PRINCIPAL COMPONENTS OF THE 180°-TURN WITH THE BALL KINEMATICS IN YOUTH SOCCER PLAYERS

The interpretation of parameters extracted from sophisticated sport-specific techniques is not always straightforward. This is the case of the 180° change of direction with the ball in soccer. The 180°-turn performance of ten Under-13, sub-elite soccer players was recorded by means of a motion capture system. Principal Component Analysis (PCA) was applied to a set of 21 anthropometrics and kinematic variables including center of mass related quantities and joints range of motion. The first three principal components, explaining 61% of the overall variance, were retained and discussed. PCA unveiled the relevant structure of the dataset, describing both movement speed and amplitude issues, and the relationship between body size and the change of direction ability

Design and Development of Flow Fields with Multiple Inlets or Outlets in Vanadium Redox Flow Batteries

In vanadium redox flow batteries, the flow field geometry plays a dramatic role on the distribution of the electrolyte and its design results from the trade-off between high battery performance and low pressure drops. In the literature, it was demonstrated that electrolyte permeation through the porous electrode is mainly regulated by pressure difference between adjacent channels, leading to the presence of under-the-rib fluxes. With the support of a 3D computational fluid dynamic model, this work presents two novel flow field geometries that are designed to tune the direction of the pressure gradients between channels in order to promote the under-the-rib fluxes mechanism. The first geometry is named Two Outlets and exploits the splitting of the electrolyte flow into two adjacent interdigitated layouts with the aim to give to the pressure gradient a more transverse direction with respect to the channels, raising the intensity of under-therib fluxes and making their distribution more uniform throughout the electrode area. The second geometry is named Four Inlets and presents four inlets located at the corners of the distributor, with an interdigitated-like layout radially oriented from each inlet to one single central outlet, with the concept of reducing the heterogeneity of the flow velocity within the electrode. Subsequently, flow fields performance is verified experimentally adopting a segmented hardware in symmetric cell configuration with positive electrolyte, which permits the measurement of local current distribution and local electrochemical impedance spectroscopy. Compared to a conventional interdigitated geometry, both the developed configurations permit a significant decrease in the pressure drops without any reduction in battery performance. In the Four Inlets flow field the pressure drop reduction is more evident (up to 50%) due to the lower electrolyte velocities in the feeding channels, while the Two Outlets configuration guarantees a more homogeneous current density distribution

A Novel Accelerated Stress Test for a Representative Enhancement of Cathode Degradation in Direct Methanol Fuel Cells

Performance decay of direct methanol fuel cells hinders technology competitiveness. The cathode electrochemical surface area loss is known to be a major reason for performance loss and it is mainly affected by cathode potential and dynamics, locally influenced by water and methanol crossover. To mitigate such phenomenon, novel materials and components need to be developed and intensively tested in relevant operating conditions. Thus, the development of representative accelerated stress tests is crucial to reduce the necessary testing time to assess material stability. In the literature, the most diffused accelerated stress tests commonly enhance a specific degradation mechanism, each resulting in limited representativeness of the complex combination and interaction of mechanisms involved during real-life operation. This work proposes a novel accelerated stress test procedure permitting a quantifiable and predictable acceleration of cathode degradation, with the goal of being representative of the real device operation. The results obtained with a 200 h accelerated stress test are validated by comparing both in situ and post mortem measurements with those performed during a 1100 h operational test, demonstrating an acceleration factor equal to 6.25x and confirming the development of consistent cathode degradation

DIFFERENCES IN LANDING STRATEGIES DURING A VERTICAL DROP JUMP TEST AS A FUNCTION OF GENDER AND AGE IN YOUTH ÉLITE SOCCER PLAYERS

We analyzed the kinematics and kinetics of vertical drop jump landing in a group of 85 (20 females) elite youth soccer players (U10-U11 and U17). Peak knee flexion and knee varus increased in male players as a function of age; normalized peak vertical ground reaction forces decreased with age and in female players. Preliminary data can be used to build reference tables for this specific population and can help to correct dangerous landing strategies, and to keep each player’s development monitored. We also suggest that education of safe landing technique would prevent dangerous knee postures

FATIGUE ALTERS THE BIOMECHANICS OF TURNS WHILE RUNNING

This study identified the effects of fatigue on lower limb kinematics while running with repeated 180°-turns. An increased stiffness of the pivoting limb was observed in terms of a reduction of hip and knee flexion angles, and an increase of hip abduction and internal rotation. We concluded that muscle fatigue can trigger a sequence of adaptations that were previously found to expose the athlete to an increasing risk of ligament injury. These results expand the base of evidence for the development of field-based prevention programs

Identifying the optimal body shape and composition associated with strength outcomes in children and adolescent according to place of residence: an allometric approach

The purpose of the study was to identify the optimal body shape and composition associated with physical fitness levels of children living in urban and rural areas of Italy. A total of 7102 children (11–14 years) were assessed for weight, height, percentage body fat (FM%), sit-and-reach flexibility (SAR), standing broad jump (SBJ) and sit-ups (SUP). A multiplicative allometric model, Y = a · massk1 · heightk2 ·ε, was used to predict the physical outcome variables Y = SBJ and SUP. The model was expanded to incorporate FM% and SAR as follows Y = a · massk1 · heightk2 · FM%k3 · exp(b· FM% + c· SAR) ·ε. Note that FM% was incorporated as a “gamma function” that allows an initial growth, and subsequent decline in Y as FM% increases in size. Although having an ectomorph body shape appears advantageous, being too thin appears detrimental to the strength outcomes. Being flexible would also benefit physical fitness levels. Finally, our results indicate that ursban children aged 11–14 have superior strength outcomes compared with rural children, having controlled for differences in body shape and composition, a finding that may be associated with rural environments having fewer exercise facilities compared with urban conurbations

A KINEMATICALLY BASED ALGORITHM TO ESTIMATE THE ENERGY COST OF VARIABLE-SPEED SHUTTLE RUNNING

Changes of direction (CoDs) have a high metabolic and mechanical impact in field and court team sports, but the estimation of the associated workload is still inaccurate. The aim of this study is to establish a kinematic-based algorithm to determine the energy cost of running at variable speed with frequent 180° CoDs. Kinematic and metabolic data were simultaneously collected during 5-minutes 5+5 m shuttle run tests. Mechanical work computation was split into positive (eccentric) and negative (concentric) contributions. When compared to the actual energy cost, the estimation algorithm returned an error of 5%. This model constitutes the theoretical basis to extend the model from the laboratory to the field, obtaining an accurate measure of the workload of training and matches

DECELERATION COUNTS: ESTIMATING THE ENERGY COST OF SHUlTLE RUNNING FROM MECHANICAL WORK

To estimate the energetic requirements of 5-m shuttle running based on kinematic data, we devised a modified version of existing models for the estimation of the energy cost of gait. In our approach, negative/eccentric work during deceleration phases was added to positive/concentric work in propulsive phases. Ten subjects performed two 5-rnin trials at 50% and 75% of their maximal aerobic speed. The metabolic cost estimated from 30 kinematics was compared to that measured by a portable metabolimeter. The estimation error was 1.2 J/kg/s (7.3%): results encourage to apply this method for the estimation of the workload in sports involving frequent turns and changes of direction