is crucial to uninterrupted task execution and requires effective regulation by the central nervous system. On the other, minimization of the energetic cost of transport is instrumental in choosing the locomotion strategy by the central nervous system. Both these concepts, (i.e. maintaining stability and optimization of energetic cost of locomotion) have a fundamental role on how and why humans move in the way they do. However, how the human central nervous system prioritizes between the different goals is unknown. In the last 20 years, ideas from scientific paradigms such as chaos theory and complex systems have given us novel tools to approach these questions. The current thesis examines the dynamic stability during human locomotion under such an approach using the concept of Lyapunov analysis. At first a methodological examination of the reliability of the maximum Lyapunov exponent in walking and running has been conducted (chapter 2). Afterwards, an examination between the habitual running condition and after removal of footwear was conducted, exhibiting a decrease in stability following the acute transition to the new condition (chapter 3). In the last study, a training intervention aiming at improvements in running energetics was performed using a short-term and a long-term intervention group (chapter 4). The results evidence that motor control errors can have a role in the energy cost of running and thus, a flexible prioritization of the motor control output.

The maximum Lyapunov exponent (MLE) has often been suggested as the prominent measure for evaluation of dynamic stability of locomotion in pathological and healthy population. Although the popularity of the MLE has increased in the last years, there is scarce information on the reliability of the method, especially during running. The purpose of the current study was, thus, to examine the reliability of the MLE during both walking and running. Sixteen participants walked and ran on a treadmill completing two measurement blocks (i.e., two trials per day for three consecutive days per block) separated by 2 months on average. Six different marker-sets on the trunk were analyzed. Intraday, interday and between blocks reliability was assessed using the intraclass correlation coefficient (ICC) and the root mean square difference (RMSD). The MLE was on average significantly higher (p < 0.001) in running (1.836 ± 0.080) compared to walking (1.386 ± 0.207). All marker-sets showed excellent ICCs (>0.90) during walking and mostly good ICCs (>0.75) during running. The RMSD ranged from 0.023 to 0.047 for walking and from 0.018 to 0.050 for running. The reliability was better when comparing MLE values between blocks (ICCs: 0.965–0.991 and 0.768–0.961; RMSD: 0.023–0.034 and 0.018–0.027 for walking and running respectively), and worse when considering trials of the same day (ICCs: 0.946–0.980 and 0.739–0.844; RMSD: 0.042–0.047 and 0.045–0.050 for walking and running respectively). Further, different marker-sets affect the reliability of the MLE in both walking and running. Our findings provide evidence that the assessment of dynamic stability using the MLE is reliable in both walking and running. More trials spread over more than 1 day should be considered in study designs with increased demands of accuracy independent of the locomotion condition.

The need to move over uneven terrain is a daily challenge. In order to face unexpected perturbations due to changes in the morphology of the terrain, the central nervous system must flexibly modify its control strategies. We analysed the local dynamic stability and the modular organisation of muscle activation (muscle synergies) during walking and running on an even- and an uneven-surface treadmill. We hypothesized a reduced stability during uneven-surface locomotion and a reorganisation of the modular control. We found a decreased stability when switching from even- to uneven-surface locomotion (p