Graphics Reference
In-Depth Information
Before considering the sequence of leg movements for a quadruped, let's look
at the action of an individual leg. This movement of a single leg during a walk
cycle may be divided into different phases. Because this movement is part of a
cycle, there is no distinct starting position, just a given point within the cycle.
I have chosen the front leg of a horse as an illustration for this action, though
the principle holds for other quadrupeds.
The two main positions of a leg during a walk cycle are the stride and the
passing position.
Passing position
is a term used for a phase within the walk
cycle when the paired legs are positioned alongside one another. One leg
is held rigid and straight and supports the weight of the animal (or person);
the other leg is held bent as it swings forward and passes the supporting
leg. Cursorial mammals and birds move their legs along parasagittal
planes. That is, they move them parallel to the length of the animal's body.
In this example of a passing position, the feet are located directly below
the position of the animal's shoulder or hips. Noncursorial animals move
with their legs held at an angle away from the body; as a consequence the
foot is not located directly beneath the shoulder or hip during the passing
position.
Stride
describes the moment within the walk cycle when the paired legs are
outstretched in opposite directions, one facing forward in anticipation of
taking on part of the duty factor of the walk while the other is held backward,
having just completed its contribution to the duty factor.
The sequence of a complete walk cycle is much more complex and involves
the transition from one position, the passing position, to the other, the stride.
Let's examine this scenario more fully.
Phase One
This phase begins with the leg stretched backward to its furthest extent. From
this position the leg begins to move forward from its backwardly extended
position and moves with the foot lifting clear from the ground and a bend at
the elbow and at the wrist. As the foot is lifted, the shoulder is also lifted to
assist in raising the foot clear off the ground. This leg finishes this phase with
the elbow located almost directly beneath the shoulder as it passes the now
upright supporting leg on the opposite side of the body. The leg is no longer
straight and the angle between radius and the third metacarpal bone is at its
most acute.
Phase Two
The front leg begins this phase as it passes the supporting leg (the passing
position). It has gained momentum and continues to swing forward, and as it
does so the leg begins to straighten at the elbow and the wrist. As the leg reaches
its forwardmost position, the foot is located well ahead of the shoulder. The foot
has swung forward to present the underside parallel to the ground and ready
for it to be placed flat on the ground. During this phase the shoulder starts to
