Re: Definitions of running (was RE: RE: Complaining)

[Vivian Allen <mrvivianallen@googlemail.com>]

Sorry, to repeat, dunno if the last one actually got through. Yes,
there is a specific definition of running:

An animal moving at steady speed on swinging legs must place it's leg
in front of it's centre of
mass for the first part of the stance period, and it's leg behind it
for the last part of the stance period. To stay upright and balanced,
this means that forces applied by the feet are pointing backwards
(i.e. braking) in the first part of stance, and pointing forwards
(i.e. accelerating) in the last part. To increase efficiency, it makes
sense to try and recover the energy lost in braking and re-use to
assist in accelerating. Walking and running are two separate classes
of energy recovery mechanism. In walking, you use the energy lost in
braking to raise your centre of mass upwards, turning your forwards
speed (ke) into potential energy (pe), vaulting over a relatively
stiff limb. In the last part of stance, you let you centre of mass
fall fowards on a relatively stiff limb, turning your pe back into
fowards speed, into ke. Hence ke and pe are out of phase with each
other in a walk - a 'pure' walk would have pe and ke fluctuating in
exactly opposite patterns, so 180 degrees out of phase (antiphase).

In a run, a different mechanism is used. Here, both the ke AND the pe
are used to compress biological springs (tendons) during the braking
phase, and it is the release of these springs that helps power the
accelerative phase. So in the braking phase, the body boths slows and
falls downwards, both of which compress the forwards/downwards facing
contact limb and stretch out it's springy tendons. In the accelerative
phase the limb recoils, speeding the body up and projecting it
forwards AND upwards. This is the 'bounce'. Hence ke and pe, working
together to compress the springs in the limb, are acting in phase with
each other. Again, a 'pure' run would have ke and pe exactly in phase
(0 degrees phase difference).

So, this phase-difference implies that there is a continuum between
walking and running, with two 'pure' endstates and a infinite variety
of intermediate states. This is true, theoretically you could use any
gait between a walk and a run, using different proportions of
gravitational vs elastic energy storage to transfer braking energy to
accelerating energy. However, at different speeds, given the size of
the animal, limb length, tendon properties etc, it will be
energetically more efficient to use one or the other. The maths can
get a bit involved, and I would be lying if I said I perfectly
understood gait transitions, but it is generally a case that animals
will switch from gravitational to elastic storage quite quickly over a
narrow range of speed differences. One thing that seems to be involved
is simply that at high speeds, at high ke, you would begin leaving the
ground if you use a walking gait anyway - the forwards energy is
greater than body weight, and so when you vault over a stiffened limb,
the energy exchange between ke and pe is enough to counteract gravity
through centripetal acceleration. Once you have left the ground, you
have a landing to collision to deal with, and it probably makes sense
to switch to a running gait and store that landing impact in springy
tendons.

But anyway, that is the biomechanically agreed-upon definition of
walking and running. Unlike footfall patterns it describes grounded
and aerial running accurately, as it is energy patterns and mechanical
limb use that is key, not whether or not you 'bounce' high enough to
leave the ground.

ELEPHANTS:
John's group found evidence of in-phase fluctuation in ke and pe in a
relatively fast moving elephant. I.E they found evidence of some
running, in the back legs AFAIK. You can use your own definition of
running by all means, but seeing as the relative phase of fluctuations
in the kinetic and potential energy of the centre of mass is the
agreed upon and used definition by those in the field of locomotion
study (and has been for what, 30 years or so?),  to do so is
unscientific.

Viv

On 1 May 2013 15:44,  <GSP1954@aol.com> wrote:
> Not really. Tetrapods with specialized limbs like elephants and sauropods
> are adapted to move at a distinctive, slow gait that is not normal for their
> groups, must of whose members can achieve a suspended phase run. Elephants
> should not be said to "run" like crocs, rhinos, horses, cats and hippos. Nor
> should elephants be said the be limited to just walking like tortoises.
> Technical terminology should always be specific, not simplistic. So elephants
> are amblers, or semi-runners.
>
> GSPaul
>
>
> In a message dated 4/30/13 9:29:34 PM, biologyinmotion@gmail.com writes:
>
> << The distinctions you propose would be particularly useful from the
> standpoint of creating animations or other visualizations where the specific
> "look" of the gait is critical, regardless of the underlying physics.  Given 
> the
> source concerns for the thread, I suspect this is exactly your point.
>
> --Mike H. >>
>
> </HTML>