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Quadrupedal Mechanics - Anatomy & Physiology (view source)
Revision as of 09:23, 29 November 2013
, 09:23, 29 November 2013no edit summary
The main movement for propulsion is protraction—retraction. It is useful to regard each limb as a pendulum, swinging in a sagittal plane. During slow locomotion, the pivot for the hindlimb is the hip joint (Fig 8.4 a). For the forelimb, the pivot is not the shoulder, but is more proximal (Figs 5.3, 8.3 b, 8.4 b).
The main movement for propulsion is protraction—retraction. It is useful to regard each limb as a pendulum, swinging in a sagittal plane. During slow locomotion, the pivot for the hindlimb is the hip joint (Fig 8.4 a). For the forelimb, the pivot is not the shoulder, but is more proximal (Figs 5.3, 8.3 b, 8.4 b).
[[File:QMFig 8.4.png|thumb|'''Fig 8.4 Limb movements of a walking cat''']]
:::::'''Fig 8.4 Limb movements of a walking cat'''
:::::'''Fig 8.4 Limb movements of a walking cat'''
::::::1 Foot leaves ground.
::::::1 Foot leaves ground.
::::::2. Foot contacts ground. Stifle, hock and elbow are extending.
::::::2. Foot contacts ground. Stifle, hock and elbow are extending.
::::::2-1 Retraction, or stance phase. Stifle, hock and elbow continue extending. Hip and shoulder are lowered slightly.
::::::2-1 Retraction, or stance phase. Stifle, hock and elbow continue extending. Hip and shoulder are lowered slightly.
The movements of the joints of each limb are small compared with the movements of the limb as a whole. The movement during walking is therefore best described as that of a pendulum with a pivot at the hip for the hindlimb. The pivot for the forelimb, however, lies proximally to the shoulder joint.
==='''The effect of size on pendulums'''===
==='''The effect of size on pendulums'''===
The centre of gravity of the limb is made as proximal as possible (Fig. 8.5 b). First, the bellies of muscles operating distal joints should lie close to the pivot, transmitting their force by means of long tendons. This is made easier if propulsive muscles operate mainly about the pivot, rather than by bending the limb distally. Fig. 8.4 shows that in walking, the limb is indeed an almost rigid pendulum. If distal joints have fewer axes of movement than proximal joints, the tendinous control apparatus is much simplified. We saw earlier in this chapter that this is the case. Also, the muscles responsible for fixing each joint lie proximally to the joint (Fig. 6.3). An additional advantage of having muscles located proximally is that they are then closer to the heart, thus reducing the length of the vascular channels and thereby improving the efficiency of the blood supply.
The centre of gravity of the limb is made as proximal as possible (Fig. 8.5 b). First, the bellies of muscles operating distal joints should lie close to the pivot, transmitting their force by means of long tendons. This is made easier if propulsive muscles operate mainly about the pivot, rather than by bending the limb distally. Fig. 8.4 shows that in walking, the limb is indeed an almost rigid pendulum. If distal joints have fewer axes of movement than proximal joints, the tendinous control apparatus is much simplified. We saw earlier in this chapter that this is the case. Also, the muscles responsible for fixing each joint lie proximally to the joint (Fig. 6.3). An additional advantage of having muscles located proximally is that they are then closer to the heart, thus reducing the length of the vascular channels and thereby improving the efficiency of the blood supply.
Secondly, the structures forming a friction contact with the ground must be light, whether they be naturally grown keratin, or metal shoes. The ungulate (hooved animal) evolved for this reason.
Secondly, the structures forming a friction contact with the ground must be light, whether they be naturally grown keratin, or metal shoes. The ungulate (hooved animal) evolved for this reason.
[[File:QMFig 8.5.png|thumb|'''Fig 8.5 Improvements to pendulum design''']]
[[File:QMFig 8.6.png|thumb|'''Fig 8.6 Phylogenetic elongation of the distal end of the mammalian limb''' ]]
Thirdly, the number of digits can be reduced, effectively to two in many artiodactyls (even toed ungulates) and to one in some perissodactyls (odd toed ungulates). A single structure is stronger than a paired structure of the same weight and length. Thus even in artiodactyls, fusion of the metapodial segment often occurs. Such a reduction in digits enhances cursorial (Latin: cursor, runner) ability and sacrifices the ability to use hands for specialised procedures.
Thirdly, the number of digits can be reduced, effectively to two in many artiodactyls (even toed ungulates) and to one in some perissodactyls (odd toed ungulates). A single structure is stronger than a paired structure of the same weight and length. Thus even in artiodactyls, fusion of the metapodial segment often occurs. Such a reduction in digits enhances cursorial (Latin: cursor, runner) ability and sacrifices the ability to use hands for specialised procedures.
:::::'''Fig 8.5 Improvements to pendulum design'''
:::::'''Fig 8.5 Improvements to pendulum design'''
:::::Modifications to a basic pendulum (a) that increase the natural frequency of oscillation (b, e) or the distance moved by the tip in each swing (c, d).
:::::Modifications to a basic pendulum (a) that increase the natural frequency of oscillation (b, e) or the distance moved by the tip in each swing (c, d).
:::::'''Fig 8.6 Phylogenetic elongation of the distal end of the mammalian limb'''
:::::'''Fig 8.6 Phylogenetic elongation of the distal end of the mammalian limb'''
:::::Comparison of the hind limb of the bear (a), dog (b) and deer (c) shows both a progressive elongation of the distal segments of the limb and a progressive incorporation of the distal segments into the limb pendulum from the plantigrade (a) to the digitigrade (b) and unguligrade (c) stance, as the limb becomes cursorily adapted.
==='''Balancing the pendulum'''===
==='''Balancing the pendulum'''===
The distal limb segments should be as long as possible (Fig. 8.5 c). This is achieved either by actual elongation of the distal segments relative to the proximal segments (Figs. 8.2 and 8.6), or by incorporation of the distal segments into the pendulum by running on the tip of the toes (Fig. 8.6).
The distal limb segments should be as long as possible (Fig. 8.5 c). This is achieved either by actual elongation of the distal segments relative to the proximal segments (Figs. 8.2 and 8.6), or by incorporation of the distal segments into the pendulum by running on the tip of the toes (Fig. 8.6).
[[File:QMFig 8.7.png|thumb|'''Fig 8.7 Adaptation of the pectoral girdle in a cursorial mammal''']]
The pivot of the pendulum should be as proximal as possible (Fig. 8.5 d). Some mammals have a complete bony pectoral girdle, which compels the forelimb to pivot from the shoulder joint (Fig. 8.7 a). A cursorial adaptation has been the reduction of bony elements of the pectoral girdle, freeing the scapula and allowing the limb to pivot from a point proximal to the shoulder (Figs. 5.3, 8.3 b, 8.4 b, 8.7 b). The dorsal border of the scapula moves cranially in limb retraction (Fig. 5.3). This enables muscles connecting the scapula to the head and neck (Mm rhomboideus, trapezius and serratus ventralis), and movements of the neck, to assist retraction. In galloping, in which the footfalls of the forelimb closely follow one another, the head is lowered during retraction and raised during protraction.
The pivot of the pendulum should be as proximal as possible (Fig. 8.5 d). Some mammals have a complete bony pectoral girdle, which compels the forelimb to pivot from the shoulder joint (Fig. 8.7 a). A cursorial adaptation has been the reduction of bony elements of the pectoral girdle, freeing the scapula and allowing the limb to pivot from a point proximal to the shoulder (Figs. 5.3, 8.3 b, 8.4 b, 8.7 b). The dorsal border of the scapula moves cranially in limb retraction (Fig. 5.3). This enables muscles connecting the scapula to the head and neck (Mm rhomboideus, trapezius and serratus ventralis), and movements of the neck, to assist retraction. In galloping, in which the footfalls of the forelimb closely follow one another, the head is lowered during retraction and raised during protraction.
The pivot is moved into the vertebral column in animals that gallop with a flexible column. This applies to forelimbs and hindlimbs, and not only elongates the pendulum but enables axial muscles to participate in propulsion (Fig. 5.3).
The pivot is moved into the vertebral column in animals that gallop with a flexible column. This applies to forelimbs and hindlimbs, and not only elongates the pendulum but enables axial muscles to participate in propulsion (Fig. 5.3).
:::::'''Fig 8.7 Adaptation of the pectoral girdle in a cursorial mammal'''
:::::'''Fig 8.7 Adaptation of the pectoral girdle in a cursorial mammal'''
:::::In the echidna (a), the clavicle and the coracoid form two complete bony bridges between the scapula and the sternum. In the dog (b), only remnants of these bridges remain, represented by the coracoid process of the scapula and a fibrous or occasionally ossified tendinous intersection within the brachiocephalicus muscle. The pivot for the forelimb can be located proximally to the shoulder.
:::::In the echidna (a), the clavicle and the coracoid form two complete bony bridges between the scapula and the sternum. In the dog (b), only remnants of these bridges remain, represented by the coracoid process of the scapula and a fibrous or occasionally ossified tendinous intersection within the brachiocephalicus muscle. The pivot for the forelimb can be located proximally to the shoulder.
==='''Reciprocating the pendulum'''===
==='''Reciprocating the pendulum'''===
==='''Short and longheaded muscles'''===
==='''Short and longheaded muscles'''===
Over most joints, there are muscles with actions limited to a particular single joint, as in Fig. 6.3, and other muscles that, in addition, act over adjacent joints (Fig. 8.8). It is convenient to call muscles acting over a single joint "shortheaded", and muscles acting over more than one joint, "longheaded". This nomenclature concurs with, for instance, that used for the long head of M. triceps brachii, as well as the short and long digital extensor muscles of the hindlimb.
Over most joints, there are muscles with actions limited to a particular single joint, as in Fig. 6.3, and other muscles that, in addition, act over adjacent joints (Fig. 8.8). It is convenient to call muscles acting over a single joint "shortheaded", and muscles acting over more than one joint, "longheaded". This nomenclature concurs with, for instance, that used for the long head of M. triceps brachii, as well as the short and long digital extensor muscles of the hindlimb.
[[File:QMFig 8.8.png|thumb|'''Fig 8.8 Short and longheaded muscles''']]
:::::'''Fig 8.8 Short and longheaded muscles'''
:::::'''Fig 8.8 Short and longheaded muscles'''
:::::The pairs of counteractive muscles shown in the hindlimb of the dog (a) are m. rectus femoris and m. semitendinosus, as longheaded muscles, and m. vastus lateralis and m. adductor as shortheaded muscles. Muscle lengths are compared between the flexed hip and stifle position (b) and the extended position (c). The longheaded muscles tend to maintain a constant length over a natural range of limb function.
:::::The pairs of counteractive muscles shown in the hindlimb of the dog (a) are m. rectus femoris and m. semitendinosus, as longheaded muscles, and m. vastus lateralis and m. adductor as shortheaded muscles. Muscle lengths are compared between the flexed hip and stifle position (b) and the extended position (c). The longheaded muscles tend to maintain a constant length over a natural range of limb function.
==='''A severe restriction on muscle function: active insufficiency'''===
==='''A severe restriction on muscle function: active insufficiency'''===
[[File:QMFig 8.9.png|thumb|'''Fig 8.9 The contribution of a longheaded muscle to the standing jump of the cat''']]
There is a problem associated with shortheaded muscles that is fundamental to the design of limbs. For any muscle, there is only one stage at which the force is maximal over the whole range over which the muscle contracts (Fig. 4.4). A shortheaded muscle is therefore properly effective at only one angle of the joint over which it acts. A muscle is therefore insufficient because of the way its contractile mechanism works. A muscle with its sarcomeres at any length other than near 2.2 µm is said to be actively insufficient.
There is a problem associated with shortheaded muscles that is fundamental to the design of limbs. For any muscle, there is only one stage at which the force is maximal over the whole range over which the muscle contracts (Fig. 4.4). A shortheaded muscle is therefore properly effective at only one angle of the joint over which it acts. A muscle is therefore insufficient because of the way its contractile mechanism works. A muscle with its sarcomeres at any length other than near 2.2 µm is said to be actively insufficient.
:::::'''Fig 8.9 The contribution of a longheaded muscle to the standing jump of the cat'''
:::::'''Fig 8.9 The contribution of a longheaded muscle to the standing jump of the cat'''
:::::The semitendinosus muscle (dashed outline) has the same length when the hindlimb is in the fully flexed position as it does with the limb in the fully extended position. This muscle therefore assists hip extension without changing from its optimal length. It is able to do this only by the counteractive action of stifle extensors.
:::::The semitendinosus muscle (dashed outline) has the same length when the hindlimb is in the fully flexed position as it does with the limb in the fully extended position. This muscle therefore assists hip extension without changing from its optimal length. It is able to do this only by the counteractive action of stifle extensors.
==='''A severe restriction on limb function: passive insufficiency'''===
==='''A severe restriction on limb function: passive insufficiency'''===
[[File:QMFig 8.10.png|thumb|'''Fig 8.10 Ligamentous adaptations of longhead muscles''']]
Even when a longheaded muscle is a strap muscle and has the longest range of contraction possible (44% of fully stretched length: see Fig. 4.3), it is not possible to allow for full independent movement of both the joints it acts over. Thus when the hip of any quadruped, and also any but the most limber human, is fully flexed, the stifle cannot be fully extended. In this case, the semitendinosus muscle is insufficient but for a different reason to that for the actively insufficient shortheaded muscle described above. Longheaded muscles are intrinsically passively insufficient, especially so when they are highly pennate.
Even when a longheaded muscle is a strap muscle and has the longest range of contraction possible (44% of fully stretched length: see Fig. 4.3), it is not possible to allow for full independent movement of both the joints it acts over. Thus when the hip of any quadruped, and also any but the most limber human, is fully flexed, the stifle cannot be fully extended. In this case, the semitendinosus muscle is insufficient but for a different reason to that for the actively insufficient shortheaded muscle described above. Longheaded muscles are intrinsically passively insufficient, especially so when they are highly pennate.
:::::'''Fig 8.10 Ligamentous adaptations of longhead muscles'''
:::::M. fibularis of the horse is entirely ligamentous. Stifle flexion (b) therefore produces hock flexion automatically, and M. gastrocnemius is kept at optimal length. The action of M. flexor digitorum superficialis also ensures that stifle extension (a) is also accompanied by hock extension. The position of the limb in (a) and (b) represent the retraction and protraction phases of a fast gallop as in Fig. 8.1.
:::::M. fibularis of the horse is entirely ligamentous. Stifle flexion (b) therefore produces hock flexion automatically, and M. gastrocnemius is kept at optimal length. The action of M. flexor digitorum superficialis also ensures that stifle extension (a) is also accompanied by hock extension. The position of the limb in (a) and (b) represent the retraction and protraction phases of a fast gallop as in Fig. 8.1.
==='''Limbs are controlled proximally by key joints and nerves'''===
==='''Limbs are controlled proximally by key joints and nerves'''===
=='''The design of the vertebral trunk'''==
=='''The design of the vertebral trunk'''==
==='''The trunk is a horizontal beam
==='''The trunk is a horizontal beam'''===
The body of the dog shown in Fig. 9.1 a is represented mechanically by a beam, the vertebral column A–D, 45 cm long with a uniformly distributed load of 0.2 kg.cm–2 (the neck and trunk), and a concentrated load cranially of 2 kg (the head). The total weight is therefore 11 kg. With the fore and hindlimbs each represented each by one simple strut and placed as shown at B and C, the opposing reaction forces can be calculated as 7.9 kg for the forelimbs and 3.1 kg for the hindlimbs. This turns out to be approximately the situation in a real dog in which the weight borne by the forelimb is about twice that borne by the hindlimb. Since the weight of the hindlimb of the dog is about the same as that of the forelimb, one must conclude that the weight of hindlimb in excess of what is needed for postural support is available for propulsion; in other words the forelimb and the hindlimb of the dog differ in their postural and propulsive roles. The forelimb supports and balances, the hindlimb pushes.
The body of the dog shown in Fig. 9.1 a is represented mechanically by a beam, the vertebral column A–D, 45 cm long with a uniformly distributed load of 0.2 kg.cm–2 (the neck and trunk), and a concentrated load cranially of 2 kg (the head). The total weight is therefore 11 kg. With the fore and hindlimbs each represented each by one simple strut and placed as shown at B and C, the opposing reaction forces can be calculated as 7.9 kg for the forelimbs and 3.1 kg for the hindlimbs. This turns out to be approximately the situation in a real dog in which the weight borne by the forelimb is about twice that borne by the hindlimb. Since the weight of the hindlimb of the dog is about the same as that of the forelimb, one must conclude that the weight of hindlimb in excess of what is needed for postural support is available for propulsion; in other words the forelimb and the hindlimb of the dog differ in their postural and propulsive roles. The forelimb supports and balances, the hindlimb pushes.
[[File:QMFig 9.1.png|thumb|'''Fig 9.1 Bending in the vertebral column''']]
In Fig. 9.1c, muscles preventing bending include (a) the epaxial muscles of the neck and thorax, (b) the hypaxial muscles of the thorax, and (c), the hypaxial muscles of the lumbar region. Props assisting the action of these muscles and the ligaments shown are (p) the spinous process of the axis, (q) the spinous process of the first thoracic vertebra (together with adjacent spinous processes), (r) the spinous process of the anticlinal vertebra (eleventh thoracic), (s) the sacrum and (t) the costal arch.
In Fig. 9.1c, muscles preventing bending include (a) the epaxial muscles of the neck and thorax, (b) the hypaxial muscles of the thorax, and (c), the hypaxial muscles of the lumbar region. Props assisting the action of these muscles and the ligaments shown are (p) the spinous process of the axis, (q) the spinous process of the first thoracic vertebra (together with adjacent spinous processes), (r) the spinous process of the anticlinal vertebra (eleventh thoracic), (s) the sacrum and (t) the costal arch.
(u) and (v) are the forelimb and hindlimb respectively. Note that the shape of the vertebral column is opposite to that of the bending moments shown in Fig. 9.1b for the horizontal beam A — D.
(u) and (v) are the forelimb and hindlimb respectively. Note that the shape of the vertebral column is opposite to that of the bending moments shown in Fig. 9.1b for the horizontal beam A — D.
:::::'''Fig 9.1 Bending in the vertebral column'''
:::::'''Fig 9.1 Bending in the vertebral column'''
:::::The trunk of a dog is represented in Fig. 9.1a by a horizontal beam A — D, 45 cm long with a uniformly distributed load of 0.2 kg.cm-1, together with a concentrated load cranially of 2 kg (the head). The total weight is 11 kg. With the limbs placed as shown, the opposing reaction forces are 7.9 kg in the forelimb and 3.1 kg in the hindlimb. Bending moments along the vertebral column are shown graphically in Fig. 9.1b.
:::::The trunk of a dog is represented in Fig. 9.1a by a horizontal beam A — D, 45 cm long with a uniformly distributed load of 0.2 kg.cm-1, together with a concentrated load cranially of 2 kg (the head). The total weight is 11 kg. With the limbs placed as shown, the opposing reaction forces are 7.9 kg in the forelimb and 3.1 kg in the hindlimb. Bending moments along the vertebral column are shown graphically in Fig. 9.1b.
==='''Forces along the beam'''===
==='''Forces along the beam'''===
[[File:QMFig 9.2.png|thumb|'''Fig 9.2 Transverse section of a pig at the thoracolumbar junction''']]
The skeleton of the trunk is a row of separate bones, the vertebral column, on its own hardly a stable bridge. Its rigidity is maintained to a large extent by a compressive force produced in the massive muscle surrounding the column (Fig. 9.2). As a bridge, therefore, this structure is comparable to prestressed concrete. We have already seen that the trabecular network is oriented axially within the vertebral bodies, to resist the tensile stress (Fig. 3.2). When the axial muscles are relaxed as during anesthesia, the vertebral column is no longer rigid.
The skeleton of the trunk is a row of separate bones, the vertebral column, on its own hardly a stable bridge. Its rigidity is maintained to a large extent by a compressive force produced in the massive muscle surrounding the column (Fig. 9.2). As a bridge, therefore, this structure is comparable to prestressed concrete. We have already seen that the trabecular network is oriented axially within the vertebral bodies, to resist the tensile stress (Fig. 3.2). When the axial muscles are relaxed as during anesthesia, the vertebral column is no longer rigid.
Rigidity of the vertebral column is attained also by the supraspinous and dorsal and ventral longitudinal ligaments (Figs. 9.2 and 9.3) and by the shape of the synovial intervertebral (zygapophyseal) joints.
Rigidity of the vertebral column is attained also by the supraspinous and dorsal and ventral longitudinal ligaments (Figs. 9.2 and 9.3) and by the shape of the synovial intervertebral (zygapophyseal) joints.
:::::'''Fig 9.2 Transverse section of a pig at the thoracolumbar junction'''
:::::'''Fig 9.2 Transverse section of a pig at the thoracolumbar junction'''
:::::Caudal aspect, showing the intervertebral joint and the structures supporting the vertebral column.
==='''The effect of gravity on the beam'''===
==='''The effect of gravity on the beam'''===
==='''Inclined spinous processes'''===
==='''Inclined spinous processes'''===
[[File:QMFig 9.3.png|thumb|'''Fig 9.3 Median section of an intervertebral joint in the thoracic region''']]
We can tell the direction of the forces on the spinous processes of the vertebrae by the way they lean, as you can easily tell in which direction the contestants in a tug-of-war are pulling, and resisting being pulled. Mechanical struts are more effective when a force is applied to them at an acute angle rather than a right angle. Therefore, the tall spines of the cranial thoracic vertebrae slope caudally. In the cat, this occurs as far caudally as the tenth thoracic vertebrae (Fig. 8.2), and the eleventh in the dog (Fig. 9.1 c).
We can tell the direction of the forces on the spinous processes of the vertebrae by the way they lean, as you can easily tell in which direction the contestants in a tug-of-war are pulling, and resisting being pulled. Mechanical struts are more effective when a force is applied to them at an acute angle rather than a right angle. Therefore, the tall spines of the cranial thoracic vertebrae slope caudally. In the cat, this occurs as far caudally as the tenth thoracic vertebrae (Fig. 8.2), and the eleventh in the dog (Fig. 9.1 c).
:::::'''Fig 9.3 Median section of an intervertebral joint in the thoracic region'''
:::::'''Fig 9.3 Median section of an intervertebral joint in the thoracic region'''
:::::Structures are shown which resist bending or hydrostatically distribute a uniform compression stress on the vertebral bodies.
==='''Flexibility of the vertebral column'''===
==='''Flexibility of the vertebral column'''===
Apart from the joint between the first two cervical vertebrae, the intervertebral joints are not synovial joints. It is important that joints in a column under compression should be able to distribute pressure evenly over the opposing surfaces of the vertebral bodies, regardless of the angle of the joint. Intervertebral movement and even pressure distribution are achieved hydrostatically by the nucleus pulposus between each body (Figs. 9.2, 9.3). The change in shape of this mucoprotein gel with movement of the joint is accommodated by the annulus fibrosis, a lamellar arrangement of collagen fibres. The lamellae pass from vertebra to vertebra with their fibres crossing each other obliquely. They are therefore constructed to withstand the tensile stress imposed by the bending of the vertebral column, and also to prevent rupture of the nucleus pulposus. The mechanical and morphological concept suggested by the term intervertebral disc is misleading.
Apart from the joint between the first two cervical vertebrae, the intervertebral joints are not synovial joints. It is important that joints in a column under compression should be able to distribute pressure evenly over the opposing surfaces of the vertebral bodies, regardless of the angle of the joint. Intervertebral movement and even pressure distribution are achieved hydrostatically by the nucleus pulposus between each body (Figs. 9.2, 9.3). The change in shape of this mucoprotein gel with movement of the joint is accommodated by the annulus fibrosis, a lamellar arrangement of collagen fibres. The lamellae pass from vertebra to vertebra with their fibres crossing each other obliquely. They are therefore constructed to withstand the tensile stress imposed by the bending of the vertebral column, and also to prevent rupture of the nucleus pulposus. The mechanical and morphological concept suggested by the term intervertebral disc is misleading.
==='''This rigid beam can be flexible'''
==='''This rigid beam can be flexible'''===
[[File:QMFig 9.4.png|thumb|'''Fig 9.4 Flexibility of the vertebral column in galloping animals''']]
In the dog, the maximum movement occurs between the segments around the tenth and the eleventh thoracic vertebrae. These segments are the most vulnerable to the injury of intervertebral structures. This is especially so in dogs with short legs which must use their vertebral column to a greater extent for propulsion than long legged dogs.
In the dog, the maximum movement occurs between the segments around the tenth and the eleventh thoracic vertebrae. These segments are the most vulnerable to the injury of intervertebral structures. This is especially so in dogs with short legs which must use their vertebral column to a greater extent for propulsion than long legged dogs.
The flexibility of the vertebral column varies between species. A galloping horse maintains an almost rigid back (Fig. 9.4 a). This is also true for some of the fastest terrestrial animals, the gazelles and antelopes, some of which are small (Fig. 9.4 b). This is in contrast with the cheetah (Fig. 9.4 c). The big cats, which have flexible vertebral columns, spend little of their time standing, as compared with herbivores of equivalent size, which have more rigid columns. The axial muscles of herbivores are presumably more postural in function than those of carnivores and the smaller animals.
The flexibility of the vertebral column varies between species. A galloping horse maintains an almost rigid back (Fig. 9.4 a). This is also true for some of the fastest terrestrial animals, the gazelles and antelopes, some of which are small (Fig. 9.4 b). This is in contrast with the cheetah (Fig. 9.4 c). The big cats, which have flexible vertebral columns, spend little of their time standing, as compared with herbivores of equivalent size, which have more rigid columns. The axial muscles of herbivores are presumably more postural in function than those of carnivores and the smaller animals.
:::::'''Fig 9.4 Flexibility of the vertebral column in galloping animals'''
:::::'''Fig 9.4 Flexibility of the vertebral column in galloping animals'''
:::::The diagrams on the left represent full extension and the right side, full flexion of the vertebral column. The horse (a) and the gazelle (b) do not flex their backs appreciably, in contrast with the cheetah, (c).
==='''The vertebral column in locomotion'''===
==='''The vertebral column in locomotion'''===
The locomotory apparatus of the trunk is driven by the caudal epaxial muscles, notably the longissimus muscle (Fig. 9.2) and its attachments. In the species in which the apparatus is well developed, such as rodents, felids and canids, the attachments of the longissimus muscle to the spinous and transverse processes of the caudal thoracic and lumbar vertebrae are functionally strengthened by an angling of these processes away from the direction of tension. Thus, the transverse processes of the lumbar vertebrae slope cranially and ventrally. For the spinous processes, the effect is opposite to that seen in the cranial segments of the vertebral column where they slope caudally against the tension in the dorsal stays of the cervicothoracic region (Fig. 9.1 c). There is therefore an anticlinal vertebra, which represents the changing point (Fig. 8.2). In species that presumably do not use their vertebral column effectively to lengthen the hindlimb pendulum in locomotion, an anticlinal vertebra is absent.
The locomotory apparatus of the trunk is driven by the caudal epaxial muscles, notably the longissimus muscle (Fig. 9.2) and its attachments. In the species in which the apparatus is well developed, such as rodents, felids and canids, the attachments of the longissimus muscle to the spinous and transverse processes of the caudal thoracic and lumbar vertebrae are functionally strengthened by an angling of these processes away from the direction of tension. Thus, the transverse processes of the lumbar vertebrae slope cranially and ventrally. For the spinous processes, the effect is opposite to that seen in the cranial segments of the vertebral column where they slope caudally against the tension in the dorsal stays of the cervicothoracic region (Fig. 9.1 c). There is therefore an anticlinal vertebra, which represents the changing point (Fig. 8.2). In species that presumably do not use their vertebral column effectively to lengthen the hindlimb pendulum in locomotion, an anticlinal vertebra is absent.
We can now proceed to consider the action of the trunk and limbs as used by quadrupeds in performing unusual tasks.
We can now proceed to consider the action of the trunk and limbs as used by quadrupeds in performing usual tasks.
=='''Locomotion'''==
=='''Locomotion'''==
==='''The standing quadruped'''===
==='''The standing quadruped'''===
[[File:QMFig 10.1.png|thumb|'''Fig 10.1 Location of the centre of gravity of a horse''' ]]
The location of the centre of gravity of a horse is shown in Fig. 10.1. In the horse, 55% of the weight of the body is supported by the forelimbs and 45% by the hindlimbs. A horse usually rests one hindlimb while standing, but can do so for a forelimb only with difficulty. Relatively more weight is presumably supported by the forelimb of the bison and by the hindlimb of the rabbit (Fig. 10.2). The dog carries two thirds of its weight on its forelimbs and one third on its hindlimbs (Fig. 9.1).
The location of the centre of gravity of a horse is shown in Fig. 10.1. In the horse, 55% of the weight of the body is supported by the forelimbs and 45% by the hindlimbs. A horse usually rests one hindlimb while standing, but can do so for a forelimb only with difficulty. Relatively more weight is presumably supported by the forelimb of the bison and by the hindlimb of the rabbit (Fig. 10.2). The dog carries two thirds of its weight on its forelimbs and one third on its hindlimbs (Fig. 9.1).
[[File:QMFig 10.2.png|thumb|'''Fig 10.2 Species differences in the location of the centre of gravity''']]
:::::'''Fig 10.1 Location of the centre of gravity of a horse'''
:::::'''Fig 10.1 Location of the centre of gravity of a horse'''
:::::This is found in a transverse plane immediately caudal to the xiphoid process, and in a dorsal plane between the ventral and middle thirds of the trunk.
:::::This is found in a transverse plane immediately caudal to the xiphoid process, and in a dorsal plane between the ventral and middle thirds of the trunk.
:::::'''Fig 10.2 Species differences in the location of the centre of gravity'''
:::::'''Fig 10.2 Species differences in the location of the centre of gravity'''
:::::Compare these probable locations in the bison (a) and rabbit (b) with that determined for the horse in Fig. 10.1.
:::::Compare these probable locations in the bison (a) and rabbit (b) with that determined for the horse in Fig. 10.1.
==='''Using the head to change the centre of gravity'''===
==='''Using the head to change the centre of gravity'''===
[[File:QMFig 10.3.png|thumb|'''Fig 10.3 Use of the head and neck to alter the centre of gravity''']]
The function of the limbs as struts in supporting the trunk, and the function of the trunk as a supporting beam, have already been discussed. The remaining major bulk of the body, the head and neck, is also important in the standing quadruped because of its influence on the centre of gravity. Raising the head will bring it nearer to the trunk in the dorsal plane, and lowering it to bring the neck parallel to the trunk axis will increase this distance. Thus raising the head moves the centre of gravity caudally, removing weight from the forelimbs, and lowering it moves the centre of gravity cranially, increasing weight on the forelimbs (Fig. 10.3). Thus when a lame forelimb takes weight, the head is raised to relieve it. When one wishes to lift a forefoot of a horse or ox, its head should be raised and move laterally away from the foot to be lifted. On the other hand, it should be lowered with a hindfoot is to be lifted. An animal can be prevented from kicking with its hindlimb by holding its head high.
The function of the limbs as struts in supporting the trunk, and the function of the trunk as a supporting beam, have already been discussed. The remaining major bulk of the body, the head and neck, is also important in the standing quadruped because of its influence on the centre of gravity. Raising the head will bring it nearer to the trunk in the dorsal plane, and lowering it to bring the neck parallel to the trunk axis will increase this distance. Thus raising the head moves the centre of gravity caudally, removing weight from the forelimbs, and lowering it moves the centre of gravity cranially, increasing weight on the forelimbs (Fig. 10.3). Thus when a lame forelimb takes weight, the head is raised to relieve it. When one wishes to lift a forefoot of a horse or ox, its head should be raised and move laterally away from the foot to be lifted. On the other hand, it should be lowered with a hindfoot is to be lifted. An animal can be prevented from kicking with its hindlimb by holding its head high.
[[File:QMFig 10.4.png|thumb|'''Fig 10.4 Curvature of the vertebral column in the sitting cat''']]
:::::'''Fig 10.3 Use of the head and neck to alter the centre of gravity'''
:::::'''Fig 10.3 Use of the head and neck to alter the centre of gravity'''
:::::'''Fig 10.4 Curvature of the vertebral column in the sitting cat'''
:::::'''Fig 10.4 Curvature of the vertebral column in the sitting cat'''
:::::The shape of the thoracic cage (doted outline) is maintained by bending the vertebral column predominantly near the thoracolumbar junction.
:::::The shape of the thoracic cage (doted outline) is maintained by bending the vertebral column predominantly near the thoracolumbar junction.
==='''Motion without change of location'''===
==='''Motion without change of location'''===
'''Standing up:''' The smaller the animal, the more easily it can spring to its feet with a sudden back movement, but even horses can manage to do this. A slower, more deliberate movement is, for the dog, cat and pig, essentially the reverse process of lying down. Cattle and sheep raise themselves up on to their hindlimbs first, then on to their flexed carpuses. The horse rolls its trunk to sternal recumbency, collects its four legs underneath, and then protracts and extends its forelimbs, raising its cranial trunk before its caudal trunk, by pushing its weight over its forelimbs with a thrust from the hindlimbs.
'''Standing up:''' The smaller the animal, the more easily it can spring to its feet with a sudden back movement, but even horses can manage to do this. A slower, more deliberate movement is, for the dog, cat and pig, essentially the reverse process of lying down. Cattle and sheep raise themselves up on to their hindlimbs first, then on to their flexed carpuses. The horse rolls its trunk to sternal recumbency, collects its four legs underneath, and then protracts and extends its forelimbs, raising its cranial trunk before its caudal trunk, by pushing its weight over its forelimbs with a thrust from the hindlimbs.
[[File:QMFig 10.5.png|thumb|'''Fig 10.5 Stable positions for the horse on its hindlimbs''']]
'''Standing on hindlimbs:''' In order to achieve this, the centre of gravity must lie vertically above the hindfeet (Fig. 10.5 a, b). This is easier for horses than cattle. Horses can prance and kick with their forelimbs, but cattle cannot. A copulating bull can maintain the position only briefly, and uses the cow for support. Standing on hindlimbs is especially easy for animals with a more caudally located centre of gravity (Fig. 10.2 b).
'''Standing on hindlimbs:''' In order to achieve this, the centre of gravity must lie vertically above the hindfeet (Fig. 10.5 a, b). This is easier for horses than cattle. Horses can prance and kick with their forelimbs, but cattle cannot. A copulating bull can maintain the position only briefly, and uses the cow for support. Standing on hindlimbs is especially easy for animals with a more caudally located centre of gravity (Fig. 10.2 b).
[[File:QMFig 10.6.png|thumb|'''Fig 10.6 Propulsion and friction forces''']]
:::::'''Fig 10.5 Stable positions for the horse on its hindlimbs'''
:::::'''Fig 10.5 Stable positions for the horse on its hindlimbs'''
:::::'''Fig 10.6 Propulsion and friction forces'''
:::::'''Fig 10.6 Propulsion and friction forces'''
:::::The insertion of a plate between the hose and the ground enables the forces acting on the body during a propulsive movement of the hindlimbs during the gallop to be visualised.
:::::The insertion of a plate between the hose and the ground enables the forces acting on the body during a propulsive movement of the hindlimbs during the gallop to be visualised.
==='''Propulsive force'''===
==='''Propulsive force'''===
A propulsive force, applied to the limb to retract it, propels the animal over the ground. Opposing the propulsive force is friction force, acting between the animal body and the ground. It is the friction force that acts in the direction of movement. This is best visualised by interposing a plate between the animal and the ground (Fig. 10.6). When there is no friction between the plate and the ground, the propulsive force accelerates the plate and not the animal body. If the friction force equals the propulsion force, the plate remains at rest, and the animal is accelerated. Friction, which is usually thought to oppose motion, is essential for locomotion. The construction of the surface contact with the ground is vitally important and is often a source of breakdown in running animals, just as it is for motor vehicles.
A propulsive force, applied to the limb to retract it, propels the animal over the ground. Opposing the propulsive force is friction force, acting between the animal body and the ground. It is the friction force that acts in the direction of movement. This is best visualised by interposing a plate between the animal and the ground (Fig. 10.6). When there is no friction between the plate and the ground, the propulsive force accelerates the plate and not the animal body. If the friction force equals the propulsion force, the plate remains at rest, and the animal is accelerated. Friction, which is usually thought to oppose motion, is essential for locomotion. The construction of the surface contact with the ground is vitally important and is often a source of breakdown in running animals, just as it is for motor vehicles.
[[File:QMFig 10.7.png|thumb|'''Fig 10.7 The propulsive apparatus of the greyhound''']]
In animals that use their vertebral column in running, the fore and hindlimb pendulums swing about a pivot near the thoracolumbar junction (Fig. 10.7). Axial muscles and extrinsic limb muscles acting over this pivot produce the propulsive thrust.
In animals that use their vertebral column in running, the fore and hindlimb pendulums swing about a pivot near the thoracolumbar junction (Fig. 10.7). Axial muscles and extrinsic limb muscles acting over this pivot produce the propulsive thrust.
:::::(a) Hindlimb. Retraction is brought about by the extensors of the vertebral column (x, m. longissimus and its dorsal aponeurosis) and the extensors of the hip (y, the middle gluteal muscle and z, the caudal thigh muscles).
:::::(a) Hindlimb. Retraction is brought about by the extensors of the vertebral column (x, m. longissimus and its dorsal aponeurosis) and the extensors of the hip (y, the middle gluteal muscle and z, the caudal thigh muscles).
:::::(b) Forelimb. Retraction is brought about by the flexors of the vertebral column (w, the external abdominal oblique muscle and x, rectus abdominis), and suitably placed muscles of the forelimb (y, latissimus dorsi and z, the deep pectoral muscle).
:::::(b) Forelimb. Retraction is brought about by the flexors of the vertebral column (w, the external abdominal oblique muscle and x, rectus abdominis), and suitably placed muscles of the forelimb (y, latissimus dorsi and z, the deep pectoral muscle).
==='''Gaits'''===
==='''Gaits'''===
All animals use the slow walk at times when stability is important, for example when walking over rough terrain and by animals bearing awkward loads. It is visualised by a gait diagram in Fig. 10.10 a.
All animals use the slow walk at times when stability is important, for example when walking over rough terrain and by animals bearing awkward loads. It is visualised by a gait diagram in Fig. 10.10 a.
[[File:QMFig 10.8.png|thumb|'''Fig 10.8 Support triangles''']]
:::::'''Fig 10.8 Support triangles'''
:::::'''Fig 10.8 Support triangles'''
:::::When a horse stands on all four feet, its centre of gravity lies as shown diagrammatically in dorsal view (a), cranial to the diagonals between the feet. Either hindfoot can now be lifted without change in weight distribution, because the centre of gravity lies within the triangle of support formed by the other three feet (b, c). Raising the head as in Fig.10.3 a directs the centre of gravity caudally. In (d), either forefoot can now be lifted. The centre of gravity need not be directed so far caudally if the head deviates to the side; if it deviates to the left, the right forefoot can be lifted (e).
:::::When a horse stands on all four feet, its centre of gravity lies as shown diagrammatically in dorsal view (a), cranial to the diagonals between the feet. Either hindfoot can now be lifted without change in weight distribution, because the centre of gravity lies within the triangle of support formed by the other three feet (b, c). Raising the head as in Fig.10.3 a directs the centre of gravity caudally. In (d), either forefoot can now be lifted. The centre of gravity need not be directed so far caudally if the head deviates to the side; if it deviates to the left, the right forefoot can be lifted (e).
[[File:QMFig 10.9.png|thumb|'''Fig 10.9 Head, neck, trunk and tail movements during the slow walk''']]
:::::'''Fig 10.9 Head, neck, trunk and tail movements during the slow walk'''
:::::'''Fig 10.9 Head, neck, trunk and tail movements during the slow walk'''
:::::If protraction of the left hind limb in (a) produces the axial displacement shown in (b), the left forefoot can be lifted and the limb protracted. Forelimb protraction straightens the body axis and prepares it for the same movement on the opposite side of the body (c, d). Using this slow gait, even a highly adapted cursorial animal uses to some extent the same axial movements for locomotion as an animal with only rudimentary limbs.
:::::If protraction of the left hind limb in (a) produces the axial displacement shown in (b), the left forefoot can be lifted and the limb protracted. Forelimb protraction straightens the body axis and prepares it for the same movement on the opposite side of the body (c, d). Using this slow gait, even a highly adapted cursorial animal uses to some extent the same axial movements for locomotion as an animal with only rudimentary limbs.
==='''The fast walk'''===
==='''The fast walk'''===
[[File:QMFig 10.10.png|thumb|'''Fig 10.10 Extreme forms of symmetrical gaits of the horse'''']]
If the sequence in the slow walk is speeded up to any extent, the next foot in the sequence will be off the ground before the previous limb moved has contacted the ground. Thus, for a time, there are only two feet left on the ground (Fig. 10.10 b). The animal would fall over if stopped at certain stages of the cycle. Stability is achieved by a dynamic rather than a static equilibrium. For the heaviest quadrupeds, such as elephants, the fast walk represents the greatest degree of static instability allowable, and it is their fastest gait. Horses, and in particular some breeds such as the Tennessee Walking Horse, can be trained to perform a variety of styles of fast walk with names such as "paso", "slow gait", "running walk", "rack" and "plantation gait", useful because of the smoothness for the rider.
If the sequence in the slow walk is speeded up to any extent, the next foot in the sequence will be off the ground before the previous limb moved has contacted the ground. Thus, for a time, there are only two feet left on the ground (Fig. 10.10 b). The animal would fall over if stopped at certain stages of the cycle. Stability is achieved by a dynamic rather than a static equilibrium. For the heaviest quadrupeds, such as elephants, the fast walk represents the greatest degree of static instability allowable, and it is their fastest gait. Horses, and in particular some breeds such as the Tennessee Walking Horse, can be trained to perform a variety of styles of fast walk with names such as "paso", "slow gait", "running walk", "rack" and "plantation gait", useful because of the smoothness for the rider.
:::::'''Fig 10.10 Extreme forms of symmetrical gaits of the horse'''
:::::'''Fig 10.10 Extreme forms of symmetrical gaits of the horse'''
:::::From a, b to c or a, b to d there are progressively shorter periods of contact of each foot with the ground.
:::::From a, b to c or a, b to d there are progressively shorter periods of contact of each foot with the ground.
:::::All these gaits are symmetrical because the time interval between footfalls of pairs of forefeet or hindfeet is always one half of a stride interval. Thus there is symmetry between right and left sides of the body. Pale blue horseshoe = left footfall; Dark blue horseshoe = right footfall.
:::::All these gaits are symmetrical because the time interval between footfalls of pairs of forefeet or hindfeet is always one half of a stride interval. Thus there is symmetry between right and left sides of the body. Pale blue horseshoe = left footfall; Dark blue horseshoe = right footfall.
==='''Two–beat symmetrical gaits'''===
==='''Two–beat symmetrical gaits'''===
==='''Asymmetrical gaits'''===
==='''Asymmetrical gaits'''===
When the footfalls of a pair of forefeet or hindfeet are unevenly spaced in time, the gait is asymmetrical. Symmetrical gaits are all some variant of the gallop, which is a general term. The beat is therefore composed of couplets, separated by pauses, and one foot is a leading foot. For the horse at least, the leading foot for the forelimbs is conventionally the second foot of the couplet to strike the ground (Fig. 10.11 c: 6th stage). The leading forelimb is always on the inside of a turn (Fig. 10.12). A galloping horse changes its lead during the stage when both forelimbs are off the ground (Fig. 10.11: 2nd stage). It may be necessary for a horse to change lead during a jump in order to prepare for a turn immediately on landing.
When the footfalls of a pair of forefeet or hindfeet are unevenly spaced in time, the gait is asymmetrical. Symmetrical gaits are all some variant of the gallop, which is a general term. The beat is therefore composed of couplets, separated by pauses, and one foot is a leading foot. For the horse at least, the leading foot for the forelimbs is conventionally the second foot of the couplet to strike the ground (Fig. 10.11 c: 6th stage). The leading forelimb is always on the inside of a turn (Fig. 10.12). A galloping horse changes its lead during the stage when both forelimbs are off the ground (Fig. 10.11: 2nd stage). It may be necessary for a horse to change lead during a jump in order to prepare for a turn immediately on landing.
[[File:QMFig 10.11.png|thumb|'''Fig 10.11 Forms of asymmetrical gaits''']]
:::::'''Fig 10.11 Forms of asymmetrical gaits'''
:::::'''Fig 10.11 Forms of asymmetrical gaits'''
:::::Dark blue footprint = right footfall.
:::::Dark blue footprint = right footfall.
[[File:QMFig 10.12.png|thumb|'''Fig 10.12 Limb sequencing at a galloping turn''']]
:::::'''Fig 10.12 Limb sequencing at a galloping turn'''
:::::'''Fig 10.12 Limb sequencing at a galloping turn'''
:::::The zebra is turning to its left. The right forefoot contacts the ground first and the leading (left) limb is on the inside of the turn.
:::::The zebra is turning to its left. The right forefoot contacts the ground first and the leading (left) limb is on the inside of the turn.
==='''Transverse and rotary sequence'''===
==='''Transverse and rotary sequence'''===
4.Finally, both extended and gathered suspension occurs in the fast springing gallop of the rabbit, carnivores (Fig. 10.11 d) and some artiodactyls.
4.Finally, both extended and gathered suspension occurs in the fast springing gallop of the rabbit, carnivores (Fig. 10.11 d) and some artiodactyls.
[[File:QMFig 10.13.png|thumb|'''Fig 10.13 Basic patterns of footfalls in asymmetric gaits''' ]]
:::::'''Fig 10.13 Basic patterns of footfalls in asymmetric gaits'''
:::::'''Fig 10.13 Basic patterns of footfalls in asymmetric gaits'''
:::::The canter is a transverse gallop, modified to a three beat gait.
:::::The canter is a transverse gallop, modified to a three beat gait.
The half bound occurs when a spring is made using both hindlimbs together, but the animal lands on one forefoot before the other, as seen in rabbits and hares.
The half bound occurs when a spring is made using both hindlimbs together, but the animal lands on one forefoot before the other, as seen in rabbits and hares.
The running jump occurs in with a springing gallop during the stage of extended suspension (Fig. 10.11 d: 4th stage). In the horse, the jump is not part of a normal gallop.
The running jump occurs in with a springing gallop during the stage of extended suspension (Fig. 10.11 d: 4th stage). In the horse, the jump is not part of a normal gallop.
[[File:QMFig 10.14.png|thumb|'''Fig 10.14 The running jump of a horse''']]
:::::'''Fig 10.14 The running jump of a horse'''
:::::'''Fig 10.14 The running jump of a horse'''
The kinetic energy of the horse has been partly stored as elastic energy in the hindlimbs; this energy can subsequently be released to oppose gravity. The necessity for a concept of elasticity in quadrupedal mechanics cannot be overstated.
The kinetic energy of the horse has been partly stored as elastic energy in the hindlimbs; this energy can subsequently be released to oppose gravity. The necessity for a concept of elasticity in quadrupedal mechanics cannot be overstated.
[[Category:Musculoskeletal System - Anatomy & Physiology]]
[[Category:Musculoskeletal System - Anatomy & Physiology]]