Comparative foot morphology

Comparative foot morphology involves comparing the form of

There is considerable variation in the scale and proportions of body and limb, as well as the nature of loading, during standing and locomotion both among and between quadrupeds and bipeds.^[1] The anterior-posterior body mass distribution varies considerably among mammalian quadrupeds, which affects limb loading. When standing, many terrestrial quadrupeds support more of their weight on their forelimbs rather than their hind limbs;^[2][3] however, the distribution of body mass and limb loading changes when they move.^[4][5][6] Humans have a lower-limb mass that is greater than their upper-limb mass. The hind limbs of the dog and horse have a slightly greater mass than the forelimbs, whereas the elephant has proportionally longer limbs. The elephant's forelimbs are longer than its hind limbs.^[7]

In the horse

Even many terrestrial vertebrates exhibit differences in the scaling of limb dimension, limb coordination and magnitude of forelimb-hind limb loading, in the dog, horse and elephant the structure of the distal forelimb is similar to that of the distal hind limb.^[7][8][12] In the human, the structures of the hand are generally similar in shape and arrangement to those of the foot. Terrestrial vertebrate quadrupeds and bipeds generally possess distal limb and foot endoskeleton structures that are aligned in series, stacked in a relatively vertical orientation and arranged in a quasi-columnar fashion in the extended limb.^[1][13][14] In the dog and horse, the bones of the proximal limbs are oriented vertically, whereas the distal limb structures of the ankle and foot have an angulated orientation. In humans and elephants, a vertical-column orientation of the bones in the limbs and feet is also evident for associated skeletal muscle-tendon units.^[6] The horse's foot contains an external nail (hoof) oriented about the perimeter in the shape of a semicircle. The underlying bones are arranged in a semi-vertical orientation.^[15][16] The dog's paw similarly contains bones arranged in a semi-vertical orientation.

In the human and the elephant, the column orientation of the foot complex is replaced in humans by a plantigrade orientation, and in elephants by a semi-plantigrade alignment of the hind limb foot structure.^[6] This difference in orientation in the foot bones and joints of humans and elephants helps them to adapt to variations in the terrain.^[17]

Distal cushion

Distal cushions on the foot of a raccoon and an elephant

Many representative terrestrial vertebrates possess a distal cushion on the under-surface of the foot. The dog's paw contains a number of visco-elastic pads oriented along the middle and distal foot. The horse possesses a centralized digital pad known as the frog, which is located at the distal aspect of the foot and surrounded by the hoof.^[12] Humans possess a tough fibro and elastic pad of fat that is anchored to the skin and bone of the rear portion of the foot.^[18][19]

The foot of the elephant possesses what is perhaps one of the most unusual distal cushions found in vertebrates. The forefoot (manus) and hindfoot (pes) contain huge pads of fat that are scaled to cope with the massive loadings imposed by the largest terrestrial vertebrate. In addition, a cartilage-like projection (prepollex in the forelimb and prehallux in the hind limb) appears to anchor the distal cushion to the bones of the elephant's foot.^[20]

The distal cushions of all these organisms (dog, horse, human and elephant) are dynamic structures during locomotion, alternating between phases of compression and expansion; it has been suggested that these structures thereby reduce the loads experienced by the skeletal system.^{[18][19][20][21]}

Organization

Arrangement of foot structures:

Because of the wide variety in body types, scaling and morphology of the distal limbs of terrestrial vertebrates, there exists a degree of controversy concerning the nature and organization of foot structures. One organizational approach to understanding foot structures makes distinctions regarding their regional anatomy. The foot structures are divided into segments from proximal to distal and are grouped according to similarity in shape, dimension and function. In this approach, the foot may be described in three segments: as the hindfoot, midfoot and forefoot.

The hindfoot is the most proximal and posterior portion of the foot.^[22] Functionally, the structures contained in this region are typically robust, possessing a larger size and girth than the other structures of the foot. The structures of the hindfoot are usually adapted for transmitting large loads between the proximal and distal aspects of the limb when the foot contacts the ground. This is apparent in the human and elephant foot, where the hindfoot undergoes greater loading during initial contact in many forms of locomotion.^[23] The hindfoot structures of the dog and horse are located relatively proximally compared to the elephant and human foot.

The midfoot is the intermediate portion of the foot between the hindfoot and forefoot. The structures in this region are intermediate in size, and typically transmit loads from the hindfoot to the forefoot. The human transverse tarsal joint of the midfoot transmits forces from the subtalar joint in the hindfoot to the forefoot joints (metatarsophalangeal and interphalangeal) and associated bones (metatarsals and phalanges).^[24] The midfoot of the dog, horse and elephant contains similar intermediate structures having similar functions to those of the human midfoot.

The forefoot represents the most distal portion of the foot. In the human and elephant, the bone structures contained in this region are generally longer and narrower. The structures of the forefoot play a role in providing leverage for terminal stance propulsion and load transfer.^[6][23]

Function

Load transmission of the foot in representative terrestrial vertebrates:

Dog paw

The paw of the dog has a

The horse's foot is in an unguligrade orientation. The columnar orientation of bones and connective tissue is similarly well-aligned to transmit loads during the weight-bearing phase of locomotion. The thick keratinized and semicircular hoof changes shape during loading and unloading. Similarly, the cushioned frog situated centrally at the rear ends of the hoof undergoes compression during loading, and expansion when unloaded. Together, the hoof and cushioned frog structures may work in concert with hoof capsule to provide shock absorption.^[21] The horse hoof also acts dynamically during loading, which may cushion the endoskeleton from high loads that would otherwise produce critical deformation.

Elephant foot

The hind limb and foot of the elephant are oriented semi-plantigrade, and closely resemble the structure and function of the human foot. The tarsals and metapodials are arranged so as to form an arch, similarly to the human foot. The six toes of each foot of the elephant are enclosed in a flexible sheath of skin.^[20][26] Similar to the dog's paw, the elephant's phalanges are oriented in a downward direction. The distal phalanges of the elephant do not directly touch the ground, and are attached to the respective nail/hoof.^[27] Distal cushions occupy the spaces between the muscle tendon units and ligaments within the hindfoot, midfoot and forefoot bones on the plantar surface.^[28] The distal cushion is highly innervated by sensory structures (Meissner's and Pacinian corpuscles), making the distal foot one of the most sensitive structures of the elephant (more so than its trunk).^[20] The cushions of the elephant's foot respond to the requirement to store and absorb mechanical loads when they are compressed, and to distribute locomotor loads over a large area in order to keep foot tissue stresses within acceptable levels.^[20] In addition, the musculoskeletal foot arch and sole cushion of the elephant act in concert, similarly to the horse's cushioned frog and hoof^[6] and the human foot.^[29] In the elephant, the nearly half-cupula-shaped arrangement of the bony elements of the metatarsals and toes has interesting similarities to the structure of the arches of human feet.^[29][30]

Recently, scientists at the

The unique

In the horse, dryness of the hoof may cause stiffening of the external foot structure. The stiffer hoof reduces the foot's load attenuation capacity, rendering the horse unable to bear much weight on the distal limb. Similar characteristic features emerge in the human foot in the form of the pes cavus alignment deformity, which is produced by tight connective tissue structures and joint congruency that create a rigid foot complex. Individuals with pes cavus display characteristic reduced load-attenuation features, and other structures proximal to the foot may compensate with increased load transfer (i.e., excessive loading to the knees, hips, lumbo-pelvic joints or lumbar vertebrae).^[24] Foot disorders are common in captive elephants. However, the cause is poorly understood.^[34]

See also

References