Changes

==='''Using the head to change the centre of gravity'''===  

==='''Using the head to change 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.

 

:::::'''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.   

 

'''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).

 

:::::'''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.

 

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.

 

:::::'''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).  

 

:::::'''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'''===  

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.

 

:::::'''Fig 10.11 Forms of asymmetrical gaits'''

:::::'''Fig 10.11 Forms of asymmetrical gaits'''

:::::Dark blue footprint = right footfall.

:::::Dark blue footprint = right footfall.

 

:::::'''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.  

    

    

 

:::::'''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.   

 

:::::'''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]]