There has been a prevalence of long-distance running footwear incorporating carbon-fiber plates within their midsoles, effectively increasing their longitudinal bending stiffness (LBS). This modification of modern racing footwear has occurred concurrently with large improvements in running times (Bermon et al., 2021), putting into question how these footwear components affect performance (Muniz-Pardos et al., 2021). The current literature has investigated this at level running, but with the increasing popularity of trail running, it is of interest to investigate whether the benefits found during level running translate to graded running. Therefore, the overall aim of this study was to investigate the effects of increased footwear midsole longitudinal bending stiffness (i.e. carbon-fiber plates) on running energetics and biomechanics at various inclines. The effects of high LBS (Nike Vaporfly 4% with midsole intact) and low LBS (Nike Vaporfly 4% with mediolateral cuts made at the forefoot of the midsole through the carbon-fiber plate) footwear conditions were compared for running at 0°, 6°, and 12° inclines. Running energetics and biomechanics data were quantified by measuring metabolic rate and lower leg joint mechanics (from motion capture and ground reaction force measurements). Results from this study suggest that increasing longitudinal bending stiffness within the footwear midsoles has limited influence on running energetics (small non-significant improvements of metabolic power at all inclines), but has considerable effects on the biomechanics of the ankle and MTP joints. However, the most important between shoe differences were independent of grade, suggesting that the benefits of modern racing shoe observed for level running can be expected to translate to steep uphill running. Nevertheless, it should be noted that this study was only able to collect and use data for analysis from a limited number of participants (n=7), and therefore is underpowered, so there may be significant differences that go undetected

Le trail running, course nature de distances, dénivelés et technicités variables, est une discipline récente à la popularité croissante. La performance en trail dépend de nombreux facteurs génétiques, énergétiques, techniques, stratégiques et motivationnels. Parmi ces facteurs, le coût énergétique de la locomotion fait débat. Les études de cette Thèse s'attachent aux corrélations plat/montée de cette variable, au calcul d'un coût en pente à partir d'un coût à plat, à l'influence de la modification forcée de la fréquence de foulée sur l'énergie consommée, et à l'impact de la fatigue générée par un ultra trail sur le coût à plat et en montée. La prise en compte des contraintes musculaires et biomécaniques en trail d'une part, des facteurs influençant l'économie de course d'autre part, ainsi que des résultats de notre principale étude de terrain, nous laissent émettre l'hypothèse que le coût énergétique et ses variations pre/post course à plat/montée, ne sont pas des indicateurs de performance de l'activité. La force, l'endurance de force et le pacing, en plus de la puissance aérobie et de l'endurance, sont les variables testées les mieux corrélées à la performance en ultra trail / Trail running is a discipline with increasing popularity over the last 2 decades. Trail performance depends on many genetic, energetic, technical, strategic and motivational factors. Among these factors, the energy cost of locomotion is debated. The studies in this Thesis focus on the level/graded correlations of this variable, on the calculation of an uphill cost from a level cost, on the influence of a forced modification of stride frequency on running economy, and on the impact of fatigue generated by an ultra trail on level and uphill costs. Taking account of the muscular and biomechanical constraints on the one hand, and the factors influencing the running economy on the other hand, as well as the results of our main field study, we hypothesize that energy cost and its pre/post fatigue variations, are not performance indicators of the activity. Force, endurance of force and pacing, in addition to aerobic power and endurance, are the tested variables best correlated to ultra trail performance