Professional runners in many cases at matches reduce the length of
their anterior extremities by flexing their arms and carrying them
on a level with their chest (fig. 28, p. 62). It would seem that in
rapid running there is not time for the arms to oscillate naturally,
and that under these circumstances the arms, if allowed to swing
about, retard rather than increase the speed. The centre of gravity
is well forward in the ostrich, and is regulated by the movements of
the head and neck, and the obliquity of the body and legs. In running
the neck is stretched, the body inclined forward, and the legs moved
alternately and with great rapidity. When the right leg is flexed and
elevated, it swings forward pendulum-fashion, and describes a curve
whose convexity is directed towards the right side. When the left leg
is flexed and elevated, it swings forward and describes a curve whose
convexity is directed towards the left side. The curves made by the
right and left legs form when united a waved line (_vide_ figs. 18, 19,
and 20, pp. 37, 39, and 41). When the right leg is flexed, elevated,
and advanced, it rotates upon the iliac portion of the trunk of the
bird, the trunk being supported for the time being by the left leg,
which is extended, and in contact with the ground. When the left leg
is flexed, elevated, and advanced, it in like manner rotates upon the
trunk, supported in this instance by the extended right leg. The leg
which is on the ground for the time being supplies the necessary lever,
the ground the fulcrum. When the right leg is flexed and elevated, it
rotates upon the iliac portion of the trunk in a forward direction,
the right foot describing the arc of a circle. When the right leg and
foot are extended and fixed on the ground, the trunk rotates upon the
right foot in a forward direction to form the arc of a circle, which is
the converse of that formed by the right foot. If the arcs alternately
supplied by the right foot and trunk are placed in opposition, a more
or less perfect circle is produced, and thus it is that the locomotion
of animals is approximated to the wheel in mechanics. Similar remarks
are to be made of the left foot and trunk. The alternate rolling of the
trunk on the extremities, and the extremities on the trunk, utilizes
or works up the inertia of the moving mass, and powerfully contributes
to continuity and steadiness of action in the moving parts. By
advancing the head, neck, and anterior parts of the body, the ostrich
inaugurates the rolling movement of the trunk, which is perpetuated by
the rolling movements of the legs. The trunk and legs of the ostrich
are active and passive by turns. The movements of the trunk and limbs
are definitely co-ordinated. But for this reciprocation the action of
the several parts implicated would neither be so rapid, certain, nor
continuous. The speed of the ostrich exceeds that of every other land
animal, a circumstance due to its long, powerful legs and great stride.
It can outstrip without difficulty the fleetest horses, and is only
captured by being simultaneously assailed from various points, or run
down by a succession of hunters on fresh steeds. If the speed of the
ostrich, which only measures six or eight feet, is so transcending,
what shall we say of the speed of the extinct _Æpyornis maximus_ and
_Dinornis giganteus_, which are supposed to have measured from sixteen
to eighteen feet in height? Incredible as it may appear, the ostrich,
with its feet reduced to a minimum as regards size, and peculiarly
organized for walking and running on solids, can also swim. Mr. Darwin,
that most careful of all observers, informs us that ostriches take to
the water readily, and not only ford rapid rivers, but also cross from
island to island. They swim leisurely, with neck extended, and the
greater part of the body submerged.
_Locomotion in Man._--The speed attained by man, although considerable,
is not remarkable. It depends on a variety of circumstances, such as
the height, age, sex, and muscular energy of the individual, the nature
of the surface passed over, and the resistance to forward motion due to
the presence of air, whether still or moving. A reference to the human
skeleton, particularly its inferior extremities, will explain why the
speed should be moderate.
On comparing the inferior extremities of man with the legs of birds,
or the posterior extremities of quadrupeds, say the horse or deer, we
find that the bones composing them are not so obliquely placed with
reference to each other, neither are the angles formed by any two
bones so acute. Further, we observe that in birds and quadrupeds the
tarsal and metatarsal bones are so modified that there is an actual
increase in the number of the angles themselves. In the extremities
of birds and quadrupeds there are four angles, which may be increased
or diminished in the operations of locomotion. Thus, in the quadruped
and bird (fig. 4, p. 21, and fig. 24, p. 47), the femur forms with the
ilium one angle (_a_); the tibia and fibula with the femur a second
angle (_b_); the cannon or tarso-metatarsal bone with the tibia and
fibula a third angle (_c_); and the bones of the foot with the cannon
or tarso-metatarsal bone a fourth angle (_d_). In man three angles only
are found, marked respectively _a_, _b_, and _c_ (figs. 26 and 27,
pp. 55 and 59). The fourth angle (_d_ of figs. 4 and 24) is absent. The
absence of the fourth angle is due to the fact that in man the tarsal
and metatarsal bones are shortened and crushed together; whereas in the
quadruped and bird they are elongated and separated.
As the speed of a limb increases in proportion to the number and
acuteness of the angles formed by its several bones, it is not
difficult to understand why man should not be so swift as the majority
of quadrupeds. The increase in the number of angles increases the power
which an animal has of shortening and elongating its extremities, and
the levers which the extremities form. To increase the length of a
lever is to increase its power at one end, and the distance through
which it moves at the other; hence the faculty of bounding or leaping
possessed in such perfection by many quadrupeds.[34] If the wing be
considered as a lever, a small degree of motion at its root produces an
extensive sweep at its tip. It is thus that the wing is enabled to work
up and utilize the thin medium of the air as a buoying medium.
[34] “The posterior extremities in both the lion and tiger are
longer, and the bones inclined more obliquely to each other than the
anterior, giving them greater power and elasticity in springing.”
Another drawback to great speed in man is his erect position. Part of
the power which should move the limbs is dedicated to supporting the
trunk. For the same reason the bones of the legs, instead of being
obliquely inclined to each other, as in the quadruped and bird, are
arranged in a nearly vertical spiral line. This arrangement increases
the angle formed by any two bones, and, as a consequence, decreases
the speed of the limbs, as explained. A similar disposition of the
bones is found in the anterior extremities of the elephant, where
the superincumbent weight is great, and the speed, comparatively
speaking, not remarkable. The bones of the human leg are beautifully
adapted to sustain the weight of the body and neutralize shock.[35]
Thus the femur or thigh bone is furnished at its upper extremity with
a ball-and-socket joint which unites it to the cup-shaped depression
(acetabulum) in the ilium (hip bone). It is supplied with a neck which
carries the body or shaft of the bone in an oblique direction from
the ilium, the shaft being arched forward and twisted upon itself
to form an elongated cylindrical screw. The lower extremity of the
femur is furnished with spiral articular surfaces accurately adapted
to the upper extremities of the bones of the leg, viz. the tibia and
fibula, and to the patella. The bones of the leg (tibia and fibula) are
spirally arranged, the screw in this instance being split up. At the
ankle the bones of the leg are applied to those of the foot by spiral
articular surfaces analogous to those found at the knee-joint. The
weight of the trunk is thus thrown on the foot, not in straight lines,
but in a series of curves. The foot itself is wonderfully adapted to
receive the pressure from above. It consists of a series of small bones
(the tarsal, metatarsal, and phalangeal bones), arranged in the form
of a double arch; the one arch extending from the heel towards the
toes, the other arch across the foot. The foot is so contrived that
it is at once firm, elastic, and moveable,--qualities which enable
it to sustain pressure from above, and exert pressure from beneath.
In walking, the heel first reaches and first leaves the ground. When
the heel is elevated the weight of the body falls more and more on
the centre of the foot and toes, the latter spreading out[36] as in
birds, to seize the ground and lever the trunk forward. It is in this
movement that the wonderful mechanism of the foot is displayed to most
advantage, the multiplicity of joints in the foot all yielding a little
to confer that elasticity of step which is so agreeable to behold, and
which is one of the characteristics of youth. The foot may be said to
roll over the ground in a direction from behind forwards. I have stated
that the angles formed by the bones of the human leg are larger than
those formed by the bones of the leg of the quadruped and bird. This is
especially true of the angle formed by the femur with the ilium, which,
because of the upward direction given to the crest of the ilium in man,
is so great that it virtually ceases to be an angle.
[35] “The pelvis receives the whole weight of the trunk and
superposed organs, and transmits it to the heads of the femurs.”
[36] The spreading action of the toes is seen to perfection in
children. It is more or less destroyed in adults from a faulty
principle in boot and shoemaking, the soles being invariably too
narrow.
The bones of the superior extremities in man merit attention from the
fact that in walking and running they oscillate in opposite directions,
and alternate and keep time with the legs, which oscillate in a similar
manner. The arms are articulated at the shoulders by ball-and-socket
joints to cup-shaped depressions (glenoid cavities) closely resembling
those found at the hip-joints. The bone of the arm (humerus) is carried
away from the shoulder by a short neck, as in the thigh-bone (femur).
Like the thigh-bone it is twisted upon itself and forms a screw. The
inferior extremity of the arm bone is furnished with spiral articular
surfaces resembling those found at the knee. The spiral articular
surfaces of the arm bone are adapted to similar surfaces existing on
the superior extremities of the bones of the forearm, to wit, the
radius and ulna. These bones, like the bones of the leg, are spirally
disposed with reference to each other, and form a screw consisting of
two parts. The bones of the forearm are united to those of the wrist
(carpal) and hand (metacarpal and phalangeal) by articular surfaces
displaying a greater or less degree of spirality. From this it follows
that the superior extremities of man greatly resemble his inferior
ones; a fact of considerable importance, as it accounts for the part
taken by the superior extremities in locomotion. In man the arms do
not touch the ground as in the brutes, but they do not on this account
cease to be useful as instruments of progression. If a man walks with
a stick in each hand the movements of his extremities exactly resemble
those of a quadruped.
The bones of the human extremities (superior and inferior) are seen to
advantage in fig. 26; and I particularly direct the attention of the
reader to the ball-and-socket or universal joints by which the arms are
articulated to the shoulders (_x_, _x´_), and the legs to the pelvis
(_a_, _a´_), as a knowledge of these is necessary to a comprehension of
the oscillating or pendulum movements of the limbs now to be described.
The screw configuration of the limbs is well depicted in the left arm
(_x_) of the present figure. Compare with the wing of the bird, fig. 6,
and with the anterior extremity of the elephant, fig. 7, p. 28. But
for the ball-and-socket joints, and the spiral nature of the bones and
articular surfaces of the extremities, the undulating, sinuous, and
more or less continuous movements observable in walking and running,
and the twisting, lashing, flail-like movements necessary to swimming
and flying, would be impossible.
[Illustration: FIG. 26.--Skeleton of Man. Compare with fig. 4, p. 21,
and fig. 24, p. 47.--_Original._]
The leg in the human subject moves by three joints, viz., the hip,
knee, and ankle joints. When standing in the erect position, the
hip-joint only permits the limb to move forwards, the knee-joint
backwards, and the ankle-joint neither backwards nor forwards. When the
body or limbs are inclined obliquely, or slightly flexed, the range
of motion is increased. The greatest angle made at the knee-joint
is equal to the sums of the angles made by the hip and ankle joints
when these joints are simultaneously flexed, and when the angle of
inclination made by the foot with the ground equals 30°.
From this it follows that the trunk maintains its erect position during
the extension and flexion of the limbs. The step in walking was divided
by Borelli into two periods, the one corresponding to the time when
both limbs are on the ground; the other when only one limb is on the
ground. In running, there is a brief period when both limbs are off the
ground. In walking, the body is alternately supported by the right and
left legs, and advanced by a sinuous movement. Its forward motion is
quickened when one leg is on the ground, and slowed when both are on
the ground. When the limb (say the right leg) is flexed, elevated, and
thrown forward, it returns if left to itself (_i.e._ if its movements
are not interfered with by the voluntary muscles) to the position from
which it was moved, viz. the vertical, unless the trunk bearing the
limb is inclined in a forward direction at the same time. The limb
returns to the vertical position, or position of rest, in virtue of the
power exercised by gravity, and from its being hinged at the hip by a
ball-and-socket joint, as explained. In this respect the human limb
when allowed to oscillate exactly resembles a pendulum,--a fact first
ascertained by the brothers Weber. The advantage accruing from this
arrangement, as far as muscular energy is concerned, is very great, the
muscles doing comparatively little work.[37] In beginning to walk, the
body and limb which is to take the first step are advanced together.
When, however, the body is inclined forwards, a large proportion
of the step is performed mechanically by the tendency which the
pendulum formed by the leg has to swing forward and regain a vertical
position,--an effect produced by the operation of gravity alone. The
leg which is advanced swings further forward than is required for the
step, and requires to swing back a little before it can be deposited on
the ground. The pendulum movement effects all this mechanically. When
the limb has swung forward as far as the inclination of the body at the
time will permit, it reverses pendulum fashion; the back stroke of the
pendulum actually placing the foot upon the ground by a retrograde,
descending movement. When the right leg with which we commenced is
extended and firmly placed upon the ground, and the trunk has assumed
a nearly vertical position, the left leg is flexed, elevated, and the
trunk once more bent forward. The forward inclination of the trunk
necessitates the swinging forward of the left leg, which, when it has
reached the point permitted by the pendulum movement, swings back again
to the extent necessary to place it securely upon the ground. These
movements are repeated at stated and regular intervals. The retrograde
movement of the limb is best seen in slow walking. In fast walking the
pendulum movement is somewhat interrupted from the limb being made to
touch the ground when it attains a vertical position, and therefore
before it has completed its oscillation.[38] The swinging forward of
the body may be said to inaugurate the movement of walking. The body
is slightly bent and inclined forwards at the beginning of each step.
It is straightened and raised towards the termination of that act.
The movements of the body begin and terminate the steps, and in this
manner regulate them. The trunk rises vertically at each step, the head
describing a slight curve well seen in the walking of birds. The foot
on the ground (say the right foot) elevates the trunk, particularly
its right side, and the weight of the trunk, particularly its left
side, depresses the left or swinging foot, and assists in placing it
on the ground. The trunk and limbs are active and passive by turns. In
walking, a spiral wave of motion, most marked in an antero-posterior
direction (although also appearing laterally), runs through the
spine. This spiral spinal movement is observable in the locomotion
of all vertebrates. It is favoured in man by the antero-posterior
curves (cervical, dorsal, and lumbar) existing in the human vertebral
column. In the effort of walking the trunk and limbs oscillate on the
ilio-femoral articulations (hip-joints). The trunk also rotates in
a forward direction on the foot which is placed upon the ground for
the time being. The rotation begins at the heel and terminates at the
toes. So long as the rotation continues, the body rises. When the
rotation ceases and one foot is placed flat upon the ground, the body
falls. The elevation and rotation of the body in a forward direction
enables the foot which is off the ground for the time being to swing
forward pendulum fashion; the swinging foot, when it can oscillate no
further in a forward direction, reversing its course and retrograding
to a slight extent, at which juncture it is deposited on the ground,
as explained. The retrogression of the swinging foot is accompanied
by a slight retrogression on the part of the body, which tends at
this particular instant to regain a vertical position. From this it
follows that in slow walking the trunk and the swinging foot advance
together through a considerable space, and retire through a smaller
space; that when the body is swinging it rotates upon the ilio-femoral
articulations (hip-joints) as an axis; and that when the leg is not
swinging, but fixed by its foot upon the ground, the trunk rotates upon
the foot as an axis. These movements are correlated and complementary
in their nature, and are calculated to relieve the muscles of the legs
and trunk engaged in locomotion from excessive wear and tear.
[37] The brothers Weber found that so long as the muscles exert the
general force necessary to execute locomotion, the velocity depends
on the size of the legs and on external forces, but _not on the
strength of the muscles_.
[38] “In quick walking and running the swinging leg never passes
beyond the vertical which cuts the head of the femur.”
Similar movements occur in the arms, which, as has been explained,
are articulated to the shoulders by ball-and-socket joints (fig. 26,
_x_ _x_´, p. 55). The right leg and left arm advance together to make
one step, and so of the left leg and right arm. When the right leg
advances the right arm retires, and _vice versâ_. When the left leg
advances the left arm retires, and the converse. There is therefore a
complementary swinging of the limbs on each side of the body, the leg
swinging always in an opposite direction to the arm on the same side.
There is, moreover, a diagonal set of movements, also complementary
in character: the right leg and left arm advancing together to form
one step; the left leg and right arm advancing together to form the
next. The diagonal movements beget a lateral twisting of the trunk and
limbs; the oscillation of the trunk upon the limbs or feet, and the
oscillation of the feet and limbs upon the trunk, generate a forward
wave movement, accompanied by a certain amount of vertical undulation.
The diagonal movements of the trunk and extremities are accompanied
by a certain degree of lateral curvature; the right leg and left
arm, when they advance to make a step, each describing a curve, the
convexity of which is directed to the right and left respectively.
Similar curves are described by the left leg and right arm in making
the second or complementary step. When the curves formed by the right
and left legs or the right and left arms are joined, they form waved
tracks symmetrically arranged on either side of a given line. The
curves formed by the legs and arms intersect at every step, as shown
at fig. 19, p. 39. Similar curves are formed by the quadruped when
walking (fig. 18, p. 37), the fish when swimming (fig. 32, p. 68), and
the bird when flying (figs. 73 and 81, pp. 144 and 157).
[Illustration:
FIG. 27 shows the simultaneous positions of both legs during a step,
divided into four groups. The first group (_A_), 4 to 7, gives the
different positions which the legs simultaneously assume while both
are on the ground; the second group (_B_), 8 to 11, shows the various
positions of both legs at the time when the posterior leg is elevated
from the ground, but behind the supported one; the third group (_C_),
12 to 14, shows the positions which the legs assume when the swinging
leg overtakes the standing one; and the fourth group (_D_), 1 to 3,
the positions during the time when the swinging leg is propelled in
advance of the resting one. The letters _a_, _b_, and _c_ indicate
the angles formed by the bones of the right leg when engaged in
making a step. The letters _m_, _n_, and _o_, the positions assumed
by the right foot when the trunk is rolling over it. _g_ Shows the
rotating forward of the trunk upon the left foot (_f_) as an axis.
_h_ Shows the rotating forward of the left leg and foot upon the
trunk (_a_) as an axis. Compare with fig. 4, p. 21; with fig. 24,
p. 47; and with fig. 26, p. 55.--After Weber.]
The alternate rotation of the trunk upon the limb and the limb upon the
trunk is well seen in fig. 27, p. 59.
At _A_ of fig. 27 the trunk (_g_) is observed rotating on the left foot
(_f_). At _D_ of fig. the left leg (_h_) is seen rotating on the trunk
(_a_, _i_): these, as explained, are complementary movements. At _A_
of fig. the right foot (_c_) is firmly placed on the ground, the left
foot (_f_) being in the act of leaving it. The right side of the trunk
is on a lower level than the left, which is being elevated, and in the
act of rolling over the foot. At _B_ of fig. the right foot (_m_) is
still upon the ground, but the left foot having left it is in the act
of swinging forward. At _C_ of fig. the heel of the right foot (_n_)
is raised from the ground, and the left foot is in the act of passing
the right. The right side of the trunk is now being elevated. At _D_
of fig. the heel of the right foot (_o_) is elevated as far as it can
be, the toes of the left foot being depressed and ready to touch the
ground. The right side of the trunk has now reached its highest level,
and is in the act of rolling over the right foot. The left side of the
trunk, on the contrary, is subsiding, and the left leg is swinging
before the right one, preparatory to being deposited on the ground.
From the foregoing it will be evident that the trunk and limbs have
pendulum movements which are natural and peculiar to them, the extent
of which depends upon the length of the parts. A tall man and a short
man can consequently never walk in step if both walk naturally and
according to inclination.[39]