Comparative foot morphology
The paw of the dog, the hoof of the horse, the manus (forefoot) and pes (hindfoot) of the elephant, and the foot of the human all share some common features of structure, organization and function.
Their foot structures function as the load-transmission platform which is essential to balance, standing and types of locomotion (such as walking, trotting, galloping and running).
The discipline of biomimetics applies the information gained by comparing the foot morphology of a variety of terrestrial vertebrates to human-engineering problems.
For instance, it may provide insights that make it possible to alter the foot's load transmission in people who wear an external orthosis because of paralysis from spinal-cord injury, or who use a prosthesis following the diabetes-related amputation of a leg.
[9] The locomotion of the elephant (which is the largest terrestrial vertebrate) displays a similar loading distribution on its hind limbs and forelimbs.
The comparative morphology of the distal limb and foot structure of some representative terrestrial vertebrates reveals some interesting similarities.
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.
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.
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.
The angled orientation of the elongated metatarsal and the digits extends the area available for storing and releasing mechanical energy in the muscle tendon units originating proximally to the ankle joint and terminating at the distal aspect of the foot bones.
These muscle tendon unit structures appear well designed to aid in the ground-reaction transmission of forces that is essential for locomotion.
Similarly, the cushioned frog situated centrally at the rear ends of the hoof undergoes compression during loading, and expansion when unloaded.
[27] Distal cushions occupy the spaces between the muscle tendon units and ligaments within the hindfoot, midfoot and forefoot bones on the plantar surface.
[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 Royal Veterinary College in the United Kingdom have discovered that the elephant possesses a sixth false toe, a sesamoid, located similarly to the giant panda's extra "thumb".
In this configuration, the foot is able to absorb and damp the large loads encountered during heel strike and early weight acceptance.
The stiffer hoof reduces the foot's load attenuation capacity, rendering the horse unable to bear much weight on the distal limb.
