This text deserves being compared with Vierordt's text, published later (1860/ 1862/ 1864/ 1871/ 1877). It is evident that Vierordt knew Longet's handbook and was widely inspired by it, but Vierordt knew more on the inputs of the postural system and he was personnaly involved in a research on upright posture and wrote about it, while Longet made also a research on the postural control, but did not write about it, in his handbook.
After researches undertaken with the aim of determining
the position of the centre of gravity of man body, Borelli said
a little bit vaguely that he was situated inter nates and pubim.
Recently, W. Weber (1) resumed this question and treated it with
all the exactness, which characterises his works on animal mechanics.
According to W. Weber, the centre of gravity of all the body of
a 1,669 metres man is situated either 87.7 mm over the twisting
axis of the hip, or 8,7 mm over the promontory. This skilful experimenter
did not content himself with this first determination; he looked
also for the solution of another question, very important, that
of the position of the centre of gravity of the trunk. Always
operating on the same corpse, he saw that the centre of gravity
of the body, separated from its two lower limbs, raised until
the level of the lower extremity of the breastbone and the ensiform
So, mobile parts on the heads of thighbones have their centre of gravity situated on a straight line led from the superior extremity to the lower extremity of the rachis, at the level of the ensiform process. We shall see that this high position of the centre of gravity of the trunk, over its basis, must be taken in serious consideration for the determination of equilibrium conditions in upright position.
Among very numerous and varied modes of station that man can adopt, we shall content with making the history of standing upright, as being the most important and the most characteristic of human race. We shall study this upright position, in two main circumstances: at first when the body resting on two thigh bones, the weight is transmitted on to two feet pressed on the ground; secondly, when the weight of the body concerns only one of two lower limbs, the foot on the opposite site, although resting on the ground, is not, or very little, used to support the body.
A. Upright position on two feet.
Let us suppose
the standing man, two feet resting on ground, the centres of heels
spread apart from the space which separates the centres of the
heads of thighbones, the knee in extension, in a way that the
thighbone and the shin, placed on the same straight line, rest
vertically on the astragal, and in a way that the line led from
an extremity to other one of the rachis is vertical and passes
by the axis of movement of the pond on thighbones. The vertical
line led from the centre of gravity of the body will fall then
in the middle of the line, which passes by the centre of ankle
joints, and balance will be possible.
The loads, which the spinal column supports, to the left and to the right of the axis, are so exactly balanced as, for lack of an outside cause of movement, nothing can disturb balance in the left-right direction. The trunk can even move on the legs from left to right, or from right to left, without balance is really compromised. Indeed, as long as the vertical line led from the centre of gravity of the body will not fall out of ankle joints, the body will rest solidly on his support basis. Now, let us suppose that side movement is rather considerable so that this vertical line falls exactly on one of two ankle joints, the lower limb on the side where movement will have been executed will be quite complete tilted outside, because the knee joint does not lend itself to any lateral flexion. But, in this position, the shin will have reached the limit of possible side movements on the astragal; of more coxo-femoral joint will have arrived at the extreme adduction, so that articular capacities and resistance of ligaments will prevent this movement of balance from going farther, will hold the body in balance in this extreme situation, and will make impossible a lateral fall. In this lateral direction, connections of the articular surfaces and resistance of ligaments are enough to maintain and to assure balance, without resorting to muscular contraction.
Is it the same in the antero-posterior direction in the case we examine now, that is when the thighbone is exactly in the direction of the shin, and when the hip and knee joints did not reach the extreme limits of extension? The centre of gravity of the trunk, situated at the level of the appendix xiphoïde and on a vertical line, which cuts the axis of rotation of the pond, is in a very high and unstable position over its support basis. Moreover flexion and extension of the coxo-femoral joint are very easy, and only muscular contraction can prevent the trunk, if slightly shaken, from falling forward or behind. What we have just said about the hip joint applies exactly to ankle and tibio-femoral joints. This position would be very precarious and extremely tiring, because it would be only by means of continual muscular contractions that the body could be maintained in balance and returned to its mean position.
But if the trunk is slightly pushed backwards, then the knee and hip joints are placed in their state of extreme extension, and this modification will be enough to return much more stable vertical position in the antero-posterior plan and much less tiring to maintain, without moreover altering its conditions of solidity in the lateral direction.
Then, indeed, the vertical line passing by the centre of gravity of the trunk placed at the level of the appendix xiphoïde will fall a little backwards from the axis of rotation of the pond on the heads of thighbones. The weight of the body will tend to increase the extension of the coxo-femoral joint and to tip over the pond behind, but this movement is made impossible by the resistance of the superior ligament and of the ileo-trochanter strip. As for the fall of the trunk forward, it could not be made without determining a light ascent of the centre of gravity, and that circumstance is enough to prevent it from being made. In this way the balance of the trunk on coxo-femoral joints is assured independently of any muscular contraction.
In this situation, the vertical line led from the centre of gravity of the body will always fall on the middle of the rotation axis of two ankle joints, but it will cross a little in front of tibio-femoral joints. With regard to knee joints, considered as support basis, the body has a very marked tendency to fall forward, but this movement is not possible any more, because the extension of these two joints reached its limits, and the big resistance of the ligamentous device is enough to assure and to maintain balance.
As long as will not arise a considerable outside cause of movement, the body will be held in balance on the hip and knee joint, without muscular fatigue and by the only effect of the tension of the ligamentous device. But quite complete weight is passed on to ankle joints. Here falling is possible and easy in the antero-posterior plane. The ligaments of this joint can oppose neither flexion nor extension, and once balance is broken forward and especially backward, the vertical line passing by the centre of gravity would be soon pulled out of the support basis by the continuation of the movement, and fall would take place. Only muscular power can, by their action, prevent these movements, limit and correct them when they are already begun.
So, in upright position on two feet, balance of the head on the spinal column is assured by the action of cervical muscles; vertebrae are held in their relative position by the elasticity of intervertebral discs and by the yellow ligament, and also by the action of the back muscles. In this condition the rachis is like an inflexible yard put on the sacrum. Then the trunk being slightly push behind, and the hip and knee joints returned to the extreme extension, the resistance of the ligamentous device is enough to assure balance of the trunk on thighs, and on legs. The only axis of rotation around of which fall of the body is possible is that of ankle joints. There, indeed, neither the articular surfaces nor the ligaments are arranged to oppose fall of the body forward or backward. Now, from the very high situation of the centre of gravity of the body over these joints, it results that any movement determines a lowering of the centre of gravity, and that, as a consequence, any begun movement tends by himself to continue, until body is returned to the horizontal position. Therefore body balance on feet is unstable; only muscular contraction can assure it, and for this purpose leg is provided with numerous powerful muscles.
B. Natural upright position, on one foot.
This way of standing
quiet, is commonly called: "Standing at ease", in French,
"Position hanchée". To fix ideas, let us suppose
that the subject stands on his right member, what, moreover, is
the most common case. The right foot bears strongly on the ground,
the knee joint is in extreme extension, the hip joint is pushed
to adduction and extreme extension. The body is slightly tilted
to the right and backwards; the left lower limb, put a little
forward, appreciably bent on the hip and knee joints, rests on
its foot slightly supported on the ground. The weight of the body
concerns the right lower limb nearly completely and exclusively.
Moreover, the vertical line passing by the centre of gravity of
the trunk falls a little backward the right coxo-femoral joint,
and the vertical line led straight ahead from the centre of gravity
of the body passes a little in front of the knee joint and falls
on the ankle joint of the same side.
In the lateral direction, movement is impossible to the right, because the three joints of the lower limb do not allow any movement of this kind. To the left, balance could be broken around the coxo-femoral joint especially, but the left lower limb, although resting about freely on the ground, offers a sufficient resistance to oppose any fall in this way.
What we said about the disposal of the centres of gravity of the trunk and the complete body, with regard to the right lower limb, indicates enough that, in the antero-posterior plane, balance is assured around the knee joint and around the hip, by the same mechanism that we explained in standing upright on two feet. It is only around the ankle joint that body can fall forward or backward, and that muscular contraction has to intervene actively to prevent body from falling. This way of standing upright has a considerable advantage on the precedent, which explains why it can be prolonged much longer and with less fatigue, why after a long station on two feet, there is real relief passing to standing at ease on a single foot.
Whatever be, indeed, the kind of upright position adopted, the marked tendency of the body is falling forward around ankle joint. Now, during the station on two feet, only the powerful calf muscles can oppose this movement, they are inevitably almost in permanent contraction and from this continuance of action results a real, considerable, inevitable fatigue. On the contrary, in the station on one foot, the lower limb opposite to the one that bears weight is slightly bent and rests on the ground in a plane slightly anterior. In any forward movement, the body falls on this inactive member; and this slightly bent member makes the effect of one real propping up, resists to the pressure of the body, maintains it in its position by a light muscular contraction. This resistance helps the action of the calf muscles, relieves them by exempting them from an active and persevering contraction, and, by this fact, decreases considerably the fatigue inseparable from standing upright.
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