100 research outputs found
Comments on point:counterpoint: skeletal muscle mechanical efficiency does/does not increase with age.
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Interplay among the changes of muscle strength, cross-sectional area and maximal explosive power : theory and facts
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BODY CENTER OF MASS TRAJECTORY SHOWS HOW RACE WALKERS ELUDE “FROUDE LAW”
No full textIntroduction
Froude number, Fr = v^2/gl (where v is the speed of progression (m/s), g is gravity acceleration (9.81m/s^2), l is the leg length (m)) is used to compare dynamic similar gait (Alexander 1989). Fr=1 defines the maximal speed value for pendulum-like locomotion such us walking. Race Walking is an Olympic discipline, which is supposed to be the fastest expression of walking. Assuming athletes’ leg length = 1m, the maximal speed according to “Froude Law” is 3.13m/s much lower than race pace (20km: 4.16m/s; 50km: 3.61m/s).
Alexander (1984) suggested a possible explanation of this discrepancy by differences in kinematic: he suggested that the straight knee at heel strike and the back movements would maximise the radius of the inverted pendular motion. The aim of the study was to analyse the race walkers’ Body Center of Mass (BCoM) pattern in order to find how they elude the “Froude Law”.
Methods
16 Athletes race-walked on a treadmill at incremental speed 2.77-4.72m/s at a step of 0.138m/s every minute. Kinematic acquisitions were made by 8 Vicon 1.3Mp Cameras at 300Hz. The mathematical description of the BCoM pattern was done with 10 harmonics Fourier Analysis and Lissajous Contour as explain by Minetti and co-workers (2011) with a custom-written software in LabVIEW (National Instrument, USA).
Results
Athletes’ leg length average 0.93±0.04m, which allowed for a theoretical (Fr=1) maximal speed of 3.02m/s. The Lissajous Contour of the BCoM during race walking showed a characteristic pattern different both from walking and running: during single support BCoM reached the lowest position, while it is highest during double support. At velocity ≤ 4.4m/s two forward ‘protrusions’ were present in the lowest part of contour.
Discussion
The pattern of race walkers’ BCoM was found to be dynamically opposite when compared to walking. In fact, during the single support phase of normal walking, BCoM shows the lowest speed and reaches the highest point of a contour resembling a circumference arc, as in an inverted pendulum. Also, the forward ‘protrusions’ of race walk contours indicate that speed increases in the middle of the support phase. This confirms that race walk rules constraint to adopt a trajectory different from walking but, by deviating from a circle arc, there is no issue of extending the radius. Although a variation of walking, race walking is not a pendulum-like gait, thus it does not undergo the “Froude Law”
The first humans travelling on ice: an energy-saving strategy?
No full textEconomy of locomotion is a constant challenge for animals, particularly when related to migrations and travelling. The present study focuses on human locomotion and particularly on the development of ice skating. The aim of our research was to understand whether an environmental feature such as a strong presence of lakes (frozen in winter) could force humans to develop ice skates in order to limit the energy cost of travelling. We hypothesized that the energy-saving principle was a determinant factor in the development of human locomotion on ice. Five healthy adult participants took part in the experiments, during which we recorded the speed (1.2 ± 0.3 m s−1) and metabolic energy cost (4.6 ± 0.9 J kg−1 m−1) associated with travelling on bone skates. Simulations were also performed to demonstrate whether the benefit given by the use of skates was different in the areas where ice skating appears to have evolved originally. The gain reachable by using bone skates could lead to an extremely high energy saving (equal to 10% of the energy needed to survive during the cold season) and differs significantly between the regions considered in the present study. An analysis of the geometrical shape of lakes associated with fractal analysis of their distribution suggests that, in order to better adapt to the severe conditions imposed by the long lasting winters, Finnish populations could benefit more than others from developing this ingenious locomotion tool. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 93, 1–7
Muscular exercise at high altitude. IV. Oxygen transport system before and after exposure to chronic hypoxia
No full textThe interplay of central and peripheral factors in limiting maximal O2 consumption in man : the effects of prolonged bed rest.
No full textHuman locomotion on ice: the evolution of ice-skating energetics through history.
No full textMore than 3000 years ago, peoples living in the cold North European regions started developing tools such as ice skates that allowed them to travel on frozen lakes. We show here which technical and technological changes determined the main steps in the evolution of ice-skating performance over its long history. An in-depth historical research helped identify the skates displaying significantly different features from previous models and that could consequently determine a better performance in terms of speed and energy demand. Five pairs of ice skates were tested, from the bone-skates, dated about 1800 BC, to modern ones. This paper provides evidence for the fact that the metabolic cost of locomotion on ice decreased dramatically through history, the metabolic cost of modern ice-skating being only 25% of that associated with the use of bone-skates. Moreover, for the same metabolic power, nowadays skaters can achieve speeds four times higher than their ancestors could. In the range of speeds considered, the cost of travelling on ice was speed independent for each skate model, as for running. This latter finding, combined with the accepted relationship between time of exhaustion and the sustainable fraction of metabolic power, gives the opportunity to estimate the maximum skating speed according to the distance travelled. Ice skates were probably the first human powered locomotion tools to take the maximum advantage from the biomechanical properties of the muscular system: even when travelling at relatively high speeds, the skating movement pattern required muscles to shorten slowly so that they could also develop a considerable amount of force
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