125,364 research outputs found
Legged Locomotion Paradigms on Earth can Teach Humans how to Safely Extend their Progression Speed when Moving on the Moon
No full textWalk (W), run/trot (R/T) and skip/gallop (S/G) represent the universal gait types for bipeds/quadrupeds on Earth. Each type has been extensively studied by both metabolic and biomechanical research. The main characterizing aspect of those gaits resides in the different strategy to save mechanical energy within the body during progression, and consequently to minimize the energy expenditure: pendulum-like exchange between gravitational potential and kinetic energies (W), storage/release of elastic energy in tendons (R/T), and a combination of the two (S/G).
Almost 50 years ago, investigators started challenging human locomotion in different gravitational environments (Margaria & Cavagna, 1964) and rightly concluded that on the Moon walking should be possible only at very low speeds, and that 'terrestrial' running would have been mechanically difficult to adopt there. Later on, a general predicting equation for the speed of dynamically equivalent walking in hetero-gravity has been proposed and validated (Minetti 2001a, 2001b).
While studying the 3rd locomotor paradigm for humans, 'skipping' was biomechanically analysed in details and found to be particularly suitable for low gravity environments (Minetti 1998). That insight was confirmed by inspection of NASA footage of Apollo missions, where astronauts frequently preferred to skip.
We propose to set up a new, more focused study on human skipping (and small quadruped gallop) in simulated hypogravity during parabolic flights. By using a corridor of dynamometric platforms and 3D motion capture we intend to specifically check the predictions about those gaits and investigate the suitability of a custom training program for humans to adapt/enhance/extend their locomotion repertoire on the Lunar environment.
Bibliography
Margaria R., Cavagna G. Human locomotion in subgravity. Aerospace Med. 35: 1140-1146, 1964.
Minetti A. E. The biomechanics of skipping gaits: a third locomotor paradigm? Proc. R. Soc. B 265: 1227-1235, 1998.
Minetti A. E. Invariant aspects of human locomotion in different gravitational environments. Acta Astron. 49(3-10): 191-198, 2001.
Minetti A. E. Walking on other planets. Nature 409: 467-469, 2001
Comments on point:counterpoint: skeletal muscle mechanical efficiency does/does not increase with age.
No full textComments to the edito
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”
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
New Approaches to Reactive Power Sharing and Voltage Control in Islanded AC Microgrids
No full textAdvanced dynamic control strategies are
presented to approach primary voltage control and reactive
power sharing among Grid-Supporting inverters interfacing
production units in AC islanded Microgrids (MG). After
defining the MG structure and the power system model used in
the analytical analysis, new methods are introduced and
analyzed. Then, a setting procedure of control parameters is
developed in order to meet the requirements of a correct power
sharing, voltage stability and maintenance of acceptable voltage
profile. Next, a comparison among the new approaches and the
conventional droop results is performed, by means of dedicated
simulations on a common benchmark network in order to show
advantages of the new approaches
NETWORK OF LONG JOSEPHSON JUNCTIONS AS A SOURCE OF STRONG PINNING MECHANISMS IN HIGH TEMPERATURE SUPERCONDUCTING FILMS
No full textMechanical work rate minimization and freely chosen stride frequency of human walking : a mathematical model
No full text(external work) and the work done to move the limbs with respect to the body (internal work) has been shown experimentally partially to determine the freely chosen stride frequency during walking. A mathematical model that estimates the two components of the mechanical work is proposed. The model, according to the criterion of work rate minimization (both positive and positive plus negative), is able to predict the natural stride frequency as a function of the average progression speed. The adequacy of the model and the validity of the assumptions have been checked against measurements of natural stride frequency in 11 subjects walking on a treadmill at several speeds (range 1-3 m s-1). Comparison with theoretical predictions shows good agreement with the minimization of positive work rate at low speeds, while at high speeds the stride frequency is better explained by the model for minimum positive plus negative work rate
Optimal interval for periodical lead limb changes during straight gallop in race horses
No full textIn transverse gallop the leading feet (i.e. the second limb of each pair on the ground) of hind and forelimbs are ipsilateral. Therefore, we can distinguish a right-leading by a left-leading gallop.
When turning, a horse leads with its inside limbs. Consequently, during direction changes, right-hand bends are covered using right-leading gallop and vice-versa. Even if there are preferences for the leading foot on straights, probably due to individual asymmetries of the body, we also observe periodical lead changes. Potential reasons for those are: a) the interaction between the shoulder and the thoraco-pulmonar complex, which is maximum during the lead leg support, and b) musculo-skeletal stress induced by the gait asymmetry (the trunk sagittal plane deviated from the progression plane by 3o, pers. obs.).
Also by considering that the lead change discontinuity on straights is expected to decrease gallop performance, we hypothesized that there should be a consistently “optimal” number of strides or distance between lead changes.
From the analysis of 84 horse-race videos of different tracks in Italy, we found that 48 ± 20 strides were covered between two successive lead changes on straights. As expected, race distance, the number of bends to be covered and, possibly, the 'rider-factor' influenced the number of changes. The predominant leading leg used on straightaways is significantly different from that employed during bends in both clockwise and counter-clockwise racetracks
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