JEB desktop wallpaper calendar 2016

JEB desktop wallpaper calendar 2016

SUMMARY

Elephants are the biggest living terrestrial animal, weighing up to five tons and measuring up to three metres at the withers. These exceptional dimensions provide certain advantages (e.g. the mass-specific energetic cost of locomotion is decreased) but also disadvantages (e.g. forces are proportional to body volume while supportive tissue strength depends on their cross-sectional area, which makes elephants relatively more fragile than smaller animals). In order to understand better how body size affects gait mechanics the movement of the centre of mass (COM) of 34 Asian elephants (Elephas maximus) was studied over their entire speed range of 0.4-5.0 m s−1 with force platforms. The mass-specific mechanical work required to maintain the movements of the COM per unit distance is ~0.2 J kg−1 m−1 (about 1/3 of the average of other animals ranging in size from a 35 g kangaroo rat to a 70 kg human). At low speeds this work is reduced by a pendulum-like exchange between the kinetic and potential energies of the COM, with a maximum energy exchange of ~60% at 1.4 m s−1. At high speeds, elephants use a bouncing mechanism with little exchange between kinetic and potential energies of the COM, although without an aerial phase. Elephants increase speed while reducing the vertical oscillation of the COM from about 3 cm to 1 cm.

  • LIST OF ABBREVIATIONS

    av, af, al
    acceleration of the COM in the vertical, forward and lateral directions, respectively
    COM
    centre of mass
    Ecom
    total mechanical energy of the COM, Ecom=Ep+Ekv+Ekf+Ekl=Ep+Ek
    Ek
    kinetic energy of the COM
    Ekv, Ekf, Ekl
    kinetic energy of the vertical, forward and lateral motions of the COM, respectively
    Ep
    potential energy of the COM
    f
    step frequency
    fh
    normalised step frequency measured from the above-body-weight contact period
    fn
    normalised natural frequency of a spring—mass model
    fsn
    normalised step frequency
    Embedded Image
    average vertical force over an integral number of steps
    Fv, Ff, Fl
    vertical, forward and lateral components of the ground reaction forces, respectively
    Fvmax, Fvmin
    maximum and minimum vertical forces, respectively
    g
    gravitational acceleration
    h
    limb length
    L
    stride length
    Lf, Lh
    normalised forward distance travelled by the COM while, respectively, the fore or hindlimb is on the ground
    Ln
    normalised stride length
    Mb
    body mass
    Pb
    body weight
    %R
    relative amount of energy recovered by the pendular exchange between the kinetic and potential energies of the COM
    Sv
    vertical displacement of the COM over one step cycle
    th
    half cycle period
    Vv, Vf, Vl
    the instantaneous velocity of the COM in the vertical, forward and lateral directions, respectively
    Embedded Image
    average forward speed
    Wext
    positive work done to maintain the movement of the COM, Wext is the sum of the increments in Ecom over an integral number of steps
    Wf
    positive work done to accelerate the COM forwards, Wf is the sum of the increments in Ekf over an integral number of steps
    Wl
    positive work done to accelerate the COM laterally, Wl is the sum of the increments in Ekl over an integral number of strides
    Wv
    positive work done against gravity, Wv is the sum of the increments in Ep+Ekv over an integral number of steps
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